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Network Working Group                                         C. KaufmanRequest for Comments: 1507                 Digital Equipment Corporation                                                          September 1993DASSDistributed Authentication Security ServiceStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard.  Discussion and   suggestions for improvement are requested.  Please refer to the   current edition of the "Internet Official Protocol Standards" for the   standardization state and status of this protocol.  Distribution of   this memo is unlimited.Table of Contents1.   Introduction ................................................21.1  What is DASS? ..........................................21.2  Central Concepts .......................................41.3  What This Document Won't Tell You .....................111.4  The Relationship between DASS and ISO Standards .......171.5  An Authentication Walkthrough .........................202.   Services Used ..............................................252.1  Time Service ..........................................252.2  Random Numbers ........................................262.3  Naming Service ........................................263.   Services Provided ..........................................373.1  Certificate Contents ..................................383.2  Encrypted Private Key Structure .......................403.3  Authentication Tokens .................................403.4  Credentials ...........................................433.5  CA State ..............................................473.6  Data types used in the routines .......................473.7  Error conditions ......................................493.8  Certificate Maintenance Functions .....................493.9  Credential Maintenance Functions ......................553.10 Authentication Procedures .............................633.11 DASSlessness Determination Functions ..................874.   Certificate and message formats ............................894.1  ASN.1 encodings .......................................894.2  Encoding Rules ........................................964.3  Version numbers and forward compatibility .............964.4  Cryptographic Encodings ...............................97    Annex A - Typical Usage ........................................101A.1  Creating a CA ........................................101Kaufman                                                         [Page 1]

RFC 1507                          DASS                    September 1993A.2  Creating a User Principal ............................102A.3  Creating a Server Principal ..........................103A.4  Booting a Server Principal ...........................103A.5  A user logs on to the network ........................103A.6  An Rlogin (TCP/IP) connection is made ................104A.7  A Transport-Independent Connection ...................104    Annex B - Support of the GSSAPI ................................104B.1  Summary of GSSAPI ....................................105B.2  Implementation of GSSAPI over DASS ...................106B.3  Syntax ...............................................110    Annex C - Imported ASN.1 definitions ...........................112    Glossary .......................................................114   Security Considerations .........................................119   Author's Address ................................................119   Figures    Figure 1 - Authentication Exchange Overview ....................241. Introduction1.1 What is DASS?   Authentication is a security service. The goal of authentication is   to reliably learn the name of the originator of a message or request.   The classic way by which people authenticate to computers (and by   which computers authenticate to one another) is by supplying a   password.  There are a number of problems with existing password   based schemes which DASS attempts to solve.  The goal of DASS is to   provide authentication services in a distributed environment which   are both more secure (more difficult for a bad guy to impersonate a   good guy) and easier to use than existing mechanisms.   In a distributed environment, authentication is particularly   challenging.  Users do not simply log on to one machine and use   resources there.  Users start processes on one machine which may   request services on another.  In some cases, the second system must   request services from a third system on behalf of the user.  Further,   given current network technology, it is fairly easy to eavesdrop on   conversations between computers and pick up any passwords that might   be going by.   DASS uses cryptographic mechanisms to provide "strong, mutual"   authentication.  Mutual authentication means that the two parties   communicating each reliably learn the name of the other.  Strong   authentication means that in the exchange neither obtains any   information that it could use to impersonate the other to a third   party.  This can't be done with passwords alone.  Mutual   authentication can be done with passwords by having a "sign" and a   "counter-sign" which the two parties must utter to assure one anotherKaufman                                                         [Page 2]

RFC 1507                          DASS                    September 1993   of their identities.  But whichever party speaks first reveals   information which can be used by the second (unauthenticated) party   to impersonate it.  Longer sequences (often seen in spy movies)   cannot solve the problem in general.  Further, anyone who can   eavesdrop on the conversation can impersonate either party in a   subsequent conversation (unless passwords are only good once).   Cryptography provides a means whereby one can prove knowledge of a   secret without revealing it.  People cannot execute cryptographic   algorithms in their heads, and thus cannot strongly authenticate to   computers directly.  DASS lays the groundwork for "smart cards":   microcomputers sealed in credit cards which when activated by a PIN   will strongly authenticate to a computer.  Until smart cards are   available, the first link from a user to a DASS node remains   vulnerable to eavesdropping.  DASS mechanisms are constructed so that   after the initial authentication, smart card or password based   authentication looks the same.   Today, systems are constructed to think of user identities in terms   of accounts on individual computers.  If I have accounts on ten   machines, there is no way a priori to see that those ten accounts all   belong to the same individual.  If I want to be able to access a   resource through any of the ten machines, I must tell the resource   about all ten accounts.  I must also tell the resource when I get an   eleventh account.   DASS supports the concept of global identity and network login.  A   user is assigned a name from a global namespace and that name will be   recognized by any node in the network.  (In some cases, a resource   may be configured as accessible only by a particular user acting   through a particular node.  That is an access control decision, and   it is supported by DASS, but the user is still known by his global   identity).  From a practical point of view, this means that a user   can have a single password (or smart card) which can be used on all   systems which allow him access and access control mechanisms can   conveniently give access to a user through any computer the user   happens to be logged into.  Because a single user secret is good on   all systems, it should never be necessary for a user to enter a   password other than at initial login.  Because cryptographic   mechanisms are used, the password should never appear on the network   beyond the initial login node.   DASS was designed as a component of the Distributed System Security   Architecture (DSSA) (see "The Digital Distributed System Security   Architecture" by M. Gasser, A. Goldstein, C. Kaufman, and B. Lampson,   1989 National Computer Security Conference).  It is a goal of DSSA   that access control on all systems be based on users' global names   and the concept of "accounts" on computers eventually be replaced   with unnamed rights to execute processes on those computers.  UntilKaufman                                                         [Page 3]

RFC 1507                          DASS                    September 1993   this happens, computers will continue to support the concept of   "local accounts" and access controls on resources on those systems   will still be based on those accounts.  There is today within the   Berkeley rtools running over the Internet Protocol suite the concept   of a ".rhosts database" which gives access to local accounts from   remote accounts.  We envision that those databases will be extended   to support granting access to local accounts based on DASS global   names as a bridge between the past and the future.  DASS should   greatly simplify the administration of those databases for the   (presumably common) case where a user should be granted access to an   account ignoring his choice of intermediate systems.1.2 Central Concepts1.2.1 Strong Authentication with Public Keys   DASS makes heavy use of the RSA Public Key cryptosystem.  The   important properties of the RSA algorithms for purposes of this   discussion are:    - It supports the creation of a public/private key pair, where      operations with one key of the pair reverse the operations of      the other, but it is computationally infeasible to derive the      private key from the public key.    - It supports the "signing" of a message with the private key,      after which anyone knowing the public key can "verify" the      signature and know that it was constructed with knowledge of      the private key and that the message was not subsequently      altered.    - It supports the "enciphering" of a message by anyone knowing      the public key such that only someone with knowledge of the      private key can recover the message.   With access to the RSA algorithms, it is easy to see how one could   construct a "strong" authentication mechanism.  Each "principal"   (user or computer) would construct a public/private key pair, publish   the public key, and keep secret the private key.  To authenticate to   you, I would write a message, sign it with my private key, and send   it to you.  You would verify the message using my public key and know   the message came from me.  If mutual authentication were desired, you   could create an acknowledgment and sign it with your private key; I   could verify it with your public key and I would know you received my   message.   The authentication algorithms used by DASS are considerably more   complex than those described in the paragraph above in order to dealKaufman                                                         [Page 4]

RFC 1507                          DASS                    September 1993   with a large number of practical concerns including subtle security   threats.  Some of these are discussed below.1.2.2 Timestamps vs. Challenge/Response   Cryptosystems give you the ability to sign messages so that the   receiver has assurance that the signer of the message knew some   cryptographic secret.  Free-standing public key based authentication   is sufficiently expensive that it is unlikely that anyone would want   to sign every message of an interactive communication, and even if   they did they would still face the threat of someone rearranging the   messages or playing them multiple times.  Authentication generally   takes place in the context of establishing some sort of "connection,"   where a conversation will ensue under the auspices of the single   peer-entity authentication.  This connection might be   cryptographically protected against modification or reordering of the   messages, but any such protection would be largely independent of the   authentication which occurred at the start of the connection.  DASS   provides as a side effect of authentication the provision of a shared   key which may be used for this purpose.   If in our simple minded authentication above, I signed the message   "It's really me!" with my private key and sent it to you, you could   verify the signature and know the message came from me and give the   connection in which this message arrived access to my resources.   Anyone watching this message over the network, however, could replay   it to any server (just like a password!) and impersonate me.  It is   important that the message I send you only be accepted by you and   only once.  I can prevent the message from being useful at any other   server by including your name in the message.  You will only accept   the message if you see your name in it.  Keeping you from accepting   the message twice is harder.   There are two "standard" ways of providing this replay protection.   One is called challenge/response and the other is called timestamp-   based.  In a challenge response type scheme, I tell you I want to   authenticate, you generate a "challenge" (generally a number), and I   include the challenge in the message I sign.  You will only accept a   message if it contains the recently generated challenge and you will   make sure you never issue the same challenge to me twice (either by   using a sequence number, a timestamp, or a random number big enough   that the probability of a duplicate is negligible).  In the   timestamp-based scheme, I include the current time in my message.   You have a rule that you will not accept messages more than - say -   five minutes old and you keep track of all messages you've seen in   the last five minutes.  If someone replays the message within five   minutes, you will reject it because you will remember you've seen it   before; if someone replays it after five minutes, you will reject itKaufman                                                         [Page 5]

RFC 1507                          DASS                    September 1993   as timed out.   The disadvantage of the challenge/response based scheme is that it   requires extra messages.  While one-way authentication could   otherwise be done with a single message and mutual authentication   with one message in each direction, the challenge/response scheme   always requires at least three messages.   The disadvantage of the timestamp-based scheme is that it requires   secure synchronized time.  If our clocks drift apart by more than   five minutes, you will reject all of my attempts to authenticate.  If   a network time service spoofer can convince you to turn back your   clock and then subsequently replays an expired message, you will   accept it again.  The multicast nature of existing distributed time   services and the likelihood of detection make this an unlikely   threat, but it must be considered in any analysis of the security of   the scheme.  The timestamp scheme also requires the server to keep   state about all messages seen in the clock skew interval.  To be   secure, this must be kept on stable storage (unless rebooting takes   longer than the permitted clock skew interval).   DASS uses the timestamp-based scheme.  The primary motivations behind   this decision were so that authentication messages could be   "piggybacked" on existing connection establishment messages and so   that DASS would fit within the same "form factor" (number and   direction of messages) as Kerberos.1.2.3 Delegation   In a distributed environment, authentication alone is not enough.   When I log onto a computer, not only do I want to prove my identity   to that computer, I want to use that computer to access network   resources (e.g., file systems, database systems) on my behalf.  My   files should (normally) be protected so that I can access them   through any node I log in through.  DASS allows them to be so   protected without allowing all of the systems that I might ever use   to access those files in my absence.  In the process of logging in,   my password gives my login node access to my RSA secret.  It can use   that secret to "impersonate" me on any requests it makes on my   behalf.  It should forget all secrets associated with me when I log   off.  This limits the trust placed in computer systems.  If someone   takes control of a computer, they can impersonate all people who use   that computer after it is taken over but no others.   Normally when I access a network service, I want to strongly   authenticate to it.  That is, I want to prove my identity to that   service, but I don't want to allow that service to learn anything   that would allow it to impersonate me.  This allows me to use aKaufman                                                         [Page 6]

RFC 1507                          DASS                    September 1993   service without trusting it for more than the service it is   delivering.  When using some services, for example remote login   services, I may want that service to act on my behalf in calling   additional services.  DASS provides a mechanism whereby I can pass   secrets to such services that allow them to impersonate me.   Future versions of this architecture may allow "limited delegation"   so that a user may delegate to a server only those rights the server   needs to carry out the user's wishes.  This version  can limit   delegation only in terms of time.  The information a user gives a   server (other than the initial login node) can be used to impersonate   the user but only for a limited period of time.  Smart cards will   permit that time limitation to apply to the initial login node as   well.1.2.4 Certification Authorities   A flaw in the strong authentication mechanism described above is that   it assumes that every "principal" (user and node) knows the public   key of every other principal it wants to authenticate.  If I can fool   a server into thinking my public key is actually your public key, I   can impersonate you by signing a message, saying it is from you, and   having the server verify the message with what it thinks is your   public key.   To avoid the need to securely install the public key of every   principal in the database of every other principal, the concept of a   "Certification Authority" was invented.  A certification authority is   a principal trusted to act as an introduction service.  Each   principal goes to the certification authority, presents its public   key, and proves it has a particular name (the exact mechanisms for   this vary with the type of principal and the level of security to be   provided).  The CA then creates a "certificate" which is a message   containing the name and public key of the principal, an expiration   date, and bookkeeping information signed by the CA's private key.   All "subscribers" to a particular CA can then be authenticated to one   another by presenting their certificates and proving knowledge of the   corresponding secret.  CAs need only act when new principals are   being named and new private keys created, so that can be maintained   under tight physical security.   The two problems with the scheme as described so far are "revocation"   and "scaleability".1.2.4.1 Certificate Revocation   Revocation is the process of announcing that a key has (or may have)   fallen into the wrong hands and should no longer be accepted as proofKaufman                                                         [Page 7]

RFC 1507                          DASS                    September 1993   of some particular identity.  With certificates as described above,   someone who learns your secret and your certificate can impersonate   you indefinitely - even after you have learned of the compromise.  It   lacks the ability corresponding to changing your password.  DASS   supports two independent mechanisms for revoking certificates. In the   future, a third may be added.   One method for revocation is using timeouts and renewals of   certificates.  Part of the signed message which is a certificate may   be a time after which the certificate should not be believed.   Periodically, the CA would renew certificates by signing one with a   later timeout.  If a key were compromised, a new key would be   generated and a new certificate signed.  The old certificate would   only be valid until its timeout.  Timeouts are not perfect revocation   mechanisms because they provide only slow revocation (timeouts are   typically measured in months for the load on the CA and communication   with users to be kept manageable) and they depend on servers having   an accurate source of the current time.  Someone who can trick a   server into turning back its clock can use expired certificates.   The second method is by listing all non-revoked certificates in the   naming service and believing only certificates found there.  The   advantage of this method is that it is almost immediate (the only   delay is for name service "skulking" and caching delays).  The   disadvantages are: (1) the availability of authentication is only as   good as the availability of the naming service and (2) the security   of revocation is only as good as the security of the naming service.   A third method for revocation - not currently supported by DASS - is   for certification authorities to periodically issue "revocation   lists" which list certificates which should no longer be accepted.1.2.4.2 Certification Authority Hierarchy   While using a certification authority as an introduction service   scales much better than having every principal learn the public key   of every other principal by some out of band means, it has the   problem that it creates a central point of trust.  The certification   authority can impersonate any principal by inventing a new key and   creating a certificate stating that the new key represents the   principal.  In a large organization, there may be no individual who   is sufficiently trusted to operate the CA.  Even if there were, in a   large organization it would be impractical to have every individual   authenticate to that single person.  Replicating the CA solves the   availability problem but makes the trust problem worse.  When   authentication is to be used in a global context - between companies   - the concept of a single CA is untenable.Kaufman                                                         [Page 8]

RFC 1507                          DASS                    September 1993   DASS addresses this problem by creating a hierarchy of CAs.  The CA   hierarchy is tied to the naming hierarchy.  For each directory in the   namespace, there is a single CA responsible for certifying the public   keys of its members.  That CA will also certify the public keys of   the CAs of all child directories and of the CA of the parent   directory.  With this cross-certification, it is possible knowing the   public key of any CA to verify the public keys of a series of   intermediate CAs and finally to verify the public key of any   principal.   Because the CA hierarchy is tied to the naming hierarchy, the trust   placed in any individual CA is limited.  If a CA is compromised, it   can impersonate any of the principals listed in its directory, but it   cannot impersonate arbitrary principals.   DASS provides mechanisms for every principal to know the public key   of its "parent" CA - the CA controlling the directory in which it is   named.  The result is the following rules for the implications of a   compromised CA:    a) A CA can impersonate any principal named in its directory.    b) A CA can impersonate any principal to a server named in its       directory.    c) A CA can impersonate any principal named in a subdirectory to       any server not named in the same subdirectory.    d) A CA can impersonate to any server in a subdirectory any       principal not named in the same subdirectory.   The implication is that a compromise low in the naming tree will   compromise all principals below that directory while a compromise   high in the naming tree will compromise only the authentication of   principals far apart in the naming hierarchy.  In particular, when   multiple organizations share a namespace (as they do in the case of   X.500), the compromise of a CA in one organization can not result in   false authentication within another organization.   DASS uses the X.500 directory hierarchy for principal naming.  At the   top of the hierarchy are names of countries.  National authorities   are not expected to establish certification authorities (at least   initially), so an alternative mechanism must be used to authenticate   entities "distant" in the naming hierarchy.  The mechanism for this   in DASS is the "cross-certificate" where a CA certifies the public   key for some CA or principal not its parent or child.  By limiting   the chains of certificates they will use to parent certificates   followed by a single "cross certificate" followed by childKaufman                                                         [Page 9]

RFC 1507                          DASS                    September 1993   certificates, a DASS implementation can avoid the need to have CAs   near the root of the tree or can avoid the requirement to trust them   even if they do exist.  A special case can also be supported whereby   a global authority whose name is not the root can certify the local   roots of independent "islands".1.2.5 User vs. Node Authentication   In concept, DASS mechanisms support the mutual authentication of two   principals regardless of whether those principals are people,   computers, or applications.  Those mechanisms have been extended,   however, to deal with a common case of a pair of principals acting   together (a user and a node) authenticating to a single principal (a   remote server).  This is done by having optionally in each   credentials structure two sets of secrets - one for the user and one   for the node.  When authentication is done using such credentials,   both secrets sign the request so the receiving party can verify that   both principals are present.   This setup has a number of advantages.  It permits access controls to   be enforced based on both the identity of the user and the identity   of the originating node.  It also makes it possible to define users   of systems who have no network wide identities who can access network   resources on the basis of node credentials alone.  The security of   such a setup is less because a node can impersonate all of its users   even when they are not logged in, but it offers an easier transition   from existing global identities for all users.1.2.6 Protection of User Keys   DASS mechanisms generally deal with authentication between principals   each knowing a private key.  For principals who are people, special   mechanisms are provided for maintaining that private key.  In   particular, it many cases it will be most convenient to keep   passwords as secrets rather than private keys.  This architecture   specifies a means of storing private keys encrypted under passwords.   This would provide security as good as hiding a private key were it   not that people tend to choose passwords from a small space (like   words in a dictionary) such that a password can be more easily   guessed than a private key.  To address this potential weakness, DASS   specifies a protocol between a login node and a login agent whereby   the login agent can audit and limit the rate of password guesses.   Use of these features is optional.  A user with a smart card could   store a private key directly and bypass all of these mechanisms.  If   users can be forced to choose "good" passwords, the login agent could   be eliminated and encrypted credentials could be stored directly in   the naming service.Kaufman                                                        [Page 10]

RFC 1507                          DASS                    September 1993   Another way in which user keys are protected is that the architecture   does not require that they be available except briefly at login.   This reduces the threat of a user walking away from a logged on   workstation and having someone take over the workstation and extract   his key.  It also makes the use of RSA based smart cards practical;   the card could keep the user's private key and execute one signature   operation at login time to authenticate an entire session.1.3 What This Document Won't Tell You   Architecture documents are by their nature difficult to read.  This   one is no exception. The reason is that an architecture document   contains the details sufficient to build interoperable   implementations, but it is not a design specification. It goes out of   its way to leave out any details which an implementation could choose   without affecting interoperability. It also does not specify all the   uses of the services provided because these services are properly   regarded as general purpose tools.   The remainder of this section includes information which is not   properly part of the authentication architecture, but which may be   useful in understanding why the architecture is the way it is.1.3.1 How DASS is Embedded in an Operating System   While architecturally DASS does not require any operating system   support in order to be used by an application (other than the   services listed inSection 2), it is expected that actual   implementations of DASS will be closely tied to the operating systems   of host computers.  This is done both for security and for   convenience.   In particular, it is expected that when a user logs into a node, a   set of credentials will be created for that user and then associated   by the operating system with all processes initiated by or on behalf   of the user.  When a user delegates to a service, the remote   operating system is expected to accept the delegation and start up   the remote process with the delegated credentials.  Most nodes are   expected to have credentials of their own and support the concept of   user accounts.  When user credentials are created, the node is   expected to verify them in its own context, determine the appropriate   user account, and add node credentials to the created credentials   set.1.3.2 Forms of Credentials   In the DASS architecture, there is a single data structure called   "Credentials" with a large number of optional parts.  In anKaufman                                                        [Page 11]

RFC 1507                          DASS                    September 1993   implementation, it is possible that not all of the architecturally   allowed subsets will be supported and credentials structures with   different subsets of the data may be implemented quite differently.   The major categories of credentials likely to be supported in an   implementation are:    - Claimant credentials  - these are the credentials which would      normally be associated with a user process in order that it be      able to create authentication tokens.  It would contain the      user's name, login ticket, session private key, and (at least      logically) local node credentials and cached outgoing      contexts.    - Verifier credentials -  these are the credentials which would      normally be associated with a server which must verify tokens      and produce mutual authentication response tokens.  Since      servers may be started by a node on demand, some      representation of verifier credentials must exist independent      of a process.  If an operating system wishes to authenticate a      request before starting a server process, the credentials must      exist in usable form.  An implementation may choose to have      all services on a "node" share a verifier credentials      structure, or it may choose to have each service have its own.    - Combined credentials - architecturally, a server may have a      structure which is both claimant credentials and verifier      credentials combined so that the server may act in either role      using a single structure.  There is some overlap in the      contents.  There is no requirement, however, that an      implementation support such a structure.    - Stub credentials - In the architecture, a credentials      structure is created whenever a token is accepted.  If delegation      took place, these are claimant credentials usable by their      possessor to create additional tokens.  If no delegation took      place, this structure exists as an architectural place holder      against which an implementation may attempt to authenticate      user and node names.  An implementation might choose to      implement  stub credentials  with a different mechanism than      claimant or verifier credentials.  In particular, it might do      whatever user and node authentication is useful itself and not      support this structure at all.Kaufman                                                        [Page 12]

RFC 1507                          DASS                    September 19931.3.3 Support for Alternative Certification Authority      Implementations   A motivating factor in much of the design of DASS is the need to   protect certification authorities from compromise. CAs are only used   to create certificates for new principals and to renew them on   expiration (expiration intervals are likely to be measured in   months). They therefore do not need to be highly available. For   maximum security, CAs could be implemented on standalone PCs where   the hardware, software, and keys can be locked in a safe when the CA   is not in use. The certificates the CA generates must be delivered to   the naming service to be registered, and a possible mechanism for   this is for the CA to have an RS232 line to an on-line component   which can pass certificates and related information but not login   sessions. The intent would be to make it implausible to mount a   network attack against the CA.  Alternatively, certificates could be   carried to the network on a floppy disk.   For CAs to be secure, a whole host of design details must be done   right. The most important of these is the design of user and system   manager interfaces that make it difficult to "trick" a user or system   manager into doing the wrong thing and certifying an impostor or   revealing a key. Mechanisms for generating keys must also be   carefully protected to assure that the generated key cannot be   guessed (because of lack of randomness) and is not recorded where a   penetrator can get it. Because a certificate contains relatively   little human intelligible information (its most important components   are UIDs and public keys), it will be a challenge to design a user   interface that assures the human operator only authorizes the signing   of intented certificates. Such considerations are beyond the scope of   the architecture (since they do not affect interoperability), but   they did affect the design in subtle ways.  In particular, it does   not assume uniform security throughout the CA hierarchy and is   designed to assure that the compromise of a CA in one part of the   hierarchy does not have global implications.   The architecture does not require that CAs be off-line. The CA could   be software that can run on any node when the proper secret is   installed.  Administrative convenience can be gained by integrating   the CA with account registration utilities and naming service   maintenance. As such, the CA would have to be on-line when in use in   order to register certificates in the naming service.  The CA key   could be unlocked with a password and the password could be entered   on each use both to authenticate the CA operator and to assure that   compromise of the host node while the CA is not in use will not   compromise the CA.  This design would be subject to attacks based on   planting Trojan horses in the CA software, but is entirely   interoperable with a more secure implementation.  Realistic tradeoffsKaufman                                                        [Page 13]

RFC 1507                          DASS                    September 1993   must be made between security, cost, and administrative convenience   bearing in mind that a system is only as secure as its weakest link   and that there is no benefit in making the CA substantially more   secure than the other components of the system.1.3.4 Services Provided vs. Application Program InterfaceSection 3 of this document specifies "abstract interfaces" to the   services provided by DASS. This means it tells what services are   provided, what parameters are supplied by the caller, and what data   is returned. It does not specify the calling interfaces.  Calling   interfaces may be platform, operating system, and language dependent.   They do not affect interoperability; different implementations which   implement completely different calling interfaces can still   interoperate over a network. They do, however, affect portability. A   program which runs on one platform can only run on another which   implements an identical API.   In order to support portability of applications - not just between   implementations of DASS but between implementations of DASS and   implementations of Kerberos - a "Generic Security Service API" has   been designed and is outlined in Annex B. This API could be the only   "published" interface to DASS services.  This interface does not,   however, give access to all the functions provided by DASS and it   provides some non-DASS services. It does not give access to the   "login" service, for example, so the login function cannot be   implemented in a portable way. Clearly an implementation must provide   some implementation of the login function, though perhaps only to one   system program and the implementation need not be portable.   Similarly, the Generic API provides no access to node authentication   information, so applications which use these services may not be   portable.   The Generic API provides services for encryption of user data for   integrity and possibly privacy. These services are not specified as a   part of the DASS architecture. This is because we envisioned that   such services would be provided by the communications network and not   in applications. These services are provided by the Generic API   because these services are provided by Kerberos, there exist   applications which use these services, and they are desired in the   context of the IETF-CAT work. The DASS architecture includes a Key   Distribution service so that the encryption functions of the Generic   API can be supported and integrated. Annex B specifies how those   services can be implemented using DASS services.   The Services Provided also differ from the GSSAPI because there are   important extensions envisioned to the API for future applications   and it was important to assure that architecturally those servicesKaufman                                                        [Page 14]

RFC 1507                          DASS                    September 1993   were available.  In particular, DASS provides the ability for a   principal to have multiple aliases and for the receiver of an   authentication token to verify any one of them.  We want DASS to   support the case where a server only learns the name it is trying to   validate in the course of evaluating an ACL.  This may be long after   a connection is accepted.  The Services Provided section therefore   separates the Accept_token function from the Verify Principal Name.   The other motivation behind a different interface is that DASS   provides node authentication - the ability to authenticate the node   from which a request originates as well as the user.  Because   Kerberos provides no such mechanism, the capability is missing from   the GSSAPI, but we expect some applications will want to make use of   it.1.3.5 Use of a Naming Service   With the exception of the syntactical representation of names, which   is tied to X.500, the DASS architecture is designed to be independent   of the particular underlying naming service.  While the intention is   that certificates be stored in an X.500 naming service in the fields   architecturally reserved for this purpose in the standard, this   specification allows for the possibility of different forms of   certificate stores.  The SPX implementation of DASS implements its   own certificate distribution service because we did not want to   introduce a dependency on an X.500 naming service.1.3.6 Key Hiding - Credentials   The abstract interfaces described insection 3 specify that   "credentials" and "keys" are the inputs and outputs of various   routines.  Credentials structures in particular contain secret   information which should not be made available to the calling   application.  In most cases, keeping this information from   applications is simply a matter of prudence - a misbehaving   application can do nearly as much damage using the credentials as it   can by using the secrets directly.  Having access to the keys   themselves may allow an application to bypass auditing or leak a key   to an accomplice who can use it on another node where a large amount   of activity is less likely to be noticed.  In some cases, most   dramatically where a "node key" is present in user credentials, it is   vital that the contents of the credentials be kept out of the hands   of applications.   To accomplish this, a concrete interface is expected to create   "credentials handles" that are passed in and out of DASS routines.   The credentials themselves would be kept in some portion of memory   where unprivileged code can't get at them.Kaufman                                                        [Page 15]

RFC 1507                          DASS                    September 1993   There is another aspect of the way credentials are used which is   important to the design of real implementations.  In normal use, a   user will create a set of credentials in the process of logging on to   a system and then use them from many processes or jobs.  When many   processes share a set of credentials, it is important for the sake of   performance that they share one set of credentials rather than having   a copy of the credentials made for each.  This is because information   is cached in credentials as a side effect of some requests and for   good performance those caches should be shared.   As an example, consider a system executing a series of copy commands   moving files from one system to another.  The credentials of the user   will have been established when the user logged on.  The first time a   copy is requested, a new process will start up, open a connection to   the destination system, and create a token to authenticate itself.   Creating that token will be an expensive operation, but information   will be computed and "cached" in the credentials structure which will   allow any subsequent tokens on behalf of that user to that server to   be computed cheaply.  After the copy completes, the connection is   closed and the process terminates.  In response to a second copy   request, another new process will be created and a new token   computed.  For this operation to get a performance benefit from the   caching, the information computed by the first process must somehow   make it to the second.   A model for how this caching might work can be seen in the way   Kerberos caches credentials.  Kerberos keeps credentials in a file   whose name can be computed from the name of the local user.  This   file is initialized as part of the login process and its protection   is set so that only processes running under the UID of the user may   read and write the file.  Processes cache information there; all   processes running on behalf of the user share the file.   There are two problems with this scheme: first, on a diskless node   putting information in a file exposes it to eavesdroppers on the   network; second, it does not accomplish the "key hiding" function   described earlier in this section.  In a more secure implementation,   the kernel or a privileged process would manage some "pool" of   credentials for all processes on a node and would grant access to   them only through the DASS calls.  Credentials structures are complex   and varying length; DASS may organize them as a set of pools rather   than as contiguous blocks of data.  All such design issues are   "beyond the scope of the architecture".  Implementations must decide   how to control access to credentials.  They could copy the Kerberos   scheme of having credentials available to processes with the UID of   the login session which created them and to privileged processes or   there may be a more elaborate mechanism for "passing" credentials   handles from process to process.  This design should probably followKaufman                                                        [Page 16]

RFC 1507                          DASS                    September 1993   the operating system mechanisms for passing around local privileges.1.3.7 Key Hiding - Contexts   The "GSSAPI" has a concept of a security context which has some of   the same key hiding problems as a credentials structure.  Security   contexts are used in calls to cryptographically protect user data   (from modification or from disclosure and modification) using keys   established during authentication.  The "services provided"   specification says that create_ and accept_token return a "shared   key" and "instance identifier".  The GSSAPI says that a context   handle is returned which is an integer.  A secure implementation   would keep the key and instance identifier in protected memory and   only allow access to them through provided interfaces.   Unlike credentials, there is probably no need to provide mechanisms   for contexts to be shared between processes.  Contexts will normally   be associated with some notion of a communications "connection" and   ends of a connection are not normally shared.  If an implementation   chooses to provide additional services to applications like message   sequencing or duplicate detection, contexts will have to contain   additional fields.  These can be created and maintained without any   additional authentication services.1.4 The Relationship between DASS and ISO Standards   This section provides an introduction to DASS authentication in terms   of the ISO Authentication Framework (DP10181-2).   The purpose of   this introduction is to give the reader an intuitive understanding of   the way DASS works and how its mechanisms and terminology relate to   standards.  Important details have been omitted here but are spelled   out insection 3.1.4.1 Concepts   The primary goal of authentication is to prevent impersonation, that   is, the pretense to a false identity. Authentication always involves   identification in some form. Without authentication, anyone could   claim to be whomever they wished and get away with it.   If it didn't matter with whom one was communicating, elaborate   procedures for authentication would be unnecessary. However, in most   systems, and in timesharing and distributed processing environments   in particular, the rights of individuals are often circumscribed by   security policy. In particular, authorization (identity based access   control) and accountability (audit) provisions could be circumvented   if masquerading attempts were impossible to prevent or detect.Kaufman                                                        [Page 17]

RFC 1507                          DASS                    September 1993   Almost all practical authentication mechanisms suitable for use in   distributed environments rely on knowledge of some secret   information. Most differences lie in how one presents evidence that   they know the secret. Some schemes, in particular the familiar simple   use of passwords, are quite susceptible to attack. Generally, the   threats to authentication may be classified as:    - forgery, attempting to guess or otherwise fabricate evidence;    - replay, where one can eavesdrop upon another's authentication      exchange and learn enough to impersonate them; and    - interception, where one slips between the communicants and is      able to modify the communications channel unnoticed.   Most such attacks can be countered by using what is known as strong   authentication. Strong authentication refers to techniques that   permit one to provide evidence that they know a particular secret   without revealing even a hint about the secret. Thus neither the   entity to whom one is authenticating, nor an eavesdropper on the   conversation can further their ability to impersonate the   authenticating principal at some future time as the result of an   authentication exchange.   Strong authentication mechanisms, in particular those used here, rely   on cryptographic techniques. In particular, DASS uses public key   cryptography. Note that interception attacks cannot be countered by   strong authentication alone, but generally need additional security   mechanisms to secure the communication channel, such as data   encryption.1.4.2 Principals and Their Roles   All authentication is on behalf of principals. In DASS the following   types of principals are recognized:    - user principals, normally people with accounts who are      responsible for performing particular tasks. Generally it is      users that are authorized to do things by virtue of having      been granted access rights, or who are to be held accountable      for specific actions subject to being audited.    - server principals, which are accessed by users.    - node principals,  corresponding to locations where users and      servers, or more accurately, processes acting on behalf of      principals can reside.Kaufman                                                        [Page 18]

RFC 1507                          DASS                    September 1993   Principals can act in one of two capacities:    - the claimant is the active entity seeking to authenticate      itself, and    - the verifier is the passive entity to whom the claimant is      authenticating.   Users normally are claimants, whereas servers are usually verifiers,   although sometimes servers can also be claimants.   There is another kind of principal:    - certification authorities (CA's) issue certificates which      attest to another principal's public key.1.4.3 Representation, Delegation and Representation Transfer   Of course, although it is users that are responsible for what the   computer does, human beings are physically unable to directly do   anything within a computer system. In point of fact, it is a   process executing on behalf of a user that actually performs   useful work. From the point of view of performing security   controlled functions, the process is the agent, or   representative, of the user, and is authorized by that user to do   things on his behalf. In the terms used in the ISO Authentication   Framework, the user is said to have a representation in the   process.   The representation has to come into existence somehow.  Delegation   refers to the act of creating a representation. A user is said to   create a representation for themselves by delegating to a process. If   the user creates another process, say by doing an rlogin on a   different computer, a representation may be needed there as well. This   may be accomplished automatically by a process known as representation   transfer.  DASS uses the term delegation to also mean the act of   creating additional representations on a remote systems.   A representation is instantiated in DASS as credentials.  Credentials   include the identity of the principal as well as the cryptographic   "state" needed to engage in strong authentication procedures. Claimant   information in credentials enable principals to authenticate   themselves to others, whereas verifier information in credentials   permit principals to verify the claims of others.  Credentials   intended primarily for use by a claimant will be referred to as   claimant credentials in the text which follows.  Credentials intended   primarily for use in verification will be referred to as verifier   credentials.  A particular set of credentials may or may not containKaufman                                                        [Page 19]

RFC 1507                          DASS                    September 1993   all of the data necessary to be used in both roles.  That will depend   on the mechanisms by which the credentials were created.   In some contexts, but not here, the concept of representation   and/or delegation is sometimes referred to as proxy. This term is   used in ECMA TR/46.  We avoid use of the term because of possible   confusion with an unrelated use of the term in the context of   DECnet.1.4.4 Key Distribution, Replay, Mutual Authentication and Trust   Strong authentication uses cryptographic techniques. The   particular mechanisms used in DASS result in the distribution of   cryptographic keys as a side effect. These keys are suitable for   use for providing a data origin authentication service and/or a   data confidentiality service between a pair of authenticated   principals.   Replay detection is provided using timestamps on relevant   authentication messages, combined with remembering previously   accepted messages until they become "stale". This is in contrast   to other techniques, such as challenge and response exchanges.   Authentication can be one-way or mutual. One-way authentication is   when only one party, in DASS the claimant, authenticates to the other.   Mutual authentication provides, in addition, authentication of the   verifier back to the claimant. In certain communications schemes, for   example connectionless transfer, only one-way authentication is   meaningful. DASS supports mutual authentication as a simple extension   of one-way authentication for use in environments where it makes   sense.   DASS potentially can allow many different "trust relationships"   to exist. All principals trust one or more CA's to safeguard the   certification process. Principals use certificates as the basis   for authenticating identities, and trust that CA's which issue   certificates act responsibly. Users expect CA's to make sure that   certificates (and related secrets) are only made for principals   that the CA knows or has properly authenticated on its own.1.5 An Authentication Walkthrough   The OSI Authentication Framework characterizes authentication as   occurring in six phases. This section attempts to describe DASS   in these terms.Kaufman                                                        [Page 20]

RFC 1507                          DASS                    September 19931.5.1 Installation   In this phase, principal certificates are created, as is the   additional information needed to create claimant and verifier   credentials. OSI defines three sub-phases:    - Enrollment. In DASS, this is the definition of a principal in      terms of a key, name and UID.    - Validation,  confirmation of identity to the satisfaction of      the CA, after which the CA generates a certificate.    - Confirmation.  In DASS, this is the act of providing the user      with the certificate and with the CA's own name, key and UID,      followed up by the user creating a  trusted authority for that      CA. A trusted authority is a certificate for the CA signed by      the user.   Included in this step in DASS is the posting of the certificate so as   to be available to principals wishing to verify the principal's   identity. In addition, the user principal saves the trusted authority   so as to be available when it creates credentials.1.5.2 Distribution   DASS distributes certificates by placing them in the name service.1.5.3 Acquisition   Whenever principals wish to authenticate to one another, they access   the Name Service to obtain whatever public key certificates they need   and create the necessary credentials. In DASS, acquisition means   obtaining credentials.   Claimant credentials implement the representation of a principal in a   process, or, more accurately, provide a representation of the   principal for use by a process. In making this representation, the   principal delegates to a temporary delegation key. In this fashion   the claimant's long term principal key need not remain in the system.   Claimant credentials are made by invoking the get credentials   primitive. Claimant credentials are a DASS specific data structure   containing:    - a name    - a ticket, a data structure containingKaufman                                                        [Page 21]

RFC 1507                          DASS                    September 1993      .  a validity interval,      .  UID, and      .  (temporary) delegation public key, along with a      .  digital signature on the above made with the principal         private key    - the delegation private key   Optionally in addition, there may be credential information relating   to the node on which the user is logged in and the account on that   node.  A detailed description of all the information found in   credentials can be found insection 3.  Verifier credentials are made   with initialize_server. Verifier credentials consist of a principal   (long term) private key. The rationale is that these credentials are   usually needed by servers that must be able to run indefinitely   without re-entry of any long term key.   In addition, claimants and verifiers have a trusted authority, which   consists of information about a trusted CA.  That information is its:    - name (this will appear in the "issuer" field in principal      certificates),    - public key (to use in verifying certificates issued by that      CA), and    - UID.   Trusted authorities are used by principals to verify certificates for   other principals' public keys.  CAs are arranged in a hierarchy   corresponding to the naming hierarchy, where each directory in the   naming hierarchy is controlled by a single CA.  Each CA certifies the   CA of its parent directory, the CAs of each of its child directories,   and optionally CAs elsewhere in the naming hierarchy (mainly to deal   with the case where the directories up to a common ancestor lack   CAs).  Even though a principal has only a single CA as a trusted   authority, it can securely obtain the public key of any principal in   the namespace by "walking the CA hierarchy".1.5.4 Transfer   The DASS exchange of authentication information is illustrated in   Figure 1-1. During the transfer phase, the DASS claimant sends an   authentication token  to the verifier. Authentication tokens are made   by invoking the create_token primitive. The authentication token isKaufman                                                        [Page 22]

RFC 1507                          DASS                    September 1993   cryptographically protected and specified as a DASS data structure in   ASN.1. The authentication token includes:    - a ticket,    - a DES authenticating key encrypted using the intended      verifier's public key    - one of the following:      . if delegation is not being performed, a digital signature on        the encrypted DES key using the delegation private key, or      . if delegation is being performed, sending the delegation        private key, DES encrypted using the DES authenticating key    - an authenticator, which is a cryptographic checksum made using      the DES authenticating key over a buffer containing      . a timestamp      . any application supplied "channel bindings". For example,        addresses or other context information. The purpose of this        field is to thwart substitution and replay attacks.    - additional optional information concerning node authentication      and context.   As a side effect, after init_authentication_context, the caller   receives a local authentication context, a data structure containing:    - the DES key, and    - if mutual authentication is being requested, the expected      response.   In order to construct an authentication token, the claimant needs to   access the verifier's public key certificate from the Name Service   (labeled CDC, for Certificate Distribution Center, in the figure).   Note that while an authenticator can only be used once, it is   permissible to re-establish the same local authentication context   multiple times. That is, the ticket and DES key establishment   components of the authentication token may have a relatively long   lifetime. This permits a performance improvement in that repeated   applications of public key operations can be alleviated if one caches   authentication contexts, along with other components from a   successfully used authentication token and the associated verifiedKaufman                                                        [Page 23]

RFC 1507                          DASS                    September 1993   principal public key value. It is a relatively inexpensive operation   to create (and verify) "fresh" authenticators based on cached   authentication context.      Claimant Actions      | Communications |  Verifier Actions                            |                |           verifier name    |                |                   |        |                |                   |        |           +---+|                   \------------------->|   ||     trusted                |           |   ||   authorities              |           |CDC||        |    +-----------+  |certificate|   ||        |    |  Verify   |<-------------|   ||        \--->|Certificate|  |           +---+|             +-----------+  |                |     Claimant        |      |                |   credentials    Verifier  |                |   Verifier        |       Public Key  |                | Credentials        |            |      |                |       |        |            V      |                |       V        |    +-----------+  | Authentication | +-----------+        |    |   Make    |  |     Token      | |   Check   |   Replay        \--->|  Token    |-------------------->|   Token   |<-->Cache             +-----------+  |                | +-----------+      DES <---/      |      |                |  |   |    \----->DES      key            |      |                | /Claimant        key                     |      |                |/Public Key                     |      |                /      |        trusted                     |      |      Claimant /|      V     authorities                     |      |+---+   Name  / | +-----------+     |            authentication  ||   |<-------/  | |  Verify   |<----/               context      ||   |certificate| |Certificate|                     |      ||CDC|------------>|           |-->accept/                     |      ||   |           | +-----------+   reject                     |      ||   |           |      |      \                     |      |+---+           |authentication\                     V      |     mutual     |   context     V             +-----------+  | authentication |      |      claimant          /--|  Accept   |  |    response    | +----------+credentials         V   |  Mutual   |<--------------------|  Make    |(delegation)     accept/ +-----------+  |                | | Response |     reject                 |                | +----------+                            |                |              Figure 1 - Authentication Exchange OverviewKaufman                                                        [Page 24]

RFC 1507                          DASS                    September 19931.5.5 Verification   Upon receipt of an authentication token, the verifier extracts the   DES key using its verifier credentials, accesses the Name Service   (labeled CDC for Certificate Distribution Center) to obtain the   certificates needed to perform cryptographic checks on the incoming   information, and verifies all of the signatures on the received   certificates and the authentication token.  Verification can result   in creation of new claimant credentials if delegation is performed.   As part of this process, verified authenticators are retained for a   suitable timeout period.1.5.6 Unenrolment   This is the removal of information from the Name Service. The only   other form of revocation supported by DASS is certificate timeout.   Every certificate contains an expiration time (expected in ordinary   use to be about a year from its signing date).  DASS does not   currently support the revocation lists in X.509.2. Services Used   Aside from operating system services needed to maintain its internal   state, DASS relies on a global distributed database in which to store   its certificates, a reliable source of time, and a source of random   numbers for creating cryptographic keys.2.1 Time Service   DASS requires access to the current time in several of its   algorithms.  Some of its uses of time are security critical.  In   others, network synchronization of clocks is required.  DASS does   not, however, depend on having a single source of time which is both   secure and tightly synchronized.   The requirements on system provided time are:    - For purposes of validating certificates and tickets, the      system needs access to know the date and time accurate to      within a few hours with no particular synchronization      requirements.  If this time is inaccurate, then valid requests      may be rejected and expired messages may be accepted.      Certificate expiration is a backup revocation mechanism, so      this can only cause a security compromise in the event of      multiple failures.  In theory, this could be provided by      having a local clock on every node accurate to within a few      hours over the life of the product to provide this function.Kaufman                                                        [Page 25]

RFC 1507                          DASS                    September 1993      If an insecure network time service is used to provide this      time, there are theoretical security threats, but they are      expected to be logistically impractical to exploit.    - For purposes of detecting replay of authentication tokens, the      system needs access to a  strictly monotonic time source which      is reasonably synchronized across the network (within a few      minutes) for the system to work, but inaccuracy does not      present a security threat except as noted below. It may      constitute an availability threat because valid requests may      be rejected.  In order to get strict monotonicity in the      presence of a rapid series of requests, time must be returned      with high precision.  There is no requirement for a high      degree of accuracy.  Inaccurate time could present a security      threat in the following scenario: if a client's clock is made      sufficiently fast that its tokens are rejected, someone      harvesting those tokens from the wire could replay them later      and impersonate the client.  In some environments, this might      be an easier threat than harvesting tokens and preventing      their delivery.    - For purposes of aging stale entries from caches, DASS requires      reasonably accurate timing of intervals.  To the extent that      intervals are reported as shorter than the actually were,      revocation of certificates from the naming service may not be      as timely as it should be.2.2 Random Numbers   In order to generate keys, DASS needs a source of "cryptographic   quality" random numbers.  Cryptographic quality means that   knowing any of the "random numbers" returned from a series and   knowing all state information which is not protected, an attacker   cannot predict any of the other numbers in the series.  Hardware   sources are ideal, but there are alternative techniques which may   also be acceptable. A 56 bit "truly random" seed (say from a   series of coin tosses) could be used as a DES key to encrypt an   infinite length known text block in CBC mode to produce a pseudo-rand   sequence provided the key and current point in the sequence were   adequately protected.  There is considerable controversy   surrounding what constitutes cryptographic quality random   numbers, and it is not a goal of this document to resolve it.2.3 Naming Service   DASS stores creates and uses "certificates" associated with every   principal in the system, and encrypted credentials associated   with most.  This information is stored in an on-line serviceKaufman                                                        [Page 26]

RFC 1507                          DASS                    September 1993   associated with the principal being certified.  The long term   vision is for DASS to use an X.500 naming service, and DASS will   from its inception authenticate X.500 names.  To avoid a   dependence on having an X.500 naming service available (and to   gain the benefits of a "login agent" that controls password   guessing), an alternative certificate  distribution center   protocol is also described.   The specific requirements DASS places on the naming service are:    - It must be highly available.  A user's naming service entry      must be available to any node where the user is to obtain      services (or service will be denied).  A server's naming      service entry must be available from any node from which the      service is to be invoked (or service will be denied).    - It must be timely.  The presence of "stale" information in the      naming service may cause some problems.  When a password      changes, the old password may remain valid (and the new      password invalid) to the extent the naming service provides      stale information.  When a user or server is added to the      network, it will not be able to participate in authentication      until the information added to the naming service is available      at the node doing the authentication.  In the unusual      circumstance that a key changes, the entity whose key has      changed will not be able to use the new key until the new      certificate is uniformly available.    - It must be secure with regard to certain specific properties.      In general, the security of DASS protected applications does      not depend on the security of the naming service.  It is      expected that the availability needs of the naming service      will prevent it from being as secure as some applications need      to be.  There are two aspects of DASS security which do depend      on the security of the naming service: timely revocation of      certificates and protection of user secrets against dictionary      based password guessing. DASS depends on the removal of      certificates from the naming service in order to revoke them      more quickly than waiting for them to time out.  For this      mechanism to provide any actual security, it must not be      possible for a network entity to "impersonate" the naming      service and the naming service must be able to enforce access      controls which prevent a revoked certificate from being      reinstated by an unauthorized entity.  In the long run, it is      expected that DASS itself will be used to secure the naming      service, which presents certain potential recursion problems      (to be addressed in the naming service design).  If the naming      service is not authenticated (as is expected in earlyKaufman                                                        [Page 27]

RFC 1507                          DASS                    September 1993      versions) attacks where a revoked certificate is "reinstated"      through impersonation of the naming service are possible.   The specific functions DASS requests of the naming service are   simple:    - Given an X.500 name, store a set of certificates associated      with that name.    - Given an X.500 name, retrieve the set of certificates      associated with that name.    - Given an X.500 name, store a set of encrypted credentials      associated with that name.    - Given and X.500 name, retrieve a set of encrypted credentials      associated with that name.   Implementation over a particular naming service may implement more   specialized functions for reasons of efficiency.  For example, the   certificates associated with a name may be separated into several   sets (child, parent, cross, self) so that only the relevant ones may   be retrieved.  In order that access to the naming service itself be   secure, the protocols should be authenticated.  Certificates should   generally be readable without authentication in order to avoid   recursion problems.  Requests to read encrypted credentials should be   specialized and should include proof of knowledge of the password in   order that the naming service can audit and slow down false password   guesses.   The following sections describe the interfaces to specific naming   services:2.3.1 Interface to X.500   Certificates associated with a particular name are stored as   attributes of the entry as specified in X.509.  X.509 defines   attributes appropriate for parent and cross certificates   (CrossCertificatePair, CACertificate) for some principals; we will   have to define a DASSUserPrincipal object class including these   attributes in order to properly use them with ordinary users.   Retrieval is via normal X.500 protocols.  Certificates should be   world readable and modifiable only by appropriate authorities.   Encrypted credentials are stored with the entry of the principal   under a yet to be defined attribute.  The credentials should be   encoded as specified insection 4.  In the absence of extensions to   the X.500 protocol to control password guessing, the encryptedKaufman                                                        [Page 28]

RFC 1507                          DASS                    September 1993   credentials should be world readable and updatable only by the named   principal and other appropriate authorities.2.3.2 Interface to CDC   The CDC (Certificate Distribution Center) is a special purpose name   server created to service DASS until an X.500 service is available in   all of the environments where DASS needs to operate.  The CDC uses a   special purpose protocol to communicate with DASS clients.  The   protocol was designed for efficiency, simplicity, and security.  CDCs   use DASS as an authentication mechanism and to protect encrypted   credentials from unaudited password guessing.   Each DASS client maintains a list of CDCs and the portion of the   namespace served by that CDC.  Each directory has a master replica   which is the only one which will accept updates.  The CDCs maintain   consistency with one another using protocols beyond the scope of this   document.  When a DASS client wishes to make a request of a CDC, it   opens a TCP or DECnet connection to the CDC and sends an ASN.1 (BER)   encoded request and receives a corresponding ASN.1 (BER) encoded   response.  Clients are expected to learn the IP or DECnet address and   port number of the CDC supporting a particular name from a local   configuration file.  To maximize performance, the requests bundle   what would be several requests if made in terms of requests for   individual certificates.  It is intended that all certificates needed   for an authentication operation be retrievable with at most two CDC   requests/responses (one to the CDC of the client and one to the CDC   of the server).   Documented here is the protocol a DASS client would use to retrieve   certificates and credentials from a CDC and update a user password.   This protocol does not provide for updates to the certificate and   credential databases.  Such updates must be supported for a practical   system, but could be done either by extensions to this protocol or by   local security mechanisms implemented on nodes supporting the CDC.   Similarly, availability can be enhanced by replicating the CDC.   Automating the replication of updates could be implemented by   extensions to this protocol or by some other mechanism.  This   specification assumes that updates and replication are local matters   solved by individual CA/CDC implementations.   Requests and responses are encoded as follows:2.3.2.1 ReadPrinCertRequest   This request asks the CDC to return the child certificates and   selected incoming cross certificates for the specified object.  The   format of the request is:Kaufman                                                        [Page 29]

RFC 1507                          DASS                    September 1993        ReadPrinCertRequest ::= [4] IMPLICIT SEQUENCE {             flags [0] BIT STRING DEFAULT {},             index [1] IMPLICIT INTEGER DEFAULT 0,             resolveFrom [2] Name OPTIONAL,             principal Name,             crossCertIssuers ListOfIssuers OPTIONAL             }        ListOfIssuers ::= SEQUENCE OF Name   The first 24 bits of flags, if present, contain a protocol version   number.  Clients following this spec should place the value 2.0.0 in   the three bytes.  Servers following this spec should accept any value   of the form 1.x.x or 2.x.x.  flags bits beyond the first 24 are   reserved for future use (should not be supplied by clients and should   be ignored by servers).   index is only used if the response exceeds the size of a single   message; in that case, the query is repeated with index set to the   value that was returned by ReadPrinCertResponse.  resolveFrom and   principal imply a set of entities for which certificates should be   retrieved.  resolveFrom (if present) must be an ancestor of principal   and child certificates will be retrieved for principal and all names   which are ancestors of principal but descendants of resolveFrom.  The   encoding of names is per X.500 and is specified in more detail insection 4.  The CDC returns the certificates in order of the object   they came from, parents before children.   crossCertIssuers is a list of cross certifiers that would be believed   in the context of this authentication.  If supplied, the CDC may   return a chain of certificates starting with one of the named   crossCertIssuers and ending with the named principal.  One of   resolveFrom or crossCertIssuers must be present in any request; if   both are present, the CDC may return either chain.2.3.2.2 ReadPrinCertResponse   This is the response a CDC sends to a ReadPrinCertRequest.  Its   syntax is:        ReadPrinCertResponse ::= [5] IMPLICIT SEQUENCE {             status [0] IMPLICIT CDCstatus DEFAULT success,             index [1] INTEGER OPTIONAL,             resolveTo [2] Name OPTIONAL,             certSequence [3] IMPLICIT CertSequence,             indexInvalidator [4] OCTET STRING (SIZE(8)) OPTIONAL,             flags [5] BIT STRING OPTIONAL             }        CertSequence ::= SEQUENCE OF CertificateKaufman                                                        [Page 30]

RFC 1507                          DASS                    September 1993   status indicates success or the cause of the failure.   index if present indicates that the request could not be fully   satisfied in a single request because of size limitations.  The   request should be repeated with this index supplied in the request to   get more.   resolveTo will be present if index is present and should be supplied   in the request for more certificates.  certSequence contains   certificates found matching the search criteria.   indexInvalidator may be present and indicates the version of the   database being read.  If a set of certificates is being read in   multiple requests (because there were too many to return in a single   message), the reader should check that the value for indexInvalidator   is the same on each request.  If it is not, the server may have   skipped or duplicated some certificates.  This field must not be   present if the version number in the request was missing or version   1.x.x.   The first 24 bits of flags, if present, indicate the protocol version   number.  Implementers of this version of the spec should supply 2.0.0   and should accept any version number of the form 1.x.x or 2.x.x.2.3.2.3 ReadOutgoingCertRequest   This requests from the CDC a list of all parent and outgoing cross   certificates for a specified object.  A CDC is capable of storing   cross certificates either with the subject or the issuer of the cross   certificate.  In response to this request, the CDC will return all   parent and cross certificates stored with the issuer for the named   principal and all of its ancestors. Its syntax is:        ReadOutgoingCertRequest ::= [6] IMPLICIT SEQUENCE {             flags [0] BIT STRING DEFAULT {},             index [1] IMPLICIT INTEGER DEFAULT 0,             principal Name             }   The first 24 bits of flags is a protocol version number and should   contain 2.0.0 for clients implementing this version of the spec.   Servers implementing this version of the spec should accept any   version number of the form 1.x.x or 2.x.x.  The remaining bits are   reserved for future use (they should not be supplied by clients and   they should be ignored by servers).   index is used for continuation (see ReadPrinCertRequest).Kaufman                                                        [Page 31]

RFC 1507                          DASS                    September 1993   principal is the name for which certificates are requested.2.3.2.4 ReadOutgoingCertResponse   This is the response to a ReadOutgoingCertRequest.  Its syntax is:        ReadOutgoingCertResponse::= [7] IMPLICIT SEQUENCE {             status [0] IMPLICIT CDCStatus DEFAULT success,             index [1] INTEGER OPTIONAL,             certSequence [2] IMPLICIT CertSequence,             indexInvalidator [3] OCTET STRING (SIZE(8))        OPTIONAL,             flags [4] BIT STRING OPTIONAL             }        CertSequence ::= SEQUENCE OF Certificate   status indicates success of the cause of failure of the operation.   index is used for continuation; see ReadPrinCertRequest.   certSequence is the list of parent and outgoing cross certificates.   indexInvalidator is used for continuation; see ReadPrinCertResponse   (the same rules apply with respect to version numbers).   The first 24 bits of flags, if present, contain the protocol version   number.  Clients implementing this version of the spec should supply   the value 2.0.0.  Servers should accept any values of the form 1.x.x   or 2.x.x.  The remaining bits are reserved for future use (they   should not be supplied by clients and should be ignored by servers).2.3.2.5 ReadCredentialRequest   This request is made to retrieve an principal's encrypted   credentials.  To prevent unaudited password guessing, this structure   includes an encrypted value that proves that the requester knows the   password that will decrypt the structure.  The syntax of the request   is:        ReadCredentialRequest ::= [2] IMPLICIT SEQUENCE {             flags [0] BIT STRING DEFAULT {}             principal Name,             logindata [2] BIT STRING DEFAULT {},             token [3] BIT STRING OPTIONAL             }Kaufman                                                        [Page 32]

RFC 1507                          DASS                    September 1993   The first 24 bits of flags contains the version number of the   protocol.  The value 2.0.0 should be supplied. Any value of the form   1.x.x or 2.x.x should be accepted. Any additional bits are reserved   for future use (should not be supplied by clients and should be   ignored by servers).   principal is the name of the principal for whom encrypted credentials   are desired.   logindata is an encrypted value.  It may only be present if the   version number is 2.0.0 or higher.  It must be present to read   credentials which are protected by the login agent functionality of   the CDC.  It is constructed as a single RSA block encrypted under the   public key of the CDC.  The public key of the CDC is learned by some   local means.  Possibilities include a local configuration file or by   using DASS to read and verify a chain of certificates ending with the   CDC [the CDC serving a directory should have its public key listed   under a name consisting of the directory name with the RDN   "CSS=X509"; the OID for the type CSS is 1.3.24.9.1].  The contents of   the block are as follows:    - The low order eight bytes contain a randomly generated DES key      with the last byte of the DES key placed in the last byte of      the RSA block.  This DES key will be used by the CDC to      encrypt the response.  Key parity bits are ignored.    - The next to last eight bytes contain a long Posix time with      the integer time encoded as a byte string using big endian      order.    - The next eight bytes (from the end) contain a hash of the      password.  The algorithm for computing this hash is listed insection 4.4.2.  The CDC never computes this hash; it simply      compares the value it receives with the value associated with      the credentials.    - The next sixteen bytes (from the end) contain zero.    - The remainder of the RSA block (which should be the same size      as the public modulus of the CDC) contains a random number.      The first byte should be chosen to be non-zero but so the      value in the block does not exceed the RSA modulus.  Servers      should ignore these bits.  This random number need not be of      cryptographic strength, but should not be the same value for      all encryptions.  Repeating the DES key would be adequate.    - The byte string thus constructed is encrypted using the RSA      algorithm by treating the string of bytes as a "big endian"Kaufman                                                        [Page 33]

RFC 1507                          DASS                    September 1993      integer and treating the integer result as "big endian" to      make a string of bytes.   token will not be present in the initial implementation but a space   is reserved in case some future implementation wants to authenticate   and audit the node from which a user is logging in.2.3.2.6 ReadCredentialProtectedResponse   This is the second possible response to a ReadPrinLoginRequest.  It   is returned when the encrypted credentials are protected from   password guessing by the CDC acting as a login agent.  Its syntax is:   ReadCredentialProtectedResponse::=[16] IMPLICIT SEQUENCE {           status [0] IMPLICIT CDCStatus DEFAULT success,           encryptedCredential [1] BIT STRING,           flags [2] BIT STRING OPTIONAL           }   status indicates that the request succeeded or the cause of the   failure.   encryptedCredential contains the DASSPrivateKey structure (defined insection 4.1) encrypted under a DES key computed from the user's name   and password as specified insection 4.4.2 and then reencrypted under   the DES key provided in the ReadPrinLoginRequest.   The first 24 bits of flags, if present, contains the version number   of the protocol.  Implementers of this version of the spec should   supply 2.0.0 and should accept any version number of the form 2.x.x.   Other bits are reserved for future use (they should not be supplied   and they should be ignored).2.3.2.7 WriteCredentialRequest   This is a request to update the encrypted credential structure.  It   is used when a user's key or password changes.  The syntax of the   request is:        WriteCredentialRequest ::= [17] IMPLICIT SEQUENCE {             flags [0] BIT STRING DEFAULT {},             authtoken [2] BIT STRING OPTIONAL,             principal [3] Name,             logindata [4] BIT STRING DEFAULT {},             furtherSensitiveStuff [5] BIT STRING             }   The first 24 bits of flags is a version number.  Clients implementingKaufman                                                        [Page 34]

RFC 1507                          DASS                    September 1993   this version of the spec should supply 2.0.0.  Servers should accept   any value of the form 2.x.x.  Additional bits are reserved for future   use (clients should not supply them and servers should ignore them).   token, if present, authenticates the entity making the request.  A   request will be accepted either from a principal proving knowledge of   the password (see logindata below) or a principal presenting a token   in this field and satisfying the authorization policy of the CDC.   This field need not be present if logindata includes the hash2 of the   password (anyone knowing the old password may set a new one).   principal is the name of the object for which encrypted credentials   should be updated.   logindata is encrypted as in ReadPrinLoginRequest.  It proves that   the requester knows the old password of the principal to be updated   (unless the token supplied is from the user's CA) and includes the   key which encrypts furtherSensitiveStuff.   furtherSensitiveStuff is an encrypted field constructed as follows:    - The first eight bytes consist of the hash2 defined insection4.4.2 with the last byte of the hash2 value stored first.  The      CDC stores this value and compares it with the values supplied      in future requests of ReadCredentialRequest and      WriteCredentialRequest.    - The next (variable number of) bytes contains a DASSPrivateKey      structure (defined insection 4.1).  This is the new      credential structure that will be returned by the CDC on      future ReadCredentialRequests.    - The result is padded with zero bytes to a multiple of eight      bytes.    - The entire padded string is encrypted using the key from      logindata or token using DES in CBC mode with zero IV.   the new eight byte "hash2" defined insection 4.4.2 concatenated with   the DASSPrivateKey structure encrypted under the new "hash1" all   encrypted under the DES key included in logindata.2.3.2.8 HereIsStatus   This is the response message to ill-formed requests and requests that   only return a status and no data.  It's syntax is:Kaufman                                                        [Page 35]

RFC 1507                          DASS                    September 1993        HereIsStatus ::= [1] IMPLICIT SEQUENCE {             status [0] IMPLICIT CDCStatus DEFAULT success             }   status indicates success or the cause of the failure.2.3.2.9 Status Codes   The following are the CDCStatus codes that can be returned by   servers.  Not all of these values are possible with all calls, and   some of the status codes are not possible with any of the calls   described in this document.        CDCStatus ::= INTEGER {             success(0),             accessDenied(1),             wrongCDC(2),     --this CDC does not store the                              --requested information             unrecognizedCA(3),             unrecognizedPrincipal(4),             decodeRequestError(5),--invalid BER             illegalRequest(6),    --request not recognised             objectDoesNotExist(7),             illegalAttribute(8),             notPrimaryCDC(9),--write requests not accepted                              --at this CDC replica             authenticationFailure(11),             incorrectPassword(12),             objectAlreadyExists(13),             objectWouldBeOrphan(15),             objectIsPermanent(16),             objectIsTentative(17),             parentIsTentative(18),             certificateNotFound(19),             attributeNotFound(20),             ioErrorOnCertifDatabase(100),Kaufman                                                        [Page 36]

RFC 1507                          DASS                    September 1993             databaseFull(101),             serverInternalError(102),             serverFatalError(103),             insufficientResources(104)             }3. Services Provided   This section specifies the services provided by DASS in terms of   abstract interfaces and a model implementation.  A particular   implementation may support only a subset of these services and may   provide them through interfaces which combine functions and supply   some parameters implicitly. The specific calling interfaces are in   some cases language and operating system specific.  An actual   implementation may choose, for example, to structure interfaces so   that security contexts are established and then passed implicitly in   calls rather than explicitly including them in every call.  It might   also bundle keys into opaque structures to be used with supplied   encryption and decryption routines in order to enhance security and   modularity and better comply with export regulations. Annex B   describes a Portable API designed so that applications using a   limited subset of the capabilities of DASS can be easily ported   between operating systems and between DASS and Kerberos based   environments.  The model implementation describes data structures   which include cached values to enhance performance.  Implementations   may choose different contents or different caching strategies so long   as the same sequence of calls would produce the same output for some   caching policy.   DASS operates on four kinds of data structures: Certificates,   Credentials, Tokens, and Certification Authority State.  Certificates   and Tokens are passed between implementations and thus their exact   format must be architecturally specified. This detailed bit-for-bit   specification is insection 4. Credentials generally exist only   within a single node and their format is therefore not specified   here. The contents of all of these data structures is listed below   followed by the algorithms for manipulating them.   There are three kinds of services provided by DASS: Certificate   Maintenance, Credential Maintenance, and Authentication. The first   two kinds exist only in support of the third. Certificate maintenance   functions maintain the database of public keys in the naming service.   These functions tend to be fairly specialized and may not be   supported on all platforms. Before authentication can take place,   both authenticating principals must have constructed credentials   structures. These are built using the Credential Maintenance calls.Kaufman                                                        [Page 37]

RFC 1507                          DASS                    September 1993   The Authentication functions use credential information and   certificates, produce and consume authentication tokens and tell the   two communicating parties one another's names.3.1 Certificate Contents   For purposes of this architecture, a certificate is a data structure   posted in the naming service which proclaims that knowledge of the   private key associated with a stated public key authenticates a named   principal. Certificates are "signed" by some authority, are readable   by anyone, and can be verified by anyone knowing the public key of   the authority.  DASS organizes the CA trust hierarchy around the   naming hierarchy. There exists a trusted authority associated with   each directory in the naming hierarchy. Generally, each authority   creates certificates stating the public keys of each of its children   (in the naming hierarchy) and the public key of its parent (in the   naming hierarchy). In this way, anyone knowing the public key of any   authority can learn the public key of any other by "walking the   tree". In order that principals may authenticate even when all of   their ancestor directories do not participate in DASS, authorities   may also create "cross-certificates" which certify the public key of   a named entity which is not a descendent.  Rules for finding and   following these cross-certificates are described in the Get_Pub_Keys   routines.  Every principal is expected to know the public key of the   CA of the directory in which it is named. This must be securely   learned when the principal is initialized and may be maintained in   some form of local storage or by having the principal sign a   certificate listing the name and public key of its parent and posting   that certificate in the naming service.   The syntax and content of DASS certificates are defined in terms of   X.509 (Directory - Authentication Framework).  While that standard   prescribes a single syntax for certificates, DASS considers   certificates to be of one of six types:    - Normal Principal certificates are signed by a CA and certify      the name and public key of a principal where the name of the      CA is a prefix of the name of the principal and is one      component shorter.    - Trusted Authority certificates are signed by an ordinary      principal and certify the name and public key of the      principal's CA (i.e., the CA whose name is a prefix of the      principal's name and is one component shorter).    - Child certificates are signed by a CA and certify the name and      public key of a CA of a descendent directory (i.e., where the      name of the issuing CA is a prefix of the name of the subjectKaufman                                                        [Page 38]

RFC 1507                          DASS                    September 1993      CA and is one component shorter).    - Parent certificates are signed by a CA and certify the name      and public key of the CA of its parent directory (i.e., whose      name is a prefix of the name of the issuer and is one      component shorter).    - Cross certificates are signed by a CA and certify the name and      public key of a CA of a directory where neither name is a      prefix of the other.    - Self certificates are signed by a principal or a CA and the      issuer and subject name are the same.  They are not used in      this version of the architecture but are defined as a      convenient data structure in which in which implementations      may insecurely pass public keys and they may be used in the      future in certain key roll-over procedures.   It is intended that some future version of the architecture relax the   restrictions above where prefixes must be one component shorter.   Being able to handle certificates where prefixes are two or more   components shorter complicates the logic of treewalking somewhat and   is not immediately necessary, so such certificates are disallowed for   now.   The syntax of certificates is defined insection 4. For purposes of   the algorithms which follow, the following is the portion of the   content which is used (names in brackets refer to the field names in   the ASN.1 encoded structure):    - UID of the issuer (optional)    - Full name of the issuer (the authority or principal signing)      [issuer]    - UID of the subject (optional)    - Full name of the subject (the authority or principal whose key      is being certified) [subject]    - Public Key of the subject [subjectPublicKey]    - Period of validity (effective date and expiration date)      [valid]    - Signature over the entire content of the certificate created      using the private key of the issuer.Kaufman                                                        [Page 39]

RFC 1507                          DASS                    September 1993   When parsing a certificate, the reader compares the two name fields   to determine what type of certificate it is. For Parent and Trusted   Authority certificates, the names are ignored for purposes of all   further processing. For Child and Normal Principal certificates, only   the suffix by which the child's name is longer than the parent's is   used for further processing. The reason for this is so that if a   branch of the namespace is renamed, all of the certificates in the   moved branch remain valid for purposes of DASS processing. The only   purposes of having full names in these certificates are (1) to comply   with X.509, (2) for possible interoperability with other   architectures using different algorithms, and (3) to allow principals   to securely store their own names in trusted authority certificates   in the case where they do not have enough local storage to keep it.3.2 Encrypted Private Key Structure   In order that humans need only remember a password rather than a full   set of credentials, and also to make installation of nodes and   servers easier, there is a defined format for encrypting RSA secrets   under a password and posting in the naming service. This structure   need only exist when passwords are used to protect RSA secrets; for   servers which keep their secrets in non-volatile memory or users who   carry smart cards, they are unnecessary.   This structure includes the RSA private/public key pair encrypted   under a DES key. The DES key is computed as a one-way hash of the   password.  This structure also optionally includes the UID of the   principal.  It is needed only if a single RSA key is shared by   multiple principals (with multiple UIDs).   Since this structure is posted in the name service and may be used by   multiple implementations, its format must be architecturally defined.   The exact encoding is listed insection 4.3.3 Authentication Tokens   This section of the document defines the contents of the   authentication tokens which are produced and consumed by Create_token   and Accept_token. With DASS, the token passed from the client to the   server is complex, with a large number of optional parts, while the   token passed from server to client (in the case of mutual   authentication only) is small and simple.   The authentication token potentially contains a large number of   parts, most of which are optional depending on the type of   authentication. The following defines the content and purpose of each   of the parts, but does not describe the actual encoding (in the   belief that such details would be distracting). The encoding is inKaufman                                                        [Page 40]

RFC 1507                          DASS                    September 1993section 4.   The authentication process begins when the initiator calls   Create_token with the name of the target. This routine returns an   authentication token, which is sent to the target. The target calls   Accept_token passing it the token. Both routines produce a second   "mutual authentication token". The target returns this to the   initiator to prove that it received the token.3.3.1 Initial Authentication Token   The components of the initial authentication token are (names in   brackets refer to the field names within the ASN.1 encoded structures   defined insection 4):    a) Encrypted Shared Key - [authenticatingKey] - This is a Shared       (DES) key encrypted under the public key of the target. Also       included in the encrypted structure is a validity interval and       a recognizable pattern so that the receiver can tell whether       the decryption was successful.    b) Login Ticket - [sourcePrincipal.userTicket] - This is a       "delegation certificate" signed by a principal's long term       private key delegating to a short term public key. Its "active       ingredients" are:      1) UID of delegating principal [subjectUID]      2) Period of validity [validity]      3) Delegation public key [delegatingPublicKey]      4) Signature by private key of principal         The existence of this signature is testimony that the         private key corresponding to the delegation public key         speaks for the user during the validity interval.         This data structure is optional and will be missing if the         authentication is only on behalf of a Local Username on a         node (i.e., proxy) rather than on behalf of a real principal         with a real key.    c) Shared Key Ticket - [sourcePrincipal.sharedKeyTicketSignature]       - This is a signature of the Encrypted Shared Key by the       Delegation Public key in the Login Ticket.  The existence of       this signature is testimony that  the DES key in the encrypted       shared key speaks for the user.       This data structure is optional and will be missing if theKaufman                                                        [Page 41]

RFC 1507                          DASS                    September 1993       authentication is only on behalf of a Local Username on a node       (i.e., proxy) rather than on behalf of a real principal with a       real key. It will also be missing if delegation is taking       place.    d) Node Ticket - [sourceNode.nodeTicketSignature] - This is a       signature of the Encrypted Shared key and a "Local Username"       on the host node by the node's private key.  The existence of       this signature is testimony by the node that the DES key in       the encrypted shared key speaks for the named account on that       node.    e) Delegator - [sourcePrincipal.delegator] - This data structure       contains the private login key encrypted under the Shared key.       It is optional and is present only if the initiator is       delegating to the destination.    f) Authenticator - [authenticatorData] - This data structure       contains a timestamp and a message digest of the channel       bindings signed by the Shared Key. It is always present.    g) Principal name - [sourcePrincipal.userName] - This is the name       of the initiating principal. It is optional and will be       missing for strong proxy where bits on the wire are at a       premium and where the destination is capable of independently       constructing the name.    h) Node name - [sourceNode.nodeName] - This is the name of the       initiating node. It is optional and will be missing for strong       proxy where bits on the wire are at a premium and the name is       present elsewhere in the message being passed.    i) Local Username - [sourceNode.username] - This is the local       user name on the initiating node. It is optional and will be       missing for strong proxy where bits on the wire are at a       premium and where the name is present elsewhere in the message       being passed.3.3.2 Mutual Authentication Token   The authentication buffer sent from the target to the initiator (in   the case of mutual authentication) is much simpler. It contains only   the timestamp taken from the authenticator encrypted under the Shared   Key.  It is ASN.1 encoded to allow for future extensions.Kaufman                                                        [Page 42]

RFC 1507                          DASS                    September 19933.4 Credentials   DASS organizes its internal state with Credentials structures.  There   are many kinds of information which can be stored in credentials.   Rather than making a different kind of data structure for each kind   of data, DASS provides a single credentials structure where most of   its fields are optional.  Operating systems must provide some   mechanism for having several processes share credentials. An example   of a mechanism for doing this would be for credentials to be stored   in a file and the name of the file is used as a "handle" by all   processes which use those credentials. Some of the calls which follow   cause credentials structures to be updated. It is important to the   performance of a system that updates to credentials (such as occur   during the routines Verify_Principal_Name and Verify_Node_Name, where   the caches are updated) be visible to all processes sharing those   credentials.   In many of the calls which follow, the credentials passed may be   labeled: claimant credentials, verifier credentials or some such.   This indicates whose credentials are being passed rather than a type   of credentials. DASS supports only one type of credentials, though   the fields present in the credentials of one sort of principal may be   quite different from those present in the credentials of another.   An implementation may choose to support multiple kinds of credentials   structures each of which will support only a subset of the functions   available if it is not implementing the full architecture.  This   would be the case, for example, if an implementation did not support   the case where a server both received requests from other principals   and made requests on its own behalf using a single set of   credentials.   The following are a list of the fields that may be contained in a   credentials structure. They are grouped according to common usage.3.4.1 Claimant information   This is the information used when the holder of these credentials is   requesting something. It includes:    a) Full X.500 name of the principal    b) Public Key of the principal    c) Login Ticket - a login ticket contains:      1) the UID of the principalKaufman                                                        [Page 43]

RFC 1507                          DASS                    September 1993      2) a period of validity (effective date & expiration date)      3) a delegation public key      4) a signature of the ticket contents by the principal's long         term key    d) Delegation Private Key (corresponding to the public key in c3)    e) Encrypted Shared Key (present only when credentials were       created by accept_token; this information is needed to verify       a node ticket after credentials are accepted)3.4.2 Verifier information   This is the information needed by a server to decrypt incoming   requests. It is also used by generate_server_ticket to generate a   login ticket.    a) RSA private key.3.4.3 Trusted Authority   This is information used to seed the walk of the CA hierarchy to   reliably find the public key(s) associated with a name.   Normally, the trusted authority in a set of credentials will be   the directory parent of the principal named in Claimant   information.  In some circumstances, it may instead be the   directory parent of the node on which the credentials reside.    a) Full X.500 name of a CA    b) Corresponding RSA Public Key    c) Corresponding UID3.4.4 Remote node authentication   This information is present only for credentials generated by   "Accept_token". It includes information about any remote node which   vouched for the request.    a) Full X.500 name of the node    b) Local Username on the node    c) Node ticket.Kaufman                                                        [Page 44]

RFC 1507                          DASS                    September 19933.4.5 Local node credentials   This information is added by Combine_credentials, and is used by   Create_token to add a node signature to outbound requests.    a) Full X.500 name of the node    b) Local Username on the node    c) RSA private key of the node3.4.6 Cached outgoing contexts   There may be one (or more) such structures for each server for which   this principal has created authentication tokens. These represent a   cache: they may be discarded at any time with no effect except on   performance. For each association, the following information is kept:    a) Destination RSA Public Key (index)    b) Encrypted Shared key    c) Shared Key Ticket (optional, included if there has been a       non-delegating connection)    d) Node Ticket    e) Delegator (optional, included if there has been a delegating       connection)    f) Validity interval    g) Shared Key3.4.7 Cached Incoming Contexts   There may be one such structure for each client from which this server   has received an authentication token.  These represent a cache: they   may be discarded at any time with no effect except on performance. (An   implementation may choose to keep one System-wide Cache (and list of   incoming timestamps). While it is unlikely that the same Encrypted   Shared Key will result from encryption of Shared keys generated by   different clients or for different servers, an implementation must   ensure that an entry made for one client/server can not be reused by   another client/server.  Similarly an implementation may choose to keep   separate caches for the Shared Key/Validity Interval/Delegation Public   Key, the Nodename/UID/key/username and the Principal name/UID/key.)   For each association, the following information is kept:Kaufman                                                        [Page 45]

RFC 1507                          DASS                    September 1993    a) Encrypted Shared key (index)    b) Shared Key    c) Validity Interval    d) Full X.500 name of Client Principal    e) UID of Client Principal    f) Public Key of Client Principal    g) Name of Client Node    h) UID of Client Node    i) Public Key of Client Node    j) Local Username on Client node    k) Delegation Public key of Client Principal's Login Ticket   The Name, UID and Public key of the Principal are all entered   together once the Login Ticket has been verified. Similarly the Node   name, Node key and Username are entered together once the Node Ticket   has been verified. These pieces of information are only present if   they have been verified.3.4.8 Received Authenticators   A record of all the authenticators received is kept. This is used to   detect replayed messages. (This list must be common to all targets   that could accept the same authenticator (channel bindings will   prevent other targets from accepting the same authenticator). This   includes different `servers' sharing the same key.)  The entries in   this list may be deleted when the timestamp is old enough that they   would no longer be accepted. This list is kept separate from the   Cached incoming context in order that the information in the cached   incoming context can be discarded at any time. An implementation   could choose to save these timestamps with the cached incoming   context if it ensures that it can never purge entries from the cache   before the timestamp has aged sufficiently. This list is accessed   based on an extract from the signature from the Authenticator. The   extract must be at least 64 bits, to ensure that it is very unlikely   that 2 authenticators will be received with matching signatures.    a) Extract from Signature from AuthenticatorKaufman                                                        [Page 46]

RFC 1507                          DASS                    September 1993    b) Timestamp   If an implementation runs out of space to store additional   authenticators, it may either reject the token which would have   overflowed the table or it may temporarily narrow the allowed clock   skew to allow it to free some of the space used to hold "old"   authenticators.  The first strategy will always falsely reject   tokens; the second may cause false rejection of tokens if the allowed   clock skew gets narrowed beyond the actual clock skew in the network.3.5 CA State   The CA needs to maintain some internal state in order to generate   certificates. This internal state must be protected at all times, and   great care must be taken to prevent its being disclosed. A CA may   choose to maintain additional state information in order to enhance   security.  In particular, it is the responsibility of the CA to   assure that the same UID is not serially reused by two holders of a   single name.  In most cases, this can be done by creating the UID at   the time the user is registered.  To securely permit users to keep   their UIDs when transferring from another CA, the CA must keep a   record of any UIDs used by previous holders of the name. Since   actions of a CA are so security sensitive, the CA should also   maintain an audit trail of all certificates signed so that a history   can be reconstructed in the event of a compromise.  Finally, for the   convenience of the CA operator, the CA should record a list of the   directories for which it is responsible and their UIDs so that these   need not be entered whenever the CA is to be used.  The state   includes at least the following information:    - Public Key of CA    - Private Key of CA    - Serial number of next certificate to be issued3.6 Data types used in the routines   There are several abstract data types used as parameters to the   routines described in this section. These are listed here    a) Integer    b) Name       Names unless otherwise noted are always X.500 names.  While       most of the design of DASS is naming service independent, the       syntax of certificates and tokens only permits X.500 names to       be used.  If DASS is to be used in an environment where someKaufman                                                        [Page 47]

RFC 1507                          DASS                    September 1993       other form of name is used, those names must be translated       into something syntactically compliant with X.500 using some       mechanism which is beyond the scope of this architecture.  The       only other form of name appearing in this architecture is a       "local user name", which corresponds to the simple name of an       "account" on a node.  As a type, such names appear in       parameter lists as "Strings".    c) String       A String is a sequence of printable characters.    d) Absolute Time       A UTC time. The precision of these Times is not stated. A       precision of the order of one second in all times is       sufficient.    e) Time Interval       A Time interval is composed of 2 times. A Start Time and an       End Time, both of which are Absolute Times    f) Timestamp       A Timestamp is a time in POSIX format. I.e., two 32 bit       Integers. The first representing seconds, and the second       representing nanoseconds.    g) Duration       A Duration is the length of a time interval.    h) Octet String       A sequence of bytes containing binary data    i) Boolean       A value of either True or False    j) UID       A UID is an bit string of 128 bits.    k) OID       An OID is an ISO Object Identifier.    l) Shared key       A Shared key is a DES key, a sequence of 8 bytes    m) CA State       A structure of the form described in '3.5    n) Credentials       A structure of the form described in '3.4Kaufman                                                        [Page 48]

RFC 1507                          DASS                    September 1993    o) Certificate       An ASN.1 encoding of the structure described in '3.1    p) Authentication Token       An ASN.1 encoding of the structure described in '3.3.1    q) Mutual Authentication Token       An ASN.1 encoding of the structure described in '3.3.2    r) Encrypted Credentials       An ASN.1 encoding of  the  structure described in '3.2    s) Public key       A representation of an RSA Public key, including all the       information needed to encode the public key in a certificate.    t) Set of Public key/UID pairs       A set of Public key/UID pairs. This Data type is only used       internally in DASS - it does not appear in any interface used       to other architectures.3.7 Error conditions   These routines can return the following error conditions (an   implementation may indicate errors with more or less precision):    a) Incomplete chain of trustworthy CAs    b) Target has no keys which can be trusted.    c) Invalid Authentication Token    d) Login Ticket Expired    e) Invalid Password    f) Invalid Credentials    g) Invalid Authenticator    h) Duplicate Authenticator3.8 Certificate Maintenance Functions   Authentication services depend on a set of data structures maintained   in the naming service. There are two kinds of information:   Certificates, which associate names and public keys and are signed by   off-line Certification Authorities; and Encrypted Credentials, whichKaufman                                                        [Page 49]

RFC 1507                          DASS                    September 1993   contain RSA Private Keys and certain context information encrypted   under passwords. Encrypted Credentials are only necessary in   environments where passwords are used. Credentials may alternatively   be stored in some other secure manner (for example on a smart card).   The certificate maintenance services are designed so that the most   sensitive - the actual signing of certificates - may be done by an   off-line authority.  Once signed, certificates must be posted in the   naming service to be believed.  The precise mechanisms for moving   certificates between off-line CAs and the on-line naming service are   implementation dependent.  For the off-line mechanisms to provide any   actual security, the CAs must be told what to sign in some reliable   manner.  The mechanisms for doing this are implementation dependent.   The abstract interface says that the CA is given all of the   information that goes into a certificate and it produces the signed   certificate.  There are requirements surrounding the auditing of a   CA's actions. The details of what actions are audited, where the   audit trail is maintained, and what utilities exist to search that   audit trail are not specified here. The functions a CA must provide   are:3.8.1 Install CA   Install_CA(                       keysize               Integer,   --inputs                       CA_state              CA State,  --outputs                       CA_Public_Key         Public Key)   This routine need only generate a public/private key pair of the   requested size. Keysize is likely to be in implementation constant   rather than a parameter.  The value is likely to be either 512 or   640.  Key sizes throughout will have to increase over time as   factoring technology and CPU speeds improve.  Both keys are stored as   part of the CA_state; the public key is returned so that other CAs   may cross-certify this one. The `Next Serial number' in the CA state   is set to 1.3.8.2 Create Certificate   Create_certificate(                                                    --inputs                       Renewal               Boolean,                       Include_UID           Boolean,                       Issuer_name           Name,                       Issuer_UID            UID,                       Effective_date        Absolute Time,                       Expiration_date       Absolute Time,                       Subject_name          Name,Kaufman                                                        [Page 50]

RFC 1507                          DASS                    September 1993                       Subject_UID           UID,                       Subject_public_key    Public Key,                                                    --updated                       CA_state              CA State,                                                    --outputs                       Certificate           Certificate)   This procedure creates and signs a certificate.  Note that the   various contents of the certificate must be communicated to the CA in   some reliable fashion.  The Issuer_name and UID are the name and UID   of the directory on whose behalf the certificate is being signed.   This routine formats and signs a certificate with the private key in   CA_state. It audits the creation of the certificate and updates the   sequence number which is part of CA_state. The Issuer and Subject   names are X.500 names.  If the CA state includes a history of what   UIDs have previously been used by what names, this call will only   succeed in the collision case if the Renewal boolean is set true.  If   the Include_UID boolean is set true, this routine will generate a   1992 format X.509 certificate; otherwise it will generate a 1988   format X.509 certificate.3.8.3 Create Principal   Create_principal(                                                    --inputs                       Password              String,                       keysize               Integer,                       Principal_name        Name,                       Principal_UID         UID,                       Parent_Public_key     Public Key,                       Parent_UID            UID,                                                    --outputs                       Encrypted_Credentials Encrypted Credentials,                       Trusted_authority_certificate Certificate)   This procedure creates a new principal by generating a new   public/private key pair, encrypting the public and private keys under   the password, and signing a trusted authority certificate for the   parent CA.  In an implementation not using passwords (e.g., smart   cards), an alternative mechanism must be used for initially creating   principals.  If a principal has protected storage for trusted   authority information, it is not necessary to create a trusted   authority certificate and store it in the naming service.  Some   procedure analogous to this one must be executed, however, in which   the principal learns the public key and UID of its CA and its own   name.Kaufman                                                        [Page 51]

RFC 1507                          DASS                    September 1993   This routine creates two output structures with the following steps:    a) Generate a public/private key pair using the indicated       keysize. An implementation will likely fix the keysize as an       implementation constant, most likely 512 or 640 bits, rather       than accepting it as a parameter.  Key sizes generally will       have to increase over time as factoring technology and CPU       speeds improve.    b) Form the encrypted credentials by using the public key,       private key, and Principal_UID and encrypting them using a       hash of the password as the key.    c) Generate a trusted authority certificate (which is identical       in format to a "parent" certificate) getting fields as       follows:      1) Certificate version is X.509 1992.      2) Issuer name is the Principal name (which is an X.500 name).      3) Issuer UID is the Principal UID.      4) Validity is for all time.      5) Subject name is constructed from the Principal name by         removing the last simple name from the hierarchical name.      6) Subject UID is the CA_UID.      7) Subject Public Key is the CA_Public_Key      8) Sequence number is 1.      9) Sign the certificate with the newly generated private key of         the principal.3.8.4 Change Password   Change_password(                                 --inputs                       Encrypted_credentials Encrypted Credentials,                       Old_password          String,                       New_password          String,                                                    --outputs                       Encrypted_credentials Encrypted Credentials)   If credentials are stored encrypted under a password, it is possible   to change the password if the old one is known.  Note that it isKaufman                                                        [Page 52]

RFC 1507                          DASS                    September 1993   insufficient to just change a user's password if the password has   been disclosed.  Anyone knowing the old password may have already   learned the user's private key.  If a password has been disclosed,   the secure recovery procedure is to call create_principal again   followed by create_certificate to certify the new key.   Using DASS, it may not be appropriate for users to periodically   change their passwords as a precaution unless they also change their   private keys by the procedure above.  The only likely use of the   change_password procedure is to handle the case where an   administrator has chosen a password for the user in the course of   setting up the account and the user wishes to change it to something   the user can remember.  A future version of the architecture may   smooth key roll-over by having the change_password command also   generate a new key and sign a "self" certificate in which the old key   certifies the new one.  As a separate step, a CA which notices a self   certificate posted in the naming service could certify the new key   instead of the old one when the user's certificate is renewed.  While   this procedure is not as rapid or as reliable as having the user   directly interact with the CA, it offers a reasonable tradeoff   between security and convenience when there is no evidence of   password compromise.   This routine simply decrypts the encrypted credentials structure   supplied using the password supplied. It returns a bad status if the   format of the decrypted information is bad (indicating an incorrect   password). Otherwise, it creates a new encrypted credentials   structure by encrypting the same data with the new password. It would   be highly desirable for the user interface to this function to   provide the capability to randomly generate passwords and prohibit   easily guessed user chosen passwords using length, character set, and   dictionary lookup rules, but such capabilities are beyond the scope   of this document.  If encrypted credentials are stored in some local   secure storage, the above function is all that is necessary (in fact,   if the storage is sufficiently secure, no password is needed;   credentials could be stored unenciphered).  If they are stored in a   naming service, this function must be coupled with one which   retrieves the old encrypted credentials from the naming service and   stores the new.  The full protocol is likely to include access   control checks that require the principal to acquire credentials and   produce tokens.  For best security, the encrypted credentials should   be accessible only through a login agent.  The role of the login   agent is to audit and limit the rate of password guessing.  If   passwords are well chosen, there is no significant threat from   password guessing because searching the space is computationally   infeasible.  In the context of a login agent, change password will be   implemented with a specialized protocol requiring knowledge of the   password and (for best security) a trusted authority from which theKaufman                                                        [Page 53]

RFC 1507                          DASS                    September 1993   public key of the login agent can be learned.  Seesection 2.3.2 for   the plans for the non-X.500 credential storage facility.3.8.5 Change Name   Change_name(                                                    --inputs                       Claimant_Credentials  Credentials,                       New_name              Name,                       CA_Public_Key         Public Key,                       CA_UID                UID,                                                    --outputs                       Trusted_Authority_Certificate Certificate)   DASS permits a principal to have many current aliases, but only one   current name.  A principal can authenticate itself as any of its   aliases but verifies the names of others relative to the name by   which it knows itself.  Aliases can be created simply by using the   create_certificate function once for each alias.  To change the name   of a principal, however, requires that the principal securely learn   the public key and UID of its new parent CA.  As with   create_principal, if a principal has secure private storage for its   trusted authority information, it need not create a certificate, but   some analogous procedure must be able to install new naming   information.   This routine produces a new Trusted Authority Certificate with   contents as follows:    a) Issuer name is New_name (an X.500 name)    b) Issuer_UID is Principal UID from Credentials.    c) Validity is for all time.    d) Subject name is constructed from the Issuer name by removing       the last simple name from the hierarchical name, and       converting to an X.500 name.    e) Subject UID is CA_UID    f) Subject Public Key is CA_Public_Key    g) Sequence number is 1.    h) The certificate is signed with the private key of the       principal from the credentials. Note that this call will only       succeed if the principal's private key is in the credentials,Kaufman                                                        [Page 54]

RFC 1507                          DASS                    September 1993       which will only be true if the credentials were created by       calling Create_server_credentials.3.9 Credential Maintenance Functions   DASS credentials can potentially have information about two   principals.  This functionality is included to support the case   where a user on a node has two identities that might be   recognized for purposes of managing access controls.  First,   there is the user's network identity; second, there is an   identity as controlling a particular "account" or "username" on   that node.  There are two reasons for recognizing this second   identity: first, access controls might be specified such that   only a user is only permitted access to certain resources when   coming through certain trusted nodes (e.g., files that can't be   accessed from a terminal at home); and second, before the   transition strategy to global identities is complete, as a way to   refer to USER@NODE in a way analogous to existing mechanisms but   with greater security.   The mapping of global usernames to local user names on a node is   outside the scope of DASS.  This is done via a "proxy database"   or some analogous local mechanism.  What DASS provides are   mechanisms for adding node oriented credentials into a user's   credentials structure, carrying the dual authentication   information in authentication tokens, and extracting the   information from the credentials structure created by   Accept_token.   Some applications of DASS will not make use of the node   authentication related extensions.  In that case, they will never   use the Combine_credentials, Create_credentials, Get_node_info,   or Verify_node_name functions.   The "normal" sequence of events surrounding a user logging into a   node are as follows:    a) When the user logs in, he types either a local user ID known       to the node or a global name (the details of the user       interface are implementation specific).  Through some sort of       local mapping, the node determines both a global name and a       local account name.  The user also enters a password       corresponding to the global name.    b) The node calls network_login specifying the user's global name       and the supplied password.  The result is credentials which       can be used to access network services but which have not yet       been verified to be valid.Kaufman                                                        [Page 55]

RFC 1507                          DASS                    September 1993    c) The node calls verify_principal_name using its own credentials       to verify the authenticity of the user's credentials (these       node credentials must have previously been established by a       call to initialize_server during node initialization).    d) If that test succeeds, the node adds its credentials to those       of the user by calling combine_credentials.   The set of facilities for manipulating credentials follow:3.9.1 Network login   Network_login(                                                    --inputs                       Name                  Name,                       password              String,                       keysize               Integer,                       expiration            Time interval,                       TA_credentials        Credentials,--optional                                                    --outputs                       Claimant_credentials  Credentials)   This function creates credentials for a principal when the principal   "logs into the network".   Name is the X.500 name of the principal.   Password is a secret which authenticates the principal to the   network.   Keysize specifies the size of the temporary "login" or "delegation"   key.  In a real implementation, it is expected to be an   implementation constant (most likely 384 or 512 bits).   Expiration sets a lifetime for the credentials created.  For a normal   login, this is likely to be an implementation constant on the order   of 8-72 hours.  Some mechanism for overriding it must be provided to   make it possible (for example) to submit a background job that might   run days or even months after they are submitted.   TA_credentials   are used if the encrypted credentials are protected   by a login agent. If they are missing, the password will be less well   protected from guessing attacks.   This routine does not (as one might expect) securely authenticate the   principal to the calling procedure.  Since the password is used to   obtain the principal's private key, this call will normally fail if   the principal supplies an invalid password.  A penetrator who hasKaufman                                                        [Page 56]

RFC 1507                          DASS                    September 1993   compromised the naming service could plant fake encrypted credentials   under any name and impersonate that name as far as this call is   concerned. A caller that wishes to authenticate the user in addition   to obtaining credentials to be able to act on the user's behalf   should call Verify_principal_name (below) with the created   credentials and the credentials of the calling process.   This routine constructs a credentials structure from information   found in the naming service encrypted using the supplied password.    a) If the encrypted credentials structure is protected with a       login agent, retrieve the public key of the login agent:      1) If TA_credentials are available, use them in a call to         Get_Pub_Keys to get the public key of the login agent (whose         name is derived from the name of the principal by truncating         the last element of the RDN and adding CSS=X509).      2) If TA_credentials are not available, look up the public key         of the login agent in the naming service.       Login agents limit and audit password guesses, and are       important when passwords may not be well chosen (as when users       are allowed to choose their own).  To fully prevent the       password guessing threat, principals may only log onto nodes       that already have TA_credentials which can be used to       authenticate the login agent.  To support nodes which have no       credentials of their own and to allow this procedure to       support node initialization, it is possible to network login       without TA credentials.       A principal who logs into a node that lacks TA credentials is       subject to the following subtle security threat:  A penetrator       who impersonates the naming service could post his own public       key and address as those of the login agent.  This procedure       would then in the process of logging in reveal the the       penetrator enough information for the penetrator to mount an       unaudited password guessing attack against the principal's       credentials.    b) Retrieve the encrypted credentials from the naming service or       login agent.  In the case of the login agent, the password is       one-way hashed to produce proof of knowledge of the password       and the hashed value is supplied to the login agent encrypted       under its public key as part of the request.    c) Decrypt the encrypted credentials structure using a the       supplied password. Verify that the decryption was successfulKaufman                                                        [Page 57]

RFC 1507                          DASS                    September 1993       by verifying that the resulting structure can be parsed       according the the ASN.1 rules for Encrypted_Credentials and       that the two included primes when multiplied together produce       the included modulus. If the decryption was unsuccessful then       the routine returns the `Invalid password' error status. The       decryption results in both the Private Key and the Public Key.    d) Generate a public/private key pair for the Delegation Key,       using the indicated keysize. Key size is likely to be an       implementation constant rather than a supplied parameter, with       likely values being 384 and 512 bits.  Key sizes generally       will have to increase over time as factoring technology and       CPU speeds improve.  Delegation keys can be relatively shorter       than long term keys because DASS is designed so that       compromise of the delegation key after it has expired does not       result in a security compromise.  An important advantage of       making key size an implementation constant is that nodes can       generate key pairs in advance, thus speeding up this procedure.       Key generation is the most CPU intensive RSA procedure and       could make login annoyingly slow.    e) Construct a Login Ticket by signing with the user's private       key a combination of the public key, a validity period       constructed from the current time and the expiration passed in       the call, and the principal UID found in the encrypted-key       structure.    f) Forget the user's private key.    g) Retrieve from the naming service any trusted authority       certificates stored with the user's entry. Discard any that       are not signed by the user's public key and UID.  An       implementation in which the login node has credentials of its       own may choose its trusted authority information instead of       retrieving and verifying trusted authority certificates from       the naming service.  This will have a subtle effect on the       security of the resulting system.    h) Construct a credentials structure from:      1) Claimant credentials:        (i)  Name of the principal from calling parameter        (ii) Login Ticket as constructed in (e)        (iii)Delegation Private key as constructed in (d)        (iv) Public key from the encrypted credentials structure      2) No verifier credentialsKaufman                                                        [Page 58]

RFC 1507                          DASS                    September 1993      3) Trusted Authorities: for the most recently signed trusted         authority certificate (There is normally only one Trusted         Authority Certificate.  If there is more than one then an         implementation may choose to maintain a list of all the valid         keys. They should all refer to the same CA (UID and name).):        (i)  Name of the CA from the subject field of the certificate        (ii) Public Key of the CA from the subject public key field        (iii)UID of the CA from the subject UID field      4) no remote node credentials      5) no local node credentials      6) no cached outgoing associations      7) no cached incoming associations3.9.2 Create Credentials   Create_credentials(                                                      --outputs                       Claimant_credentials  Credentials)   This routine creates an "empty" credentials structure.  It is needed   in the case of a user logging into a node and obtaining node oriented   credentials but no global username credentials.  Because the   "combine_credentials" call wants to modify a set of user credentials   rather than create a new set, this call is needed to produce the   "shell" for combine_credentials to fill in.   It is unlikely that any real implementation would support this   function, but rather would have some functions which combine   network_login, create_credentials, and combine_credentials in   whatever ways are supported by that node.3.9.3 Combine Credentials   Combine_credentials(                                                    --inputs                       node_credentials      Credentials,                       localusername         String,                                                    --updated                       user_credentials      Credentials)   This routine is provided by implementations which support the notion   of local node credentials.  After the node has verified to its ownKaufman                                                        [Page 59]

RFC 1507                          DASS                    September 1993   satisfaction that the user_credentials are entitled to access to a   particular local account, this call adds node credential information   to the user_credential structure.  This function may be applied to   user_credentials created by network_login, create_credentials, or   accept_token.    a) Fill in the local node credentials substructure of       user_credentials as follows:      1) Full name of the node: from Full name of the Principal in         node_credentials      2) Local username on the node: from proxy lookup      3) RSA private key of the node: from verifier credentials in         node_credentials    b) Optionally,  change the trusted authorities to match the       trusted authorities from the node credentials.  This is an       implementation option, done most likely as a performance       optimization.  The only case where this option is required is       where no trusted authorities existed in the user credentials       (because they were created by create_credentials of       accept_token).  Server credentials should generally keep their       own trusted authorities.   It is likely that an implementation will choose not to replicate its   node credentials in every credentials structure that it supports, but   rather will maintain some sort of pointer to a single copy.  This   algorithm is stated as it is only for ease of specification.3.9.4 Initialize_server   initialize_server(                                                    --inputs                       Name                  Name,                       password              String,                       TA_credentials        Credentials, --optional                                                    --outputs                       Server_credentials    Credentials)   Somehow a server must get access to its credentials. One way is for   the credentials to be stored in the naming service like user   credentials encrypted under a service password. The service then   needs to gain at startup time access to a service password. This may   be easier to manage and is not insecure so long as the service   password is well chosen. Alternately, the service needs some   mechanism to gain access directly to its credentials. The credentialsKaufman                                                        [Page 60]

RFC 1507                          DASS                    September 1993   created by this call are intended to be very long lived. They do not   time out, so a node or server might store them in Non-Volatile memory   after "initial installation" rather than calling this routine at each   "boot". These credentials are shared between all servers which use   the same key. This routine works as follows:    a) Retrieve from the naming service or login agent the encrypted       credentials structure corresponding to the supplied name. See       Network_login for a discussion of the use of TA_credentials       and login agents.    b) Decrypt that structure using a one-way hash of the supplied       password. Verify that the decryption was successful. Verify       that the public key in the structure matches the private key.    c) Retrieve from the naming service any trusted authority       certificates stored under the supplied name. Discard any which       do not contain the UID from the encrypted credentials       structure or are not signed by the key in the encrypted       credentials structure.    d) Construct a credentials structure from:      1) Claimant credentials:        (i)   Name of the principal from the calling parameter        (ii)  UID of the principal from the encrypted-key structure        (iii) No login ticket        (iv)  No login secret key      2) Verifier credentials:        (i)   Server secret key from the encrypted-key structure      3) Trusted Authorities: from the most recently signed Trusted         Authority Certificate:        (i)   Name of CA from the Subject Name field        (ii)  UID of the CA from the Subject UID field        (iii) Public Key of the CA from the Subject Public Key field      4) no node credentials      5) no cached outgoing associations      6) no cached incoming associationsKaufman                                                        [Page 61]

RFC 1507                          DASS                    September 19933.9.5 Generate Server Ticket   generate_server_ticket(                                                    --inputs                       expiration            Time interval,                                                    --updated                       Server_credentials    Credentials)   Server credentials created by initialize_server can be used to accept   incoming authentication tokens and can act as node_credentials for   outgoing authentications, but cannot create user_credentials of their   own. If a server initiates connections on its own behalf, it must   have a ticket just like any other user might have. That ticket has   limited lifetime and the right to act on behalf of the server can be   delegated. The server cannot, however, delegate the right to receive   connections intended for it. An implementation must come up with a   policy for the expiration of server tickets and how long before   expiration they are renewed.  A likely policy is for this procedure   to be implicitly called by Create_token if there is no current ticket   present in the credentials.  If so, this interface need not be   exposed.   This routine is implemented as follows:    a) Generate an RSA public/private key pair.    b) Compute a validity interval from the current time and the       expiration supplied.    c) Construct a login ticket from the RSA public key (from a),       validity interval (from b), the UID from the credentials, and       signed with the server key in the credentials. (Discard       previous Login Ticket if there was one).    d) Discard all information in the  Cached Outgoing Contexts.3.9.6 Delete Credentials   delete_credentials(                                                    --updated                       credentials           Credentials)   Erases the secrets in the credentials structure and deallocates the   storage.Kaufman                                                        [Page 62]

RFC 1507                          DASS                    September 19933.10 Authentication Procedures   The guts of the authentication process takes place in the next two   calls. When one principal wishes to authenticate to another, it calls   Create_token and sends the token which results to the other. The   recipient calls Accept_token and creates a new set of credentials.   The other calls in this section manipulate the received credentials   in order to retrieve its contents and verify the identity of the   token creator.3.10.1  Create Token   Create_token(                                                    --inputs                       target_name            Name,                       deleg_req_flag         Boolean,                       mutual_req_flag        Boolean,                       replay_det_req_flag    Boolean,                       sequence_req_flag      Boolean,                       chan_bindings          Octet String,                       Include_principal_name Boolean,                       Include_node_name      Boolean,                       Include_username       Boolean,                                                      --updated                       claimant_credentials   Credentials,                                                    --outputs                       authentication_token   Authentication token,                       mutual_authentication_token                                   Mutual Authentication token,                       Shared_key             Shared Key,                       instance_identifier    Timestamp)   This routine is used by the initiator of a connection to create an   authentication token which will prove its identity. If the claimant   credentials includes node/account information, the token will include   node authentication.   target_name is the X.500 name of the intended recipient of the token.   Only an entity with access to the private key associated with that   name will be able to verify the created token and generate the   mutual_authentication_token.   deleg_req_flag indicates whether the caller wishes to delegate to the   recipient of the token. If it is set, the delegated_credentials   returned by Accept_token will be capable of generating tokens on   behalf of the caller. Node based authentication information cannot be   delegated. The mutual_req_flag, replay_det_req_flag , and   sequence_req_flag are put in the authentication token and passed toKaufman                                                        [Page 63]

RFC 1507                          DASS                    September 1993   the target.  This information is included in the token to make it   easier to implement the GSSAPI over DASS.  DASS itself makes no use   of this information.   In most applications, the purpose of a token exchange is to   authenticate the principals controlling the two ends of a   communication channel.  chan_bindings contains an identifier of the   channel which is being authenticated, and thus its format and content   should be tied to the underlying communication protocol.  DASS only   guarantees that the information has been communicated reliably to the   named target. If DASS is used with a cryptographically protected   channel (such as SP4), this data should contain a one-way hash of the   key used to encrypt the channel. If that channel is multiplexed, the   data should also include the ID of the subchannel.  If the channel is   not encrypted, the network must be trusted not to modify data on a   connection.  The source and target network addresses and a connection   ID should be included in the chan_bindings at the source and checked   at the target.  A token exchange also results in the two ends sharing   a key and an instance identifier.  If that key and instance   identifier are used to cryptographically protect subsequent   communications, then chan_bindings need not have any cryptographic   significance but may be used to differentiate multiple entities   sharing the public keys of communicating principals.  For example, if   a service is replicated and all replicas share a public key,   chan_bindings should include something that identifies a single   instance of the service (such as current address) so that the token   cannot be successfully presented to more than one of the servers.   include_principal_name, include_node_name, and include_username are   flags which determine whether the principal name, node name, and/or   username from the credentials structure are to be included in the   token.  This information is made optional in a token so that   applications which communicate this information out of band can   produce "compressed" tokens.  If this information is included in the   token, it will be used to populate the corresponding fields in the   credentials structure created by Accept_token.  claimant_credentials   are the credentials of the calling procedure.  The secrets contained   therein are used to sign the token and the trusted authorities are   used to securely learn the public key of the target.  The cached   outgoing contexts portion of the credentials may be updated as a side   effect of this call.   The major output of this routine is an  authentication_token which   can be passed to the target in order to authenticate the caller.   In addition to returning an authentication token, this routine   returns a mutual_authentication_token,  a shared_key, and an   instance_identifier. The mutual authentication token is the same asKaufman                                                        [Page 64]

RFC 1507                          DASS                    September 1993   the one generated by the Accept_token call at the target. If the   protocol using DASS wishes mutual authentication, the target should   return this token to the source. The source will compare it to the   one returned by this routine using Compare_Mutual_Token (below) and   know that the token was accepted at its proper destination.   The DES key and instance identifier can be used to encrypt or sign   data to be sent to this target. The key and instance will be given to   the target by Accept_token, and the key will only be known by the two   parties to the authentication. If a single set of credentials is used   to authenticate to the same target more than once, the same DES key   is likely to be returned each time.  If the parties wish to protect   against the possibility of an outside agent mixing and matching   messages from one authenticated session with those of another, they   should include the instance identifier in the messages. The instance   identifier is a timestamp and it is guaranteed that the DES   key/instance identifier pair will be unique.   An implementation may wish to "hide" the DES key from calling   applications by placing it in system storage and providing calls   which encrypt/decrypt/sign/verify using the key.   The primary tasks of this routine are to create its output   parameters. As a side effect, it may also update claimant_credentials   It's algorithm is as follows:    a) The login ticket is checked. If it has passed the end of its       lifetime an `Login Ticket Expired' error is returned. If there       is a login ticket, but no corresponding private key then an       `Invalid credentials' error is returned (this is the case if       the credentials were created by an authentication-without-       delegation operation).  If there is no login ticket or an       expired one and if the long term private key is present in the       credentials, an implementation may choose to automatically call       create_server_ticket to renew the ticket.    b) Create new timestamp using the current time.  (This timestamp       must be unique for this Shared Key. The timestamp is a 64 bit       POSIX time, with a resolution of 1 nanosecond An implemen tation       must ensure that timestamps cannot be reused.)    c) The public key and UID of target_name are looked up by calling       get_pub_keys, using the target_name and the Trusted Authority       section of the claimant_credentials structure. If none is       found, an error status is returned. Otherwise, the cached       outbound connections portion of credentials are searched       (indexed by target Public Key) for a cached Shared key with a       validity interval which has not expired. If a suitable one isKaufman                                                        [Page 65]

RFC 1507                          DASS                    September 1993       found skip to step g, else create a cache entry as follows:    d) Destination Public Key is the one found looking up the target.       A Shared Key is generated at random. A validity interval is       chosen according to node policy but not to exceed the validity       interval of the ticket in the credentials (if any).    e) Create the Encrypted Shared Key, using the public key of the       Target, and place in the cache.    f) If node authentication credentials are available in the       credentials structure, create a "Node Ticket" signature using       the node secret and include it in the cache.    g) If delegation is requested and no delegator is present in the       cache, create one by encrypting the delegation private key       under the Shared key. The delegation private key is       represented as an ASN.1 data structure containing only one of       the primes (p).    h) If delegation is not requested and no Shared Key Ticket is in       the cache, create one by signing the requisite information       with the delegation private key.    i) Create the Authenticator.  The contents of the Authenticator       (including the channel bindings) are encoded into ASN.1, and       the signature is computed. The Authenticator is then       re-encoded, without including the Channel Bindings but using       the same signature.    j) Create output_token as follows:      1) Encrypted Shared Key from cache      2) Login Ticket from Claimant Credentials (if present)      3) Shared Key Ticket from cache (if no delegation and if         present)      4) Node Ticket from cache (if present)      5) Delegator from cache (if delegation and if present)      6) Authenticator      7) Principal name from credentials (if present and parameter         requests this)      8) Node name from credentials (if present and parameter request         this)      9) Local Username from credentials (if present and parameter         requests this)    k) Compute Mutual_authentication_token by encrypting the       timestamp from the authenticator using the Shared key.Kaufman                                                        [Page 66]

RFC 1507                          DASS                    September 1993    l) The instance_identifier is the timestamp. This and the Shared       key are returned for use by the caller for further encryption       operations (if these are supported).3.10.2 Accept_token   Accept_token(                                                    --inputs                       authentication_token  Authentication Token,                       chan_bindings         Octet String,                                                     --updated                       verifying_credentials Credentials,                                                    --outputs                       accepted_credentials  Credentials,                       deleg_req_flag        Boolean,                       mutual_req_flag       Boolean,                       replay_det_req_flag   Boolean,                       sequence_req_flag     Boolean,                       mutual_authentication_token                                        Mutual authentication token                       shared_key            Shared Key,                       instance_identifier   Timestamp)   This routine is used by the recipient of an authentication token to   validate it.  authentication_token is the token as received;   chan_bindings is the identifier of the channel being authenticated.   See the description of Create_token for information on the   appropriate contents for chan_bindings.  DASS does not enforce any   particular content, but checks to assure that the same value is   supplied to both Create_token and Accept_token.   Verifying_credentials are the credentials of the recipient of the   token.  They must include the private key of the entity named as the   target in Create_token or the call will fail.  The cached incoming   contexts section of the verifying credentials may be modified as a   side effect of this call.   Accepted_credentials will contain additional information about the   token creator. If delegation was requested, these credentials can be   used to make additional calls to Create_token on the creator's   behalf. Whether or not delegation was requested, they can also be   used in the calls which follow to gain additional information about   the token creator.   The deleg_req_flag indicates whether the accepted_credentials include   delegation which can be used by the recipient to act on behalf of the   principal.  Mutual_req_flag, replay_det_req_flag, and   sequence_req_flag are passed through from Create_token in support ofKaufman                                                        [Page 67]

RFC 1507                          DASS                    September 1993   the GSSAPI.  DASS makes no use of these fields.   The mutual_authentication_token can be returned to the token creator   as proof of receipt. In many protocols, this will be used by a client   to authenticate a server. Only the genuine server would be able to   compute the mutual_authentication_token from the token.   The shared_key and instance_identifier can be used to encrypt or sign   data between the two authenticating parties. See Create_token.   This routine verifies the contents of the authentication token in the   context of the verifying credentials (In particular, the Private Key   of the server is used.  Also, the Cached Incoming Contexts and   Incoming Timestamp list is used.) and returns information about it.   The algorithm updates a cache of information. This cache is not   updated if the algorithm exits with an error. The algorithm is as   follows:    a) If there is a Login Ticket, but no Shared Key Ticket or       Delegator then exit with error `Invalid Authenticator'. If       there is a Shared Key Ticket or Delegator, but no Login Ticket       then exit with error `Invalid Authentication Token'.       Look up the Encrypted Shared key in the Cached Incoming Contexts       of the credentials structure. (This cache entry is used during       the execution of this routine. An implementation must ensure that       references to the cache entry can not be affected by other users       modifying the cache.  One way is to use a copy of the cache entry,       and update it at exit.)  If it is not found then create       a new cache entry as follows:      1) Encrypted Shared Key, from the Authentication Token.      2) Shared Key and Validity Interval, by decrypting the         Encrypted Shared Key using the server private key in         credentials. If the decryption fails then exit with error         `Invalid Authentication Token'.    b) Check that the Validity Interval (in the cache entry) includes       the current time; return `Invalid Authentication Token' if not.       Check the Timestamp is within max-clock-skew of the current       time, return `invalid Authentication Token' if not.       Reconstruct the Authenticator including the Channel Bindings       passed as a parameter.Kaufman                                                        [Page 68]

RFC 1507                          DASS                    September 1993       Check that the reconstructed Authenticator is signed by the       Shared key. If not then exit with error `Invalid       Authentication Token'.       Look up the Authenticator Signature in the Received       Authenticators. If the same Signature is found in the list       then exit with error `Duplicate Authenticator'. Otherwise add       the Signature and timestamp to the list.       If there is a Login Ticket and the Delegation Public key is in       the cache entry, then check that the same key is specified in       the Login Ticket, if not then exit with error `Invalid       Authentication Token'. Place the Delegation Public key in the       cache if it is not already there.       If there is a Login Ticket, the Delegation Public key was not       previously in the cache entry, and there is a Shared Key       Ticket in the Authentication Token, then check that the Shared       Key Ticket is signed by the Delegation Public Key in the Login       Ticket. If not then exit with error `Invalid Authentication       Token'.       If a delegator is present in the message then decrypt the       delegator using the Shared key. If the private key does not       match the Delegation Public key then exit with error       `Invalid Authentication Token' (The prime in the delegator       is used to find the other prime (from the modulus). The division       must not have a remainder.  Neither prime may be 1. The two       primes are then used to reconstruct any other information       needed to perform cryptographic operations.).       Build the delegation credentials data structure as follows:       1) Claimant credentials:        (i)  Login Ticket from the Authentication token        (ii) Delegation Private key from the decrypted delegator if              the token is delegating.        (iii)Encrypted Shared Key from the Authentication token.       2) There are no verifier credentials.       3) Trusted authorities are copied from the verifying_credentials          passed to this routine (If an implementation is able to          obtain the original Trusted Authorities of the Principal then          it may do so instead of using the server's Trusted          Authorities.).       4) Remote node credentials (Node name, Username, Node Ticket)       5) There are no local node credentials.       6) There are no cached contexts.Kaufman                                                        [Page 69]

RFC 1507                          DASS                    September 1993    c) The returned boolean values are obtained from the       Authenticator.    d) Mutual_authentication_token is computed by encrypting the       timestamp from the Authenticator with the Shared key from the       cache.    e) Instance_identifier is the timestamp from the Authenticator.       This and the Shared key are returned to the caller for further       encryption operations (if these are supported).3.10.3 Compare Mutual Token   Compare_mutual_token(                                                    --inputs                       Generated_token    Mutual authentication token,                       Received_token     Mutual authentication token,                                                     --outputs                       equality_flag         Boolean)   This routine compares two mutual authentication tokens and tells   whether they match.  In the expected use, the first is the token   generated by Create_token at the initiating end and the second is the   token generated by Accept_token at the accepting end and returned to   the initiating end.  This routine can be implemented as a byte by   byte comparison of the two parameters.3.10.4 Get Node Info   get_node_info(                                                    --inputs                       accepted_credentials  Credentials,                                                    --outputs                       nodename              Name,                       username              String)   This routine extracts from accepted credentials the name of the node   from which the authentication token came and the named account on   that node. Because this information is not cryptographically   protected within the token, this information can only be regarded as   a "hint" by the receiving application.  It can, however, be verified   using Verify_node_name in a cryptographically secure manner.  This   information will only be present if these are accepted credentials   and it the caller of Create_token set the include_node_name and/or   include_username flags.   An actual implementation is not likely to have get_node_info and   verify_node_name as separate calls.  They are specified this wayKaufman                                                        [Page 70]

RFC 1507                          DASS                    September 1993   because there are different ways this information might be used.  For   most applications, the nodename and username will be included in the   token, and a single function might extract and verify them (it might   in fact be part of accept token).  For other applications, the   nodename and username will not be in the token but rather will be   computed from other information passed during connection initiation   so a call would have to take these as inputs.  Still other   applications such as ACL evaluators that want to support the renaming   and aliasing capabilities of DASS would defer verifying node   information until they came upon an ACL which allowed access only   from a particular node.  They would then verify that the name on the   ACL was an authenticatable alias for the node which created the   token.  All of these uses can be defined in terms of calls to   get_node_info and verify_node_name.3.10.5 Get Principal UID   get_principal_uid(                                                    --inputs                       accepted_credentials  Credentials,                                                    --outputs                       uid                   UID)   This routine extracts a principal UID from a set of credentials.   As with Get_Node_Info, this interface is not likely to appear in an   actual implementation, but rather will be bundled with other   routines.  It is specified this way because there might be a variety   of algorithms by which credentials are evaluated and all of them can   be defined in terms of these primitives.   In DASS, it is possible for a principal to have many aliases.  This   can happen either because the principal was given multiple names to   limit the number of CAs that need to be trusted when authenticating   to different servers or because the principal's name has changed and   the old name remains behind as an alias.  Accept_token returns the   name by which the principal identified itself when creating its   credentials. A service may know the user by some alias. The normal   way to handle this is for the service to know the principal's UID   (which is constant over name changes) and to compare it with the UID   in the token to identify a likely alias situation. It gets the UID   from the token using this routine. It then confirms the alias by   calling verify_principal_name.   The UID is in a signed portion of accepted credentials, but the   signature may not have been verified at the time this call is issued.   The information returned by this routine must therefore be regarded   as a hint.  If a call to Verify_principal_name succeeds, however,Kaufman                                                        [Page 71]

RFC 1507                          DASS                    September 1993   then the caller can securely know that the name given to that routine   and the UID returned by this one are the authenticated source of the   token.3.10.6 Get Principal Name   get_principal_name(                                                    --inputs                       accepted_credentials  Credentials,                                                    --outputs                       name                  Name)   This routine extracts a principal name from a set of credentials.   This name is the name most recently associated with the principal. It   may be the name that the principal supplied when the credentials were   created (in which case it may not have been verified yet) or it may   be a different name that has been verified.   As with Get_Node_Info and Get_Principal_UID, this routine is not   likely to appear in an actual implementation, but will be bundled in   some fashion with related procedures.  The name returned by this   procedure is not guaranteed to have been cryptographically verified.   Verify_Principal_Name performs that function.3.10.7 Get Lifetime   get_lifetime(                                                    --inputs                       Claimant_credentials  Credentials,                                                    --outputs                       lifetime              Duration)   This routine computes the life remaining in a set of credentials.   Its most common use would be to know to renew credentials before they   expire.   Returns the remaining lifetime of the login ticket in the   credentials. This can either be the done on the node where the   original login took place, or at a server which has been delegated   to. It indicates how much longer these credentials can be used for   further delegations. This routine will return 0 if the login ticket   has passed the end of its life, if there is no login ticket, or if   the credentials do not contain the private key certified by the   ticket (i.e., where they were created by an authentication-without-   delegation operation).Kaufman                                                        [Page 72]

RFC 1507                          DASS                    September 19933.10.8 Verify Node Name   Verify_node_name(                                                    --inputs                       nodename              Name,                       username              String,                                                     --updated                       verifying_credentials Credentials,                       accepted_credentials  Credentials,                                                    --outputs                       Name matches          Boolean)   This routine tests whether the originating node of an authentication   token can be authenticated as having the provided name. Like a   principal, a node may have multiple aliases. One of them may be   returned by Get_node_info, but this call allows a suspected alias to   be verified.  The verifying credentials supplied with this call must   be the same credentials as were used in the Accept_token call. The   procedure for completing this request is as follows:    a) If there is no Node Ticket in the claimant credentials then       return False.    b) Search the incoming context cache of the verifying credentials       for an entry containing the same encrypted shared key as the       encrypted shared key subfield of the claimant information of       the accepted credentials.  In the steps which follow,       references to "the cache" refer to this entry.  If none is       found, initialize such an entry as follows:      1) Encrypted shared key from the encrypted shared key subfield         of the claimant information of the accepted credentials.      2) The shared key and validity interval are determined by         decrypting the encrypted shared key using the RSA private         key in the verifier information of the server credentials.         If this procedure is called after a call to Accept_token         using the same server credentials (as is required for         correct use), the shared key and validity interval must         correctly decrypt.  If called in some other context, the         results are undefined.  The validity interval is not         checked.      3) Initialize all other entries in the cache to missing.    c) If there is a "local username on client node" in the cache and       it does not match the username supplied as a parameter, return       False.Kaufman                                                        [Page 73]

RFC 1507                          DASS                    September 1993    d) If there is a "name of client node" in the cache and it       matches the nodename supplied as a parameter:      1) Set the "Full name of the node" subfield of the remote node         authentication field of the accepted credentials to be the         nodename supplied as a parameter.      2) Set the "Local Username on the node" subfield of the remote         node authentication field of the accepted credentials to be         the username supplied as a parameter.      3) return True.    e) Call the Get_Pub_Keys subroutine with the server_credentials,       the nodename supplied as a parameter, and Try_Hard=False.    f) If "Public Key of Client Node" is missing from the cache,       check all of the Public keys returned to see if one verifies       the node ticket.  If one does, set the "Public Key of Client       Node" and "UID of Client Node" fields in the cache to be the       PK/UID pair that verified the ticket and set the "Local       Username on Client node" field to be the username supplied as       a parameter..    g) If any of the Public Key/UID pairs match the "Public Key of       Client Node" and "UID of Client Node" fields in the cache,       then:      1) Set the "name of client node" in the cache equal to the         nodename supplied as a parameter.      2) Set the "Full name of the node" subfield of the remote node         authentication field of the accepted credentials to be the         nodename supplied as a parameter.      3) Set the "Local Username on the node" subfield of the remote         node authentication field of the accepted credentials to be         the username supplied as a parameter.      4) Return True.    h) If none of them match, call Get_Pub_Keys again with       Try_Hard=True and repeat steps 6 & 7.  If Step 7 fails a       second time, return False.Kaufman                                                        [Page 74]

RFC 1507                          DASS                    September 19933.10.9 Verify Principal Name   Verify_principal_name(                                                    --inputs                       principal_name        Name,                                                     --updated                       verifier_credentials  Credentials,                       claimant_credentials  Credentials,                                                    --outputs                       Name matches          Boolean)   This routine tests (in the context of the verifier credentials)   whether the claimant credentials are authenticatable as being those   of the named principal.  This procedure is called with a set of   accepted credentials to authenticate their source, or with a set of   credentials produced by network_login to authenticate the creator of   those credentials.  If the claimant credentials were created by   Accept_token, then the verifier credentials supplied in this call   must be the same as those used in that call.  The procedure for   completing this request is as follows:    a) If there is no Login Ticket in the claimant credentials, then       return False.    b) If the current time is not within the validity interval of the       Login Ticket, then return False.    c) If there is an Encrypted Shared Key present in the Claimant       information field of the claimant credentials, then find or       create a matching cache entry in the Cached Incoming Contexts       of the verifier credentials.  In the description which       follows, references to "the cache" refer to this entry.  If       the cache entry must be created, its contents is set to be as       follows:      1) Encrypted shared key from the encrypted shared key subfield         of the claimant information of the accepted credentials.      2) The shared key and validity interval are determined by         decrypting the encrypted shared key using the RSA private         key in the verifier information of the server credentials.         If this procedure is called after a call to Accept_token         using the same server credentials (as is required for         correct use), the shared key and validity interval must         correctly decrypt.  If called in some other context, the         results are undefined.  The validity interval is not         checked.Kaufman                                                        [Page 75]

RFC 1507                          DASS                    September 1993      3) Initialize all other entries in the cache to missing.    d) If there is a cache entry and if the "Public Key of Client       Principal" field is present and if the "UID of Client       Principal" field is present and matches the UID in the Login       Ticket, then:      1) Set the Public Key of the principal field in the Claimant         information to be the Public Key of Client Principal.      2) If the "Full name of the principal" field is missing from         the claimant information of the claimant credentials, then         set it to the "Name of Client Principal" field from the         cache.    e) If there is a cache entry and if the "Name of Client       Principal" field is present and if it matches the principal       name supplied to this routine and if the UID in the cache       matches the UID in the Login Ticket, return True.    f) Call the Get_Pub_Keys subroutine with the name and verifier       credentials supplied to this routine and Try_Hard=FALSE.       Ignore any keys retrieved where the corresponding UID does not       match the UID in the claimant credentials.    g) If the Public Key of the principal is missing from the       claimant information of the claimant credentials, then attempt       to verify the signature on the login ticket with each public       key returned by Get_Pub_Keys.  If verification succeeds:      1) Set the Public Key of the principal in the claimant         information of the claimant credentials to be the Public Key         that verified the ticket.      2) If the Full name of the principal in the claimant         information of the claimant credentials is missing, set it         to the name supplied to this routine.      3) If there is a cache entry, set the Name of Client Principal         to be the name supplied to this routine, the UID of Client         Principal to be the UID from the Login Ticket, and the         Public Key of Client Principal to be the Public Key that         verified the ticket.      4) Return True.    h) If the Public Key of the principal is present in the claimant       information of the claimant credentials, then see if itKaufman                                                        [Page 76]

RFC 1507                          DASS                    September 1993       matches any of the public keys returned by Get_Pub_Keys.  If       one of them matches:      1) If the Full name of the principal in the claimant         information of the claimant credentials is missing, set it         to the name supplied to this routine.      2) If there is a cache entry, set the Name of Client Principal         to be the name supplied to this routine, the UID of Client         Principal to be the UID from the Login Ticket, and the         Public Key of Client Principal to be the Public Key that         verified the ticket.      3) Return True.    i) If steps 7 & 8 fail, retry the call to Get_Pub_Keys with       Try_Hard=TRUE, and retry steps 7 & 8.  If they fail again,       return false.3.10.10 Get Pub Keys   Get_Pub_Keys(                                                    --inputs                       TA_credentials     Credentials                       Try_Hard           Boolean,                       Target Name        Name,                                                    --outputs                       Pub_keys           Set of Public key/UID pairs   This common subroutine is used in the execution of Create_Token,   Verify_Principal_Name, and Verify_Node_Name.  Given the name of a   principal, it retrieves a set of public key/UID pairs which   authenticate that principal (normally only one pair).  It does this   by retrieving from the naming service a series of certificates,   verifying the signatures on those certificates, and verifying that   the sequence of certificates constitute a valid "treewalk".   The credentials structure passed into this procedure represent a   starting point for the treewalk.  Included in these credentials will   be the public key, UID, and name of an authority that is trusted to   authenticate all remote principals (directly or indirectly).   The "Try_Hard" bit is a specification anomaly resulting from the fact   that caches maintained by this routine are not transparent to the   calling routines.  It tells this procedure to bypass caches when   doing all name service lookups because the information in caches is   believed to be stale.  In general, a routine will call Get_Pub_Keys   with Try_Hard set false and try to use the keys returned.  If use ofKaufman                                                        [Page 77]

RFC 1507                          DASS                    September 1993   those keys fails, the calling routine may call this routine again   with Try_Hard set true in hopes of getting additional keys.   Routinely calling this routine with Try_Hard set true is likely to   have adverse performance implications but would not affect the   correctness or the security of the operation.   The name supplied is the full X.500 name of the principal for whom   public keys are needed as part of some authentication process.   This procedure securely learns the public keys and UIDs of foreign   principals by constructing a valid chain of certificates between its   trusted TA and the certificate naming the foreign principal.  In the   simplest case, where the TA has signed a certificate for the foreign   principal, the chain consists of a single certificate.  Otherwise,   the chain must consist of a series of certificates where the first is   signed by the TA, the last is a certificate for the foreign   principal, and the subject of each principal in the chain is the   issuer of the next.  What follows is first a definition of what   constitutes a valid chain of certificates followed by a model   algorithm which constructs all of (and only) the valid chains which   exist between the TA and the target name.   In order to limit the implications of the compromise of a single CA,   and also to limit the complexity of the search of the certificate   space, there are restrictions on what constitutes a valid chain of   certificates from the TA to the Name provided.  The only CAs whose   compromise should be able to compromise an authentication are those   controlling directories that are ancestors of one of the two names   and that are not above a common ancestor.  Therefore, only   certificates signed by those CAs will be considered valid in a   certificate chain.  Normally, the CA for a directory is expected to   certify a public key and UID for the CA of each child directory and   one parent directory.  A CA may also certify another CA for some   remote part of the naming hierarchy, and such certificates are   necessary if there are no CAs assigned to directories high in the   naming hierarchy.   A certificate chain is considered valid if it meets the following   criteria:    a) It must consist of zero or more  parent certificates, followed       by zero or one   cross certificates, followed by zero or more       child certificates.    b) The number of parent certificates may not exceed the number of       levels in the naming hierarchy between the TA name and the       name of the least common ancestor in the naming hierarchy       between the TA name and the target name.Kaufman                                                        [Page 78]

RFC 1507                          DASS                    September 1993    c) Each  parent certificate must be stored in the naming service       under the entry of its issuer.    d) The subject of the cross certificate (if any) must be an       ancestor of the target name but must be a longer name than the       least common ancestor of the TA name and the target name.    e) The cross certificate (if any) must have been stored in the       naming service under the entry of its issuer or there must       have been an indication in the naming service that       certificates signed by this issuer may be stored with their       subjects.    f) The issuer of each parent certificate does not have stored       with it in the naming service a cross certificate with the       same issuer whose subject is an ancestor of the target name.    g) Each child certificate must be stored in the naming service       under the entry of its subject.    h) The subject of each child certificate does not have associated       with it in the naming service a cross certificate with the       same subject whose issuer is the same as the issuer of any of       the parent certificates or the cross certificate of the chain.    i) The subject of each certificate must be the issuer of the       certificate that follows in the chain.  The equality test can       be met by either of two methods:      1) The public key of the subject in the earlier certificate         verifies the signature of the later and the subject UID in         the earlier certificate is equal to the issuer UID in the         later; or      2) The public key of the subject in the earlier certificate         verifies the signature of the later, the earlier lacks a         subject UID and/or the later lacks an issuer UID and the         name of the subject in the earlier certificate is equal to         the name of the issuer in the later.    j) The Public Key of the TA verifies the signature of the first       certificate.    k) The UID of the TA equals the UID of the issuer of the first       certificate  or the UID is missing on one or both places and       the name of the TA equals the name of the issuer of the first       certificate.Kaufman                                                        [Page 79]

RFC 1507                          DASS                    September 1993    l) All of the certificates are valid X.509 encodings and the       current time is within all of their validity intervals.   If a chain is valid, the name which it authenticates can be   constructed as follows:    a) If the chain contains a cross certificate, the name       authenticated can be constructed by taking the subject name       from the cross certificate and appending to it a relative name       for each child certificate which follows.  The relative name       is the extension by which the subject name in the child       certificate extends the issuer name.    b) If the chain does not contain a cross certificate, the name       authenticated can be constructed by taking the TA name,       truncating from it the last  n name components where  n is the       number of  parent certificates in the chain, and appending to       the result a relative name for each child certificate.  The       relative name is the extension by which the subject name in       the child certificate extends the issuer name.   In the common case, the authenticated name will be the subject   name in the last certificate.  The authenticated name is   constructed by the rules above to deal with namespace   reorganization.  If a branch of the namespace is renamed (due to,   for example, a corporate acquisition or reorganization), only the   certificates around the break point need to be regenerated.   Certificates below the break will continue to contain the old   names (until renewed), but the algorithms above assure the   principals in that branch will be able to authenticate as their   new names.  Further, if the certificates at the branch point are   maintained for both the old and new names for an interim period,   principals in the moved branch will be able to authenticate as   either their old or new names for that interim period without   having duplicate certificates.   A final complication that the algorithm must deal with is the   location of  cross certificates.  If a key is compromised or for   some other reason it is important to revoke a certificate ahead   of its expiration, it is removed from the naming service.  This   algorithm will only use certificates that it has recently   retrieved from the naming service, so revocation is as effective   as the mechanisms that prevent impersonation of the naming   service.   There are plans to eventually use DASS mechanisms to   secure access to the naming service; until they are in place,   name service impersonation is a theoretical threat to the   security of revocation.  Opinions differ as to whether it is a   practical threat.   Child certificates are always stored with theKaufman                                                        [Page 80]

RFC 1507                          DASS                    September 1993   subject and will not be found unless stored in the name server of   the subject.    Parent  certificates are always stored with the   issuer and will not be found unless stored in the name server of   the issuer.  For best security, cross certificates should be   stored with the issuer because the name server for the subject   may not be adequately trustworthy to perform revocation.  There   are performance and availability penalties, however, in doing so.   The architecture and the algorithm therefore support storing   cross certificates with either the issuer or the subject.  There   must be some sort of flag in the name service associated with the   issuer saying whether cross certificates from that issuer are   permitted to be stored in the subject's name service entry, and   if that flag is set such certificates will be found and used.   In order to make revocation effective, DASS must assure that   naming service caches do not become arbitrarily stale (the   allowed age of a cache entry is included in the sum of times with   together make up the revocation time).  If DASS uses a naming   service such as DNS that does not time out cache entries, it must   bypass cache on all calls and (to achieve reasonable performance)   maintain its own naming service cache.  It may be advantageous to   maintain a cache in any case so the that the fact that the   certificates have been verified can be cached as well as the fact   that they are current.3.10.10.1 Basic Algorithm   For ease of exposition, this first description will ignore the   operation of any caches.  Permissible modifications to take   advantage of caches and enhance performance will be covered in   the next section.  This path will be followed if the Try_Hard bit   is set True on the call.   Rather than trying construct all possible chains between the TA   and the name to be authenticated (in the event of multiple   certificates per principal, there could be exponentially many   valid chains), this algorithm computes a set of PK/UID/Name   triples that are valid for each principal on the path between the   TA and the name to be authenticated.  By doing so, it minimizes   the processing of redundant information.    a) Determining path and initialization       Several state variables are manipulated during the tree walk.       These are called:Kaufman                                                        [Page 81]

RFC 1507                          DASS                    September 1993      1) Current-directory-name         This is the name indicating the current place in the tree         walk.  Initially, this is the name of the TA.      2) Least-Common-Ancestor-Name         This is the portion of the names which is common to both the         CA and the Target.  This is computed at initialization and         does not change during the treewalk.      3) Trusted-Key-Set         For each name which is an ancestor of either the TA or the         Target but not of the Least-Common-Ancestor, a list of         PK/UID/Name triples.  This is initialized to a single triple         from the TA information in the supplied credentials.      4) Search-when-descending         This is a list of PK/UID/Name triples of issuers that will         be trusted when descending the tree.  This set is initially         empty.      5) Saved-RDNs         This is a sequence of Relative Distinguished Names (RDNs)         stripped off the right of the target name to form         Least-common-ancestor-name.  This "stack" is initially empty         and is populated during Step 3.    b) Ascending the "TA side" of the tree       While Current-directory-name is not identical to       Common-point-Name the algorithm moves up the tree. At each       step it does the following operations.      1) Find all cross certificates stored in the naming service         under Current-directory-name in which the subject is an         ancestor of the principal to be authenticated or an         indication that cross certificates from this issuer are         stored in the subject entry.  If there is an indication that         such certificates are stored in the subject entry, copy all         triples in Trusted-Key-Set for Current-directory-name into         the "Search-when-descending" list.  If any such certificates         are found, filter them to include only those which meet the         following criteria:        (i)  For some triple in the Trusted-Key-Set corresponding to             the Current-directory-name, the public key in the triple             verifies the signature on the certificate  and either the             UID in the triple matches the issuer UID in the             certificate  or the UID in the triple and/or theKaufman                                                        [Page 82]

RFC 1507                          DASS                    September 1993             certificate is missing and the name in the triple matches             the issuer name in the certificate.        (ii) No certificates were found signed by this issuer in which             the subject name is longer than the subject name in this             certificate (i.e., if there are cross certificates to two             different ancestors, accept only those which lead to the             closest ancestor).        (iii)The current time is within the validity interval of the             certificate.      2) If any cross certificates were found (whether or not they         were all eliminated as part of the filtering process), set         Current-directory-name to the longest name that was found in         any certificate and construct a set of PK/UID/Name triples         for that name from the certificates which pass the filter         and place them in the Trusted Key Set associated with their         subject.  Exit the ascending tree loop at this point and         proceed directly to step 3.  Note that this means that if         there are cross certificates to an ancestor of the target         but they are all rejected (for example if they have         expired), the treewalk will   not construct a chain through         the least common ancestor and will ultimately fail unless a         crosslink from a lower ancestor is found stored with its         subject.  This is a security feature.      3) If no cross certificates are found, find all the parent         directory certificates for the directory whose name is in         the Current-directory-name.  Filter these to find only those         which meet the following criteria:        (i)  The current time is within the validity interval.        (ii) For some triple corresponding to the             Current-directory-name, the public key in the triple             verifies the signature on the certificate  and either  the             UID in the triple matches the issuer UID in the             certificate  or the UID in the triple and/or the             certificate is missing and the name in the triple matches             the issuer name in the certificate.      4) Construct PK/UID/Name triples from the remaining         certificates for the directory whose name is constructed by         stripping the rightmost simple name from the         Current-directory-name and place them in the Trusted-Key-Set.Kaufman                                                        [Page 83]

RFC 1507                          DASS                    September 1993      5) Strip the rightmost simple name of the         Current-directory-name.      6) Repeat from step (a) (testing to see if         current-directory-name is the same as Common-point-Name).    c) Searching the "target side" of the tree for a crosslink:      1) Initialization: set Current-directory-name to the name         supplied as input to this procedure.      2) Retrieve from the naming service all cross certificates         associated with Current-directory-name.  Filter to only         those that meet the following criteria:        (i)  The current time is within their validity interval.        (ii) The subject name is equal to Current-directory-name.        (iii)For some PK/UID/Name triple in the             "Search-when-descending" list compiled while ascending             the tree, the Public Key verifies the signature on the             certificate and  either the UID matches the issuer UID in             the certificate   or a UID is missing from the triple             and/or the certificate and the Name in the triple matches             the issuer name in the certificate.        (iv) There are no certificates found meeting criteria (ii) and             (iii) matching a PK/UID/Name triple in the             Search-when-descending list whose subject is a directory             lower in the naming hierarchy.      3) If any qualifying certificates are found, construct         PK/UID/Name triples for each of them; these should replace         rather than supplement any triples already in the         Trusted-key-set for that directory.      4) If after steps (b) and (c), there are no PK/UID/Name triples         corresponding to Current-directory-name in Trusted-Key-Set,         shorten Current-directory-name by one RDN (pushing it onto         the Saved-RDNs stack) and repeat this process until         Current-directory-name is equal to         Least-common-ancestor-name  or there is at least one triple         in Trusted-key-set corresponding to Current-directory-name.    d) Descending the tree       While the list Saved-RDNs is not Empty the algorithm movesKaufman                                                        [Page 84]

RFC 1507                          DASS                    September 1993       down the tree. At each step it does the following operations.      1) Remove the first RDN from Saved-RDNs and append it to the         Current-directory-name.      2) Find all the child directory certificates for the directory         whose name is in the current-directory-name.      3) Filter these certificates to find only those which meet the         following criteria:        (i)  The current time is within the validity interval.        (ii) For some PK/UID/Name triple in the Current-key-set for             the parent directory, the Public Key verifies the             signature on the certificate and either the UID matches             the issuer UID of the certificate   or the UID is missing             from the triple and/or the certificate and the Name in             the triple matches the issuer name in the certificate.        (iii)The issuer name in the certificate is a prefix of the             subject name and the difference between the two names is             the final RDN of Current-directory-name.      4) Take the key, UID, and name from each remaining certificate         and form a new triple corresponding to         Current-directory-name in Trusted-Key-Set. If this set is         empty then the algorithm exits with the         'Incomplete-chain-of-trustworthy-CAs' error condition.      5) repeat from step (a), appending a new simple name to         Current-directory-name.    e) Find public keys:       If there are no triples in the Trusted-Key-Set for the named       principal, then the algorithm exits with the `Target-has-no-keys-w       error condition. Otherwise, the Public Key and UID are       extracted from each pair, duplicates are eliminated, and this       set is returned as the Pub_keys.3.10.10.2 Allowed Variations - Caching   Some use of caches can be implemented without affecting the semantics   of the Get_Pub_Keys routine.  For example, a crypto-cache could   remember the public key that verified a signature in the past and   could avoid the verification operation if the same key was used to   verify the same data structure again.  In some cases, however, it is   impossible (or at least inconvenient) for a cache implementation toKaufman                                                        [Page 85]

RFC 1507                          DASS                    September 1993   be completely transparent.   In particular, for good performance it is important that certificates   not be re-retrieved from the naming service on every authentication.   This must be balanced against the need to have changes to the   contents of the naming service be reflected in DASS calls on a timely   basis.  There are two cases of interest: changes which cause an   authentication which previously would have succeeded to fail and   changes which cause an authentication which previously would have   failed to succeed.  These two cases are subject to different time   constraints.   In general, changes that cause authentication to succeed must be   reflected quite quickly - on the order of minutes.  If a user   attempts an operation, it fails, the user tracks down a system   manager and causes the appropriate updates to take place, and the   user retries the operation, it is unacceptable for the operation to   continue to fail for an extended period because of stale caches.   Changes that cause authentication to fail must be reflected reliably   within a bounded period of time for security reasons.  If a user   leaves the company, it must be possible to revoke his ability to   authenticate within a relatively short period of time - say hours.   These constraints mean that a naming service cache which contains   arbitrarily old information is unacceptable.  To meet the second   constraint, naming service cache entries must be timed out within a   reasonable period of time unless in implementation verifies that the   certificate is still present (a crypto-cache which lasted longer   would be legal; rather than deleting a name service cache entry, in   implementation might instead verify that the entry was still present   in the naming service.  This would avoid repeating the cryptographic   "verify").   In order to assure that information cached for even a few hours not   deny authentication for that extended period, it must be possible to   bypass caches when the result would otherwise be a failure.  Since   the performance of authentication failures is not a serious concern,   it is acceptable to expect that before an operation fails a retry   will be made to the naming service to see if there are any new   relevant certificates (or in certain obscure conditions, to see if   any relevant certificates have been deleted).   If on a call to Get_Pub_Keys, the Try_Hard bit is True, then this   procedure must return results based on the contents of the naming   service no more than five minutes previous (this would normally be   accomplished by ignoring name service caches and making all   operations directly to the naming service).  If the Try_Hard bit isKaufman                                                        [Page 86]

RFC 1507                          DASS                    September 1993   False, this procedure may return results based on the contents of the   naming service any time in the previous few hours, in the sense that   it may ignore any certificate added in the previous few hours and may   use any certificate deleted in the previous few hours.  Procedures   which call this routine with Try_Hard set to false must be prepared   to call it again with Try_Hard True if their operation fails possibly   from this result.   The exact timer values for "five minutes" and "a few hours" are   expected to be implementation constants.   In the envisioned implementation, the entire "ascending treewalk" is   retrieved, verified, and its digested contents cached when a   principal first establishes credentials.  A mechanism should be   provided to refresh this information periodically for principals   whose sessions might be long lived, but it would probably be   acceptable in the unlikely event of a user's ancestor's keys changing   to require that the user log out and log back in.  This is consistent   with the observed behavior of existing security mechanisms.   The descending treewalk, on the other hand, is expected to be   maintained as a more conventional cache, where entries are kept in a   fixed amount of memory with a "least recently used" replacement   policy and a watchdog timer that assures that stale information is   not kept indefinitely.  A call to Get_Pub_Keys with Try_Hard set   false would first check that cache for relevant certificates and only   if none were found there would it go out to the naming service.  If   there were newer certificates in the naming service, they might not   be found and an authentication might therefore fail.   When Try_Hard is false, an implementation may assume that   certificates not in the cache do not exist so long as that assumption   does not cause an authentication to falsely succeed.  In that case,   it may only make that assumption if the certificates have been   verified to not exist within the revocation time (a few hours).3.11 DASSlessness Determination Functions   In order to provide better interoperability with alternative   authentication mechanisms and to provide backward compatibility with   older (insecure) authentication mechanisms, it is sometimes important   to be able to determine in a secure way what the appropriate   authentication mechanism is for a particular named principal.  For   some applications, this will be done by a local mechanism, where   either the person creating access control information must know and   specify the mechanism for each principal or a system administrator on   the node must maintain a database mapping names to mechanisms.  Three   applications come to mind where scaleability makes such mechanismsKaufman                                                        [Page 87]

RFC 1507                          DASS                    September 1993   implausible:    a) To transparently secure proxy-based applications (like rlogin)       in an environment where some hosts have been upgraded to       support DASS and some have not, a node must be willing to       accept connections authenticated only by their network       addresses but only if they can be assured that such nodes do       not have DASS installed.  Access to a resource becomes secure       without administrative action when all nodes authorized to       access it have been upgraded.       In this scenario, the server node must be able to determine       whether the client node is DASSless in a secure fashion.    b) Similarly, in a mixed environment where some servers are       running DASS and some are not, it may be desirable for clients       to authenticate servers if they can but it would be       unacceptable for a client to stop being able to access a       DASSless server once DASS is installed on the client.  In such       a situation where server authentication is desirable but not       essential, the client would like to determine in a secure       fashion whether the server can accept DASS authentication.    c) In a DASS/Kerberos interoperability scenario, a server may       decide that Kerberos authentication is "good enough" for       principals that do not have DASS credentials without       introducing trust in on-line authorities when DASS credentials       are available.  In parallel with case 1, we want it to be true       that when the last principal with authority to access an       object is upgraded to DASS, we automatically cease to trust       PasswdEtc servers without administrative action on the part of       the object owner.  For this purpose, the authenticator must       learn in a secure fashion that the principal is incapable of       DASS authentication.   Reliably determining DASSlessness is optional for implementations of   DASS and for applications.  No other capabilities of DASS rely on   this one.   The interface to the DASSlessness inquiry function is specified as a   call independent of all others.  This capability must be exposed to   the calling application so that a server that receives a request and   no token can ask whether the named principal should be believed   without a token.  It might improve performance and usability if in   real interfaces DASSlessness were returned in addition to a bad   status on the function that creates a token if the token is targeted   toward a server incapable or processing it.  An application could   then decide whether to make the request without a token (and give upKaufman                                                        [Page 88]

RFC 1507                          DASS                    September 1993   server authentication) or to abort the request.3.11.1 Query DASSlessness   Query_DASSlessness(                                                      --inputs                       verifying_credentials Credentials,                       principal_name        Name,                                                      --outputs                       alternate_authentication Set of OIDs)   This function uses the verifying credentials to search for an   alternative authentication mechanism certificate for the named   principal or for any CA on the path between the verifying credentials   and the named principal.  Such a certificate is identical to an DASS   X.509 certificate except that it lists a different algorithm   identifier for the public key of the subject than that expected by   DASS.   This function is implemented identically to Get_Pub_Keys except:    a) If in any set of certificates found, no valid DASS certificate       is found and one or more certificates are found that would       otherwise be valid except for an invalid subject public key       OID, the OID from that certificate or certificates is returned       and the algorithm terminates.    b) On initial execution, Try_Hard=False.  If the first execution       fails to retrieve any valid PK/UID pairs but also fails to       find any invalid OID certificates, repeat the execution with       Try_Hard=True.    c) If the either execution finds PK/UID pairs or if neither finds       and invalid OID certificates, fail by returning a null set.4. Certificate and message formats4.1 ASN.1 encoding   Some definitions are taken from X.501 and X.509.   Dass DEFINITIONS ::=   BEGIN   --CCITT Definitions:   joint-iso-ccitt      OBJECT IDENTIFIER ::= {2}Kaufman                                                        [Page 89]

RFC 1507                          DASS                    September 1993   ds                   OBJECT IDENTIFIER ::= {joint-iso-ccitt 5}   algorithm            OBJECT IDENTIFIER ::= {ds 8}   encryptionAlgorithm  OBJECT IDENTIFIER ::= {algorithm 1}   hashAlgorithm        OBJECT IDENTIFIER ::= {algorithm 2}   signatureAlgorithm   OBJECT IDENTIFIER ::= {algorithm 3}   rsa                  OBJECT IDENTIFIER ::= {encryptionAlgorithm 1}   iso                  OBJECT IDENTIFIER ::= {1}   identified-organization OBJECT IDENTIFIER ::= {iso 3}   ecma               OBJECT IDENTIFIER ::= {identified-organization 12}   member-company       OBJECT IDENTIFIER ::= {ecma 2}   digital              OBJECT IDENTIFIER ::= {member-company 1011}   --1989 OSI Implementors Workshop "Stable" Agreements   oiw                OBJECT IDENTIFIER ::= {identified-organization 14}   dssig                  OBJECT IDENTIFIER ::= {oiw 7}   oiwAlgorithm           OBJECT IDENTIFIER ::= {dssig 2}   oiwEncryptionAlgorithm OBJECT IDENTIFIER ::= {oiwAlgorithm 1}   oiwHashAlgorithm       OBJECT IDENTIFIER ::= {oiwAlgorithm 2}   oiwSignatureAlgorithm  OBJECT IDENTIFIER ::= {oiwAlgorithm 3}   oiwMD2                 OBJECT IDENTIFIER ::= {oiwHashAlgorithm 1}                                                  --null parameter   oiwMD2withRSA          OBJECT IDENTIFIER ::= {oiwSignatureAlgorithm 1}                                                  --null parameter   --X.501 definitions   AttributeType ::= OBJECT IDENTIFIER   AttributeValue ::= ANY   AttributeValueAssertion ::= SEQUENCE {AttributeType,AttributeValue}   Name ::= CHOICE {       --only one for now                   RDNSequence                       }   RDNSequence ::= SEQUENCE OF RelativeDistinguishedName   DistinguishedName ::= RDNSequence   RelativeDistinguishedName ::= SET OF AttributeValueAssertion   --X.509 definitions (with proposed 1992 extensions presumed)   ENCRYPTED MACRO ::=   BEGIN   TYPE NOTATION   ::= type(ToBeEnciphered)   VALUE NOTATION  ::= value(VALUE BIT STRING)   END     -- of ENCRYPTEDKaufman                                                        [Page 90]

RFC 1507                          DASS                    September 1993   SIGNED MACRO    ::=   BEGIN   TYPE NOTATION   ::= type (ToBeSigned)   VALUE NOTATION  ::= value (VALUE   SEQUENCE{           ToBeSigned,           AlgorithmIdentifier,    --of the algorithm used to                                   --generate the signature           ENCRYPTED OCTET STRING  --where the octet string is the                                   --result of the hashing of the                                   --value of "ToBeSigned"           }                           )   END     -- of SIGNED   SIGNATURE MACRO ::=   BEGIN   TYPE NOTATION   ::= type (OfSignature)   VALUE NOTATION  ::= value (VALUE   SEQUENCE {           AlgorithmIdentifier,    --of the algorithm used to compute           ENCRYPTED OCTET STRING  -- the signature where the octet                                   -- string is a function (e.g., a                                   -- compressed or hashed version)                                   -- of the value 'OfSignature',                                   -- which may include the                                   -- identifier of the algorithm                                   -- used to compute the signature           }                           )   END     -- of SIGNATURE   Certificate ::= SIGNED SEQUENCE {           version [0]             Version DEFAULT v1988,           serialNumber    CertificateSerialNumber,           signature               AlgorithmIdentifier,           issuer          Name,           valid           Validity,           subject         Name,           subjectPublicKey        SubjectPublicKeyInfo,           issuerUID [1]   IMPLICIT UID OPTIONAL,  -- v1992           subjectUID [2]  IMPLICIT UID OPTIONAL   -- v1992           }           --The Algorithm Identifier for both the signature fieldKaufman                                                        [Page 91]

RFC 1507                          DASS                    September 1993           --and in the signature itself is:           --      oiwMD2withRSA (1.3.14.7.2.3.1)   Version ::= INTEGER {v1988(0), v1992(1)}   CertificateSerialNumber ::= INTEGER   Validity ::= SEQUENCE {           NotBefore       UTCTime,           NotAfter        UTCTime           }   AlgorithmIdentifier ::= SEQUENCE {           algorithm       OBJECT IDENTIFIER,           parameter       ANY DEFINED BY algorithm OPTIONAL           }   --The algorithms we support in one context or another are:           --oiwMD2withRSA (1.3.14.7.2.3.1) with parameter NULL           --rsa (2.5.8.1.1) with parameter keysize INTEGER which is           --           the keysize in bits           --decDEA (1.3.12.1001.7.1.2) with optional parameter           --           missing           --decDEAMAC (1.3.12.2.1011.7.3.3) with optional parameter           --           missing   SubjectPublicKeyInfo  ::=  SEQUENCE {           algorithm       AlgorithmIdentifier,     -- rsa (2.5.8.1.1)           subjectPublicKey        BIT STRING                   -- the "bits" further decode into a DASS public key           }   UID ::= BIT STRING   -- the following definitions are for Digital specified Algorithms   cryptoAlgorithm OBJECT IDENTIFIER ::= {digital 7}   decEncryptionAlgorithm  OBJECT IDENTIFIER ::= {cryptoAlgorithm 1}   decHashAlgorithm        OBJECT IDENTIFIER ::= {cryptoAlgorithm 2}   decSignatureAlgorithm   OBJECT IDENTIFIER ::= {cryptoAlgorithm 3}   decDASSLessness         OBJECT IDENTIFIER ::= {cryptoAlgorithm 6}   decMD2withRSA   OBJECT IDENTIFIER ::= {decSignatureAlgorithm 1}   decMD4withRSA   OBJECT IDENTIFIER ::= {decSignatureAlgorithm 2}   decDEAMAC       OBJECT IDENTIFIER ::= {decSignatureAlgorithm 3}Kaufman                                                        [Page 92]

RFC 1507                          DASS                    September 1993   decDEA          OBJECT IDENTIFIER ::= {decEncryptionAlgorithm 2}   decMD2          OBJECT IDENTIFIER ::= {decHashAlgorithm 1}   decMD4          OBJECT IDENTIFIER ::= {decHashAlgorithm 2}   ShortPosixTime ::= INTEGER   -- number of seconds since base time   LongPosixTime ::= SEQUENCE {           INTEGER,             -- number of seconds since base time           INTEGER              -- number of nanoseconds since second           }   ShortPosixValidity ::=  SEQUENCE {           notBefore       ShortPosixTime,           notAfter        ShortPosixTime }   -- Note: Annex C of X.509 prescribes the following format for the   -- representation of a public key, but does not give the structure   -- a name.   DASSPublicKey ::=  SEQUENCE {           modulus         INTEGER,           exponent        INTEGER           }   DASSPrivateKey ::= SEQUENCE {           p       INTEGER ,                      -- prime p           q [0]   IMPLICIT INTEGER OPTIONAL ,    -- prime q           mod[1]  IMPLICIT INTEGER OPTIONAL,     -- modulus           exp [2] IMPLICIT INTEGER OPTIONAL,     -- public exponent           dp [3]  IMPLICIT INTEGER OPTIONAL ,    -- exponent mod p           dq [4]  IMPLICIT INTEGER OPTIONAL ,    -- exponent mod q           cr [5]  IMPLICIT INTEGER OPTIONAL ,    -- Chinese                                              --remainder coefficient           uid[6]  IMPLICIT UID OPTIONAL,           more[7] IMPLICIT BIT STRING OPTIONAL   --Reserved for                                                  --future use           }   LocalUserName   ::= OCTET STRING   ChannelId               ::= OCTET STRING   VersionNumber           ::= OCTET STRING (SIZE(3))                           -- first octet is major version                           -- second octet is minor version                           -- third octet is ECO rev.   versionZero  VersionNumber ::= '000000'HKaufman                                                        [Page 93]

RFC 1507                          DASS                    September 1993   Authenticator ::= SIGNED SEQUENCE {           type            BIT STRING,                    -- first bit `delegation required'                    -- second bit `Mutual Authentication Requested'           whenSigned      LongPosixTime ,           channelId  [3]  IMPLICIT ChannelId OPTIONAL                   -- channel bindings are included when doing the                   -- signature, but excluded when transmitting the                   -- Authenticator           }                   -- uses decDEAMAC (1.3.12.2.1011.7.3.3)   EncryptedKey ::= SEQUENCE {           algorithm               AlgorithmIdentifier,                           -- uses rsa (2.5.8.1.1)           encryptedAuthKey        BIT STRING                           -- as defined insection 4.4.5           }   SignatureOnEncryptedKey ::=  SIGNATURE EncryptedKey                -- uses oiwMD2withRSA (1.3.14.7.2.3.1)                -- Signature bits computed over EncryptedKey structure   LoginTicket ::= SIGNED SEQUENCE {           version [0]         IMPLICIT VersionNumber DEFAULT versionZero,           validity            ShortPosixValidity ,           subjectUID          UID ,           delegatingPublicKey SubjectPublicKeyInfo           }           -- uses oiwMD2withRSA (1.3.14.7.2.3.1)   Delegator ::= SEQUENCE {           algorithm               AlgorithmIdentifier                           -- decDEA encryption (1.3.12.1001.7.1.2)           encryptedPrivKey        ENCRYPTED  DASSPrivateKey,                           -- (only p is included)           }   UserClaimant ::=  SEQUENCE {           userTicket [0]  IMPLICIT LoginTicket,           evidence  CHOICE {                   delegator [1]   IMPLICIT Delegator ,                                -- encrypted delegation private key                                -- under DES authenticating key                                -- present if delegating                   sharedKeyTicketSignature [2]Kaufman                                                        [Page 94]

RFC 1507                          DASS                    September 1993                           IMPLICIT SignatureOnEncryptedKey                                -- present if not delegating                   } ,           userName [3]    IMPLICIT Name OPTIONAL                                -- name of user principal           }   EncryptedKeyandUserName ::= SEQUENCE {           encryptedKey    EncryptedKey ,           username                LocalUserName           }   SignatureOnEncryptedKeyandUserName ::=           SIGNATURE EncryptedKeyandUserName                   -- uses oiwMD2withRSA (1.3.14.7.2.3.1)                   -- Signature bits computed over                   -- EncryptedKeyandUserName structure                   -- using node private key           }   NodeClaimant ::= SEQUENCE {           nodeTicket Signature[0] IMPLICIT                   SignatureOnEncryptedKeyandUserName,           nodeName  [1]   IMPLICIT Name OPTIONAL,           username  [2]   IMPLICIT LocalUserName OPTIONAL           }   AuthenticationToken ::= SEQUENCE {           version [0]    IMPLICIT VersionNumber DEFAULT versionZero,           authenticator [1]       IMPLICIT Authenticator ,           encryptedKey  [2]       IMPLICIT EncryptedKey OPTIONAL ,                    -- required if initiating token           userclaimant  [3]       IMPLICIT UserClaimant OPTIONAL ,                    -- missing if only doing node authentication                    -- required if not doing node authentication           nodeclaimant [4]        IMPLICIT NodeClaimant OPTIONAL                    -- missing if only doing principal authentication                    -- required if not doing principal authentication           }   MutualAuthenticationToken ::= CHOICE {           v1Response [0] IMPLICIT  OCTET STRING (SIZE(6))                 -- Constructed as follows:  A single DES block                 -- of eight octets is constructed from the two                 -- integers in the timestamp.  First four bytes                 -- are the high order integer encoded MSB                 -- first; Last four bytes are the low orderKaufman                                                        [Page 95]

RFC 1507                          DASS                    September 1993                 -- integer encoded MSB first.  The block is                 -- encrypted using the shared DES key, and                 -- the first six bytes are the OCTET STRING.                 -- With the [0] type and 6-byte length, the                 -- MutualAuthenticationToken has a fixed                 -- length of eight bytes.           }   END4.2 Encoding Rules   Whenever a structure is to be signed it must always be constructed   the same way. This is particularly important where a signed structure   has to be reconstructed by the recipient before the signature is   verified. The rules listed below are taken from X.509.    - the definite form of length encoding shall be used, encoded in      the minimum number of octets;    - for string types, the constructed form of encoding shall not      be used;    - if the value of a type is its default value, it shall be      absent;    - the components of a Set type shall be encoded in ascending      order of their tag value;    - the components of a Set-of type shall be encoded in ascending      order of their octet value;    - if the value of a Boolean type is true, the encoding shall      have its contents octet set to `FF'16;    - each unused bits in the final octet of the encoding of a      BitString value, if there are any, shall be set to zero;    - the encoding of a Real type shall be such that bases 8, 10 and      16 shall not  be used, and the binary scaling factor shall be      zero.4.3 Version numbers and forward compatibility   The LoginTicket and AuthenticationToken structures contain a   three octet version identifier which is intended to ease   transition to future revisions of this architecture.  The default   value, and the value which should always be supplied by   implementations of this version of the architecture is 0.0.0Kaufman                                                        [Page 96]

RFC 1507                          DASS                    September 1993   (three zero octets).  The first octet is the major version.  An   implementation of this version of the architecture should refuse   to process data structures where it is other than zero, because   changing it indicates that the interpretation of some subsidiary   data structure has changed.  The second octet is the minor   version.  An implementation of this version of the architecture   should ignore the value of this octet.  Some future version of   the architecture may set a value other than zero and may specify   some different processing of the remainder of the structure based   on that different value.  Such a change would be backward compatible   and interoperable.  The third octet is the ECO revision.  No   implementation should make any processing decisions based on the   value of that octet.  It may be logged, however, to help in   debugging interoperability problems.   In the CDC protocol, there is also a three octet version   numbering scheme, where versions 1.0.0 and 2.0.0 have been   defined.  Implementations should follow the same rules above and   reject major version numbers greater than 2.   ASN.1 is inherently extensible because it allows new fields to be   added "onto the end" of existing data structures in an   unambiguous way.  Implementations of DASS are encouraged to   ignore any such additional fields in order to enhance backwards   compatibility with future versions of the architecture.   Unfortunately, commonly available ASN.1 compilers lack this   capability, so this behavior cannot reasonably be required and   may limit options for future extensions.4.4 Cryptographic Encoding   Some of the substructures listed in the previous sections are   specified as ENCRYPTED OCTET STRINGs containing encrypted   information.  DASS uses the DES, RSA, and MD2 cryptosystems  Each   of those cryptosystems specifies a function from octet string   into another in the presence of a key (except MD2, which is   keyless).  This section describes how to form the octet strings   on which the DES and RSA operations are performed.4.4.1 Algorithm Independence vs. Key Parity   All of the defined encodings for DASS for secret key encryption   are based on DES.  It is intended, however, that other   cryptosystems could be substituted without any other changes for   formats or algorithms.  The required "form factor" for such a   cryptosystem is that it have a 64 bit key and operate on 64 bit   blocks (this appears to be a common form factor for a   cryptosystem).  For this reason, DES keys are in all placesKaufman                                                        [Page 97]

RFC 1507                          DASS                    September 1993   treated as though they were 64 bits long rather than 56.  Only in   the operation of the algorithm itself are eight bits of the key   dropped and key parity bits substituted. Choosing a key always   involves picking a 64 bit random number.4.4.2 Password Hashing   Encrypted credentials are encrypted using DES as described in the   next section.  The key for that encryption is derived from the   user's password and name by the following algorithm:    a) Put the rightmost RDN of the user's name in canonical form       according to BER and the X.509 encoding rules.  For any string       types that are case insensitive, map to upper case, and where       matching is independent of number of spaces collapse all       multiple spaces to a single space and delete leading and       trailing spaces.       Note:  the RDN is used to add "salt" to the hash calculation       so that someone can't precompute the hash of all the words in       a dictionary and then apply them against all names.  Deriving       the salt from the last RDN of the name is a compromise.  If it       were derived from the whole name, all encrypted keys would be       obsoleted when a branch of the namespace was renamed.  If it       were independent of name, interaction with a login agent would       take two extra messages to retrieve the salt.  With this       scheme, encrypted keys are obsoleted by a change in the last       RDN and if a final RDN is common to a large number of users,       dictionary attacks against them are easier; but the common       case works as desired.    b) Compute TEMP as the MD2 message digest of the concatenation of       the password and the RDN computed above.    c) Repeat the following 40 times:  Use the first 64 bits of TEMP       as a DES key to encrypt the second 64 bits;  XOR the result       with the first 64 bits of TEMP; and compute a new TEMP as MD2       of the 128 bit result.    d) Use the final 64 bits of the result (called hash1) as the key       to decrypt the encrypted credentials.  Use the first 64 bits       (called hash2) as the proof of knowledge of the password for       presentation to a login agent (if any).Kaufman                                                        [Page 98]

RFC 1507                          DASS                    September 19934.4.3 Digital DEA encryption   DES encryption is used in the following places:    - In the encryption of the encrypted credentials structure    - To encrypt the delegator in authentication tokens    - To encrypt the time in the mutual authenticator   In the first two cases, a varying length block of information   coded in ASN.1 is encrypted.  This is done by dividing the block   of information into 8 octet blocks, padding the last block with   zero bytes if necessary, and encrypting the result using the CBC   mode of DES.  A zero IV is used.   In the third case, a fixed length (8 byte) quantity (a timestamp)   is encrypted.  The timestamp is mapped to a byte string using   "big endian" order and the block is encrypted using the ECB mode   of DES.4.4.4  Digital MAC Signing   DES signing is used in the Authenticator.  Here, the signature is   computed over an ASN.1 structure.  The signature is the CBC residue   of the structure padded to a multiple of eight bytes with zeros.  The   CBC is computed with an IV of zero.4.4.5 RSA Encryption   RSA encryption is used in the Encrypted Shared Key.  RSA encryption   is best thought of as operating on blocks which are integers rather   than octet strings and the results are also integers.  Because an RSA   encryption permutes the integers between zero and (modulus-1), it is   generally thought of as acting on a block of size (keysizeinbits-1)   and producing a block of size (keysizeinbits) where keysizeinbits is   the smallest number of bits in which the modulus can be represented.   DASS only supports key sizes which are a multiple of eight bits (This   restriction is only required to support interoperation with certain   existing implementations.  If the key size is not a multiple of eight   bits, the high order byte may not be able to hold values as large as   the mandated '64'.  This is not a problem so long as the two high   order bytes together are non-zero, but certain early implementations   check for the value '64' and will not interoperate with   implementations that use some other value.).   The encrypted shared key structure is laid out as follows:Kaufman                                                        [Page 99]

RFC 1507                          DASS                    September 1993    - The DES key to be shared is placed in the last eight bytes    - The POSIX format creation time encoded in four bytes using big      endian byte order is placed in the next four (from the end)      bytes    - The POSIX format expiration time encoded in four bytes using      big endian byte order is placed in the next four (from the      end) bytes    - Four zero bytes are placed in the next four (from the end)      bytes    - The first byte contains the constant '64' (decimal)    - All remaining bytes are filled with random bytes (the security      of the system does not depend on the cryptographic randomness      of these bytes, but they should not be a frequently repeating      or predictable value.  Repeating the DES key from the last      bytes would be good).   The RSA algorithm is applied to the integer formed by treating the   bytes above as an integer in big endian order and the resulting   integer is converted to a BIT STRING by laying out the integer in   'big endian' order.   On decryption, the process is reversed; the decryptor should verify   the four explicitly zero bytes but should not verify the contents of   the high order byte or the random bytes.4.4.6 oiwMD2withRSA Signatures   RSA-MD2 signatures are used on certificates, login tickets, shared   key tickets, and node tickets.  In all cases, a signature is computed   on an ASN.1 encoded string using an RSA private key.  This is done as   follows:    - The MD2 algorithm is applied to the ASN.1 encoded string to      produce a 128 bit message digest    - The message digest is placed in the low order bytes of the RSA      block (big endian)    - The next two lowest order bytes are the ASN.1 'T' and 'L' for      an OCTET STRING.    - The remainder of the RSA block is filled with zerosKaufman                                                       [Page 100]

RFC 1507                          DASS                    September 1993    - The RSA operation is performed, and the resulting integer is      converted to an octet string by laying out the bytes in big      endian order.   On verification, a value like the above  or one where the message   digest is present but the 'T' and 'L' are missing (zero) should be   accepted for backwards compatibility with an earlier definition of   this crypto algorithm.4.4.7 decMD2withRSA Signatures   This algorithm is the same as the oiwMD2withRSA algorithm as defined   above.  We allocated an algorithm object identifier from the Digital   space in case the definition of that OID should change.  It will not   be used unless the meaning of oiwMD2withRSA becomes unstable.Annex ATypical Usage   This annex describes one way a system could use DASS services (as   described insection 3) to provide security services.  While this   example provided motivation for some of the properties of DASS, it is   not intended to represent the only way that DASS may be used.  This   goes through the steps that would be needed to install DASS "from   scratch".A.1 Creating a CA   A CA is created by initializing its state. Each CA can sign   certificates that will be placed in some directory in the name   service. Before these certificates will be believed in a wider   context than the sub-tree of the name space which is headed by that   directory, the CA must be certified by a CA for the parent directory.   The procedure below accomplishes this. For most secure operation, the   CA should run on an off-line system and communicate with the rest of   the network by interchanging files using a simple specialized   mechanism such as an RS232 line or a floppy disk. It is assumed that   access to the CA is controlled and that the CA will accept   instructions from an operator.    - Call Install_CA to create the CA State.      This state is saved within the CA system and is never      disclosed.    - If this is the first CA in the namespace and the CA is      intended to certify only members of a single directory, we are      done.  Otherwise, the new CA must be linked into the CAKaufman                                                       [Page 101]

RFC 1507                          DASS                    September 1993      hierarchy by cross-certifying the parent and children of this      CA.  There is no requirement that CA hierarchies be created      from the root down, but to simplify exposition, only this case      will be described.  The newly created CA must learn its name,      its UID, the UID of its parent directory, and the public key      of the parent directory CA by some out of band reliable means.      Most likely, this would be done by looking up the information      in the naming service and asking the CA operator to verify it.      The CA then forms this information into a   parent certificate      and signs it using the Create_certificate function.  It      communicates the certificate to the network and posts it in      the naming service.    - This name, UID, and public key of the new CA are taken to the      CA of the parent directory, which verifies it (again by some      unspecified out-of-band mechanism) and calls      Create_Certificate to create a child  certificate using its own      Name and UID in the issuer fields. This certificate is also      placed in the naming service.   A CA can sign certificates for more than one directory. In this case   it is possible that a single CA will take the role of both CAs in the   example above. The above procedure can be simplified in this case, as   no interchange of information is required.A.2 Creating a User Principal   A system manager may create a new user principal by invoking the   Create_principal function supplying the principal's name, UID, and   the public key/UID of the parent CA.  The public key and UID must be   obtained in a reliable out of band manner.  This is probably by   having knowledge of that information "wired into" the utility which   creates new principals.  At account creation time, the system manager   must supply what will become the user's password.  This might be done   by having the user present and directly enter a password or by having   the password selected by some random generator.   The trusted authority certificate and corresponding user public key   generated by the Create_principal function are sent to the CA which   verifies its contents (again by an out-of-band mechanism) and signs a   corresponding certificate.  The encrypted credentials, CA signed   certificate, and trusted authority certificates are all placed in the   naming service.  The process by which the password is made known to   the user must be protected by some out-of-band mechanism.   In some cases the principal may wish to generate its own key, and not   use the Encrypted_Credentials. (e.g., if the Principal is represented   by a Smart Card). This may be done using a procedure similar to theKaufman                                                       [Page 102]

RFC 1507                          DASS                    September 1993   one for creating a new CA.A.3 Creating a Server Principal   A server also has a public/private key pair. Conceptually, the same   procedure used to create a user principal can be used to create a   server.  In practice, the most important difference  is likely to be   how the password is protected when installing it on a server compared   to giving it to a user.   A server may wish to retrieve (and store) its Encrypted Credentials   directly and never have them placed in the naming service. In this   case some other mechanism can be used (e.g., passing the floppy disk   containing the encrypted credentials to the server). This would   require a variant of the Initialize_Server routine which does not   fetch the Encrypted Credentials from the naming service.A.4 Booting a Server Principal   When the server first boots it needs its name (unreliably) and   password (reliably). It then calls Initialize_Server to obtain its   credentials and trusted authority certificates (which it will later   need in order to authenticate users).  These credentials never time   out, and are expected to be saved for a long time.  In particular the   associated Incoming Timestamp List must be preserved while there are   any timestamps on it. It is desirable to preserve the Cached Incoming   Contexts as long as there are any contexts likely to be reused.   If a server wants to initiate associations on its own behalf then it   must call Generate_Server_Ticket.  It must repeat this at intervals   if the expiration period expires.   A node that wishes to do node authentication (or which acts as a   server under its own name) must be created as a server.A.5 A user logs on to the network   The system that the user logs onto finds the user's name and   password. It then calls Network_Login to obtain credentials for the   user. These credentials are saved until the user wants to make a   network connection. The credentials have a time limit, so the user   will have to obtain new credentials in order to make connections   after the time limit. The credentials are then checked by calling   Verify_Principal_Name, in order to check that the key specified in   the encrypted credentials has been certified by the CA.   If the system does source node authentication it will call   Combine_credentials, once the local username has been found.  (ThisKaufman                                                       [Page 103]

RFC 1507                          DASS                    September 1993   can either be found by looking the principal's global name up in a   file, or the user can be asked to give the local name directly.   Alternatively the user can be asked to give his local username, which   the system looks up to find the global name).A.6 An Rlogin (TCP/IP) connection is made   When the user calls a modified version of the rlogin utility, it   calls Create_token in order to create the Initial Authentication   Token, which is passed to the other system as part of the rlogin   protocol.  The rlogind utility at the destination node calls   Accept_token to verify it.  It then looks up in a local rhosts-like   database to determine whether this global user is allowed access to   the requested destination account.  It calls Verify_principal_name   and/or Verify_node_name to confirm the identity of the requester.  If   access is allowed, the connection is accepted and the Mutual   Authentication Token is returned in the response message.   The source receives the returned Mutual Authentication Token and uses   it to confirm it communicating with the correct destination node.   Rlogind then calls Combine_credentials to combine its node/account   information with the global user identification in the received   credentials in case the user accesses any network resources from the   destination system.A.7 A Transport-Independent Connection   As an alternative to the description in A.6, an application wishing   to be portable between different underlying transports may call   create_token to create an authentication token which it then sends to   its peer.  The peer can then call accept_token and   verify_principal_name and learn the identity of the requester.Annex BSupport of the GSSAPI   In order to support applications which need to be portable across a   variety of underlying security mechanisms, a "Generic Security   Service API" (or GSSAPI) was designed which gives access to a common   core of security services expected to be provided by several   mechanisms.  The GSSAPI was designed with DASS, Kerberos V4, and   Kerberos V5 in mind, and could be written as a front end to any or   all of those systems.  It is hoped that it could serve as an   interface to other security systems as well.   Application portability requires that the security services supportedKaufman                                                       [Page 104]

RFC 1507                          DASS                    September 1993   be comparable.  Applications using the GSSAPI will not be able to   access all of the features of the underlying security mechanisms.   For example, the GSSAPI does not allow access to the "node   authentication" features of DASS.  To the extent the underlying   security mechanisms do not support all the features of GSSAPI,   applications using those features will not be portable to those   security mechanisms.  For example, Kerberos V4 does not support   delegation, so applications using that feature of the GSSAPI will not   be portable to Kerberos V4.   This annex explains how the GSSAPI can be implemented using the   primitive services provided by DASS.B.1 Summary of GSSAPI   The latest draft of the GSSAPI specification is available as an   internet draft.  The following is a brief summary of that evolving   document and should not be taken as definitive.  Included here are   only those aspects of GSSAPI whose implementation would be DASS   specific.   The GSSAPI provides four classes of functions: Credential Management,   Context-Level Calls, Per-message calls, and Support Calls; two types   of objects: Credentials and Contexts; and two kinds of data   structures to be transmitted as opaque byte strings: Tokens and   Messages. Credentials hold keys and support information used in   creating tokens.  Contexts hold keys and support information used in   signing and encrypting messages.   The Credential Management functions of GSSAPI are "incomplete" in the   sense that one could not build a useful security implementation using   only GSSAPI.  Functions which create credentials based on passwords   or smart cards are needed but not provided by GSSAPI.  It is   envisioned that such functions would be invoked by security mechanism   specific functions at user login or via some separate utility rather   than from within applications intended to be portable.  The   Credential Management functions available to portable applications   are:    - GSS_Acquire_cred:  get a handle to an existing credential      structure based on a name or process default.    - GSS_Release_cred:  release credentials after use.   The Context-Level Calls use credentials to establish contexts.   Contexts are like connections: they are created in pairs and are   generally used at the two ends of a connection to process messages   associated with that connection.  The Context-Level Calls of interestKaufman                                                       [Page 105]

RFC 1507                          DASS                    September 1993   are:    - GSS_Init_sec_context:  given credentials and the name of a      destination, create a new context and a token which will      permit the destination to create a corresponding context.    - GSS_Accept_sec_context:  given credentials and an incoming      token, create a context corresponding to the one at the      initiating end and provide information identifying the      initiator.    - GSS_Delete_sec_context:  delete a context after use.   The Per-Message Calls use contexts to sign, verify, encrypt, and   decrypt messages between the holders of matching contexts.  The Per-   Message Calls are:    - GSS_Sign:  Given a context and a message, produces a string of      bytes which constitute a signature on a provided message.    - GSS_Verify:  Given a context, a message, and the bytes      returned by GSS_Sign, verifies the message to be authentic      (unaltered since it was signed by the corresponding context).    - GSS_Seal:  Given a context and a message, produces a string of      bytes which include the message and a signature; the message      may optionally be encrypted.    - GSS_Unseal:  Given a context and the string of bytes from      GSS_Seal, returns the original message and a status indicating      its authenticity.   The Support Calls provide utilities like translating names and status   codes into printable strings.B.2 Implementation of GSSAPI over DASSB.2.1 Data Structures   The objects and data structures of the GSSAPI do not map neatly into   the objects and data structures of the DASS architecture.   This section describes how those data structures can be implemented   using the DASS data structures and primitives   Credential handles correspond to the credentials structures in DASS,   where the portable API assumes that the credential structures   themselves are kept from applications and handles are passed to andKaufman                                                       [Page 106]

RFC 1507                          DASS                    September 1993   from the various subroutines.   Context initialization tokens correspond to the tokens of DASS.  The   GSSAPI prescribes a particular ASN.1 encoded form for tokens which   includes a mechanism specific bit string within it.  An   implementation of GSSAPI should enclose the DASS token within the   GSSAPI "wrapper".   Context handles have no corresponding structure in DASS. The   Create_token and Accept_token calls of DASS return a shared key and   instance identifier. An implementation of the GSSAPI must take those   values along with some other status information and package it as a   "context" opaque structure.  These data structures must be allocated   and freed with the appropriate calls.   Per-message tokens and sealed messages have no corresponding data   structure within DASS.  To fully support the GSSAPI functionality,   DASS must be extended to include this functionality.  These data   structures are created by cryptographic routines given the keys and   status information in context structures and the messages passed to   them.  While not properly part of the DASS architecture, the formats   of these data structures are included in section C.3.B.2.2 Procedures   This section explains how the functions of the GSSAPI can be provided   in terms of the Services Provided by DASS.  Not all of the DASS   features are accessible through the GSSAPI.B.2.2.1 GSS_Acquire_cred   The GSSAPI does not provide a mechanism for logging in users or   establishing server credentials. It assumes that some system specific   mechanism created those credentials and that applications need some   mechanism for getting at them. A model implementation might save all   credentials in a node-global pool indexed by some sort of credential   name. The credentials in the pool would be access controlled by some   local policy which is not concern of portable applications. Those   applications would simply call GSS_Acquire_cred and if they passed   the access control check, they would get a handle to the credentials   which could be used in subsequent calls.B.2.2.2 GSS_Release_cred   This call corresponds to the "delete_credentials" call of DASS.Kaufman                                                       [Page 107]

RFC 1507                          DASS                    September 1993B.2.2.3 GSS_Init_sec_context   In the course of a normal mutual authentication, this routine will be   called twice. The procedure can determine whether this is the first   or second call by seeing whether the "input_context_handle" is zero   (it will be on the first call).  On the first call, it will use the   DASS Create_token service to create a token and it will also allocate   and populate a "context" structure. That structure will hold the key,   instance identifier, and mutual authentication token returned by   Create_token and will in addition hold the flags which were passed   into the Init_sec_context call. The token returned by   Init_sec_context will be the DASS token included in the GSSAPI token   "wrapper".  The DASS token will include the optional principal name.   If mutual authentication is not requested in the GSSAPI call, the   mutual authentication token returned by DASS will be ignored and the   initial call will return a COMPLETE status. If mutual authentication   is requested, the mutual authentication token will be stored in the   context information and a CONTINUE_NEEDED status returned.   On the second call to GSS_Init_sec_context (with input_context_handle   non-zero), the returned token will be compared to the one in the   context information using the Compare_mutual_token procedure and a   COMPLETE status will be returned if they match.B.2.2.4 GSS_Accept_sec_context   This routine in GSSAPI accepts an incoming token and creates a   context.  It combines the effects of a series of DASS functions.  It   could be implemented as follows:    - Remove the GSSAPI "wrapper" from the incoming token and pass      the rest and the credentials to "Accept_token".  Accept_token      produces a mutual authentication token and a new credentials      structure.  If delegation was requested, the new credentials      structure will be an output of GSS_Accept_sec_context.  In any      case, it will be used in the subsequent steps of this      procedure.    - Use the DASS Get_principal_name function to extract the      principal name from the credentials produced by Accept_token.      This name is one of the outputs of "GSS_Accept_sec_context.    - Apply the DASS Verify_principal_name to the new credentials      and the retrieved name to authenticate the token as having      come from the named principal.    - Create and populate a context structure with the key andKaufman                                                       [Page 108]

RFC 1507                          DASS                    September 1993      timestamp returned by Accept_token and a status of COMPLETE.      Return a handle to that context.    - If delegation was requested, return the new credentials from      GSS_Accept_sec_context.  Otherwise, call Delete_credentials.    - If mutual authentication was requested, wrap the mutual      authentication token from Accept_token in a GSSAPI "wrapper"      and return it.  Otherwise return a null string.B.2.2.5 GSS_Delete_sec_context   This routine simply deletes the context state.  No calls to DASS are   required.B.2.2.6 GSS_Sign   This routine takes as input a context handle and a message. It   creates a per_msg_token by computing a digital signature on the   message using the key and timestamp in the context block.  No DASS   services are required. If additional cryptographic services were   requested (replay detection or sequencing), a timestamp or sequence   number must be prepended to the message and sent with the signature.   The syntax for this message is listed in section C.3.B.2.2.7 GSS_Verify   This routine repeats the calculation of the sign routine and verifies   the signature provided. If replay detection or sequencing services   are provided, the context must maintain as part of its state   information containing the sequence numbers or timestamps of messages   already received and this one must be checked for acceptability.B.2.2.8 GSS_Seal   This routine performs the same functions as Sign but also optionally   encrypts the message for privacy using the shared key and   encapsulates the whole thing in a GSSAPI specified ASN.1 wrapper.B.2.2.9 GSS_Unseal   This routine performs the same functions as GSS_Verify but also   parses the data structure including the signature and message and   decrypts the message if necessary.Kaufman                                                       [Page 109]

RFC 1507                          DASS                    September 1993B.3 Syntax   The GSSAPI specification recommends the following ASN.1 encoding for   the tokens and messages generated through the GSSAPI:        --optional top-level token definitions to frame        -- different mechanisms        GSSAPI DEFINITIONS ::=        BEGIN        MechType ::= OBJECT IDENTIFIER        -- data structure definitions        ContextToken ::=        -- option indication (delegation, etc.) indicated        -- within mechanism-specific token        [APPLICATION 0] IMPLICIT SEQUENCE {             thisMech MechType,             responseExpected BOOLEAN,             innerContextToken ANY DEFINED BY MechType               -- contents mechanism-specific             }        PerMsgToken ::=        -- as emitted by GSS_Sign and processed by        -- GSS_Verify        [APPLICATION 1] IMPLICIT SEQUENCE {             thisMech MechType,             innerMsgToken ANY DEFINED BY MechType               -- contents mechanism-specific             }        SealedMessage ::=        -- as emitted by GSS_Seal and processed by        -- GSS_Unseal        [APPLICATION 2] IMPLICIT SEQUENCE {             sealingToken PERMSGTOKEN,             confFlag BOOLEAN,             userData OCTET STRING               -- encrypted if confFlag TRUE             }   The object identifier for the DASS MechType is 1.3.12.2.1011.7.5.   The innerContextToken of a token is a DASS token or mutual   authentication token.   The innerMsgToken is a null string in the case where the message is   encrypted and the token is included as part of a SealedMessage.Kaufman                                                       [Page 110]

RFC 1507                          DASS                    September 1993   Otherwise, it is an eight octet sequence computed as the CBC residue   computed using a key and string of bytes defined as follows:    - Pad the message provided by the application with 1-8 bytes of      pad to produce a string whose length is a multiple of 8      octets.  Each pad byte has a value equal to the number of pad      bytes.    - Compute the key by taking the timestamp of the association      (two four byte integers laid out in big endian order with the      most significant integer first), complementing the high order      bit (to avoid aliasing with mutual authenticators), and      encrypting the block in ECB mode with the shared key of the      association.   The userData field of a SealedMessage is exactly the application   provided byte string if confFlag=FALSE.  Otherwise, it is the   application supplied message encrypted as follows:    - Pad the message provided by the application with 1-8 bytes of      pad to produce a string whose length = 4 (mod 8).  Each pad      byte has a value equal to the number of pad bytes.    - Append a four byte CRC32 computed over the message + pad.    - Compute a key by taking the timestamp of the association (two      four byte integers laid out in big endian order with the most      significant integer first), complementing the high order bit      (to avoid aliasing with mutual authenticators), and encrypting      the block in ECB mode with the shared key of the association.    - Encrypt the message + pad + CRC32 using CBC and the key      computed in the previous step.   A note of the logic behind the above:    - Because the shared key of an association may be reused by many      associations between the same pair of principals, it is      necessary to bind the association timestamp into the messages      somehow to prevent messages from a previous association being      replayed into a new sequence.  The technique above of      generating an association key accomplishes this and has a side      benefit.  An implementation may with to keep the long term      keys out of the hands of applications for purposes of      confinement but may wish to put the encryption associated with      an association in process context for reasons of performance.      Defining an association key makes that possible.Kaufman                                                       [Page 111]

RFC 1507                          DASS                    September 1993    - The reason that the association specific key is not specified      as the output of Create_token and Accept_token is that the DCE      RPC security implementation requires that a series of      associations between two principals always have the same key      and we did not want to have to support a different interface      in that application.    - The CRC32 after pad constitutes a cheap integrity check when      data is encrypted.    - The fact that padding is done differently for encrypted and      signed messages means that there are no threats related to      sending the same message encrypted and unencrypted and using      the last block of the encrypted message as a signature on the      unencrypted one.Annex CImported ASN.1 definitions   This annex contains extracts from the ASN.1 description of X.509 and   X.500 definitions referenced by the DASS ASN.1 definitions.   CCITT DEFINITIONS ::=   BEGIN joint-iso-ccitt          OBJECT IDENTIFIER ::= {2} ds   OBJECT IDENTIFIER ::= {joint-iso-ccitt 5} algorithm   OBJECT IDENTIFIER ::= {ds 8}   iso                      OBJECT IDENTIFIER ::= {1} identified-   organization  OBJECT IDENTIFIER ::= {iso 3} ecma            OBJECT   IDENTIFIER ::= {identified-organization 12} digital   OBJECT IDENTIFIER ::= { ecma 1011 }   -- X.501 definitions   AttributeType ::= OBJECT IDENTIFIER AttributeValue ::= ANY           -- useful ones are                   --      OCTET STRING ,                   --      PrintableString ,                   --      NumericString ,                   --      T61String ,                   --      VisibleString   AttributeValueAssertion ::= SEQUENCE {AttributeType,                                                 AttributeValue}   Name ::= CHOICE {-- only one possibility for now --                   RDNSequence}Kaufman                                                       [Page 112]

RFC 1507                          DASS                    September 1993   RDNSequence ::= SEQUENCE OF RelativeDistinguishedName   DistinguishedName ::= RDNSequence   RelativeDistinguishedName ::= SET OF AttributeValueAssertion   -- X.509 definitions   Certificate ::= SIGNED SEQUENCE {                   version [0]             Version DEFAULT 1988 ,                   serialNumber            SerialNumber ,                   signature               AlgorithmIdentifier ,                   issuer                  Name,                   valid                   Validity,                   subject                 Name,                   subjectPublicKey        SubjectPublicKeyInfo }   Version ::=      INTEGER { 1988(0)} SerialNumber ::= INTEGER Validity   ::=     SEQUENCE{           notBefore               UTCTime,           notAfter                UTCTime}   SubjectPublicKeyInfo  ::=  SEQUENCE {           algorithm               AlgorithmIdentifier ,           subjectPublicKey        BIT STRING           }   AlgorithmIdentifier ::= SEQUENCE {           algorithm       OBJECT IDENTIFIER ,                       parameters ANY DEFINED BY algorithm OPTIONAL}   ALGORITHM MACRO BEGIN TYPE NOTATION   ::= "PARAMETER" type VALUE   NOTATION  ::= value (VALUE OBJECT IDENTIFIER) END -- of ALGORITHM   ENCRYPTED MACRO BEGIN TYPE NOTATION   ::=type(ToBeEnciphered) VALUE   NOTATION  ::= value(VALUE BIT STRING)           -- the value of the bit string is generated by           -- taking the octets which form the complete           -- encoding (using the ASN.1 Basic Encoding Rules)           -- of the value of the ToBeEnciphered type and           -- applying an encipherment procedure to those octets-- END   SIGNED MACRO    ::= BEGIN TYPE NOTATION   ::= type (ToBeSigned) VALUE   NOTATION  ::= value(VALUE SEQUENCE{           ToBeSigned,           AlgorithIdentifier, -- of the algorithm used to generate                               -- the signature           ENCRYPTED OCTET STRING           -- where the octet string is the resultKaufman                                                       [Page 113]

RFC 1507                          DASS                    September 1993           -- of the hashing of the value of "ToBeSigned" END -- of   SIGNED   SIGNATURE MACRO ::= BEGIN TYPE NOTATION   ::= type(OfSignature) VALUE   NOTATION  ::= value(VALUE           SEQUENCE{                   AlgorithmIdentifier,                   -- of the algorithm used to compute the signature                   ENCRYPTED OCTET STRING                   -- where the octet string is a function (e.g., a                   -- compressed or hashed version) of the value                   -- "OfSignature", which may include the identifier                   -- of the algorithm used to compute                   -- the signature--}                           ) END -- of SIGNATURE   -- X.509 Annex H (not part of the standard)   encryptionAlgorithm OBJECT IDENTIFIER ::= {algorithm 1} rsa ALGORITHM           PARAMETER KeySize           ::= {encryptionAlgorithm 1}   KeySize ::= INTEGER   ENDGlossary   authentication        The process of determining the identity        (usually the name) of the other party in some communication        exchange.   authentication context        Cached information used during a particular instance of        authentication and including a shared symmetric (DES) key as        well as components of the authentication token conveyed        during establishment of this context.   authentication token        Information conveyed during a strong authentication exchange        that can be used to authenticate its sender. An        authentication token can, but is not necessarily limited to,        include the claimant identity and ticket, as well as signed        and encrypted secret key exchange messages conveying a        secret key to be used in future cryptographic operations. AnKaufman                                                       [Page 114]

RFC 1507                          DASS                    September 1993        authentication token names a particular protocol data        structure component.   authorization        The process of determining the rights        associated with a particular principal.   certificate        The public key of a particular principal, together        with some other information relating to the names of the        principal and the certifying authority, rendered unforgeable        by encipherment with the private key of the certification        authority that issued it.   certification authority        An authority trusted by one or more principals to create and        assign certificates.   claimant        The party that initiates the authentication process.        In the DASS architecture, claimants possess credentials        which include their identity, authenticating private key and        a ticket certifying their authenticating public key.   credentials        Information "state" required by principals in order        to for them to authenticate.   Credentials may contain        information used to initiate the authentication process        (claimant information), information used to respond to an        authentication request (verifier information), and cached        information useful in improving performance.   cryptographic checksum        Information which is derived by performing a cryptographic        transformation on the data unit. This information can be        used by the receiver to verify the authenticity of data        passed in cleartext   decipher        To reverse the effects of encipherment and render a        message comprehensible by use of a cryptographic key.   delegation        The granting of temporary credentials that allow a        process to act on behalf of a principal.Kaufman                                                       [Page 115]

RFC 1507                          DASS                    September 1993   delegation key        A short term public/private key pair used by a claimant        to act on behalf of a principal for a bounded period. The        delegation public key appears in the ticket, whereas the        delegation private key is used to sign secret key exchange        messages.   DES        Data Encryption Standard: a symmetric (secret key)        encryption algorithm used by DASS. An alternate encryption        algorithm could be substituted with little or no disruption        to the architecture.   DES key        A 56-bit secret quantity used as a parameter to the        DES encryption algorithm.   digital signature        A value computed from a block of data        and a key which could only be computed by someone knowing        the key. A digital signature computed with a secret key can        only be verified by someone knowing that secret key.  A        digital signature computed with a private key can be        verified by anyone knowing the corresponding public key.   encipher        To render incomprehensible except to the holder of a        particular key. If you encipher with a secret key, only the        holder of the same secret can decipher the message. If you        encipher with a public key, only the holder of the        corresponding private key can decipher it.   initial trust certificate        A certificate signed by a principal for its own use which        states the name and public key of a trusted authority.   global user name        A hierarchical name for a user which is        unique within the entire domain of discussion (typically the        network).   local user name        A simple (non-hierarchical) name by        which a user is known within a limited context such as on a        single computer.Kaufman                                                       [Page 116]

RFC 1507                          DASS                    September 1993   principal        Abstract entity which can be authenticated by name.        In DASS there are user principals and server principals.   private key        Cryptographic key used in asymmetric (public key)        cryptography to decrypt and/or sign messages. In asymmetric        cryptography, knowing the encryption key is independent of        knowing the decryption key. The decryption (or signing)        private key cannot be derived from the encrypting (or        verifying) public key.   proxy        A mapping from an external name to a local account        name for purposes of establishing a set of local access        rights. Note that this differs from the definition in ECMA        TR/46.   public key        Cryptographic key used in asymmetric cryptography to        encrypt messages and/or verify signatures.   RSA        The Rivest-Shamir-Adelman public key cryptosystem        based on modular exponentiation where the modulus is the        product of two large primes.  When the term RSA key is used,        it should be clear from context whether the public key, the        private key, or the public/private pair is intended.   secret key        Cryptographic key used in symmetric cryptography to        encrypt, sign, decrypt and verify messages. In symmetric        cryptography, knowledge of the decryption key implies        knowledge of the encryption key, and vice-versa.   sign        A process which takes a piece of data and a key and        produces a digital signature which can only be calculated by        someone with the key. The holder of a corresponding key can        verify the signature.   source        The initiator of an authentication exchange.   strong authentication        Authentication by means of cryptographically derived        authentication tokens and credentials. The actual working        definition is closer to that of "zero knowledge" proof:Kaufman                                                       [Page 117]

RFC 1507                          DASS                    September 1993        authentication so as to not reveal any information usable by        either the verifier, or by an eavesdropping third party, to        further their potential ability to impersonate the claimant.   target        The intended second party (other than the source) to        an authentication exchange.   ticket        A data structure certifying an authenticating        (public) key by virtue of being signed by a user principal        using their (long term) private key. The ticket also        includes the UID of the principal.   trusted authority        The public key, name and UID of a        certification authority trusted in some context to certify        the public keys of other principals.   UID        A 128 bit unique identifier produced according to OSF        standard specifications.   user key        A "long term" RSA key whose private portion        authenticates its holder as having the access rights of a        particular person.   verify        To cryptographically process a piece of data and a        digital signature to determine that the holder of a        particular key signed the data.   verifier        The party who will perform the operations necessary        to verify the claimed identity of a claimant.Kaufman                                                       [Page 118]

RFC 1507                          DASS                    September 1993Security Considerations   Security issues are discussed throughout this memo.Author's Address   Charles Kaufman   Digital Equipment Corporation   ZKO3-3/U14   110 Spit Brook Road   Nashua, NH 03062   Phone: (603) 881-1495   Email: kaufman@zk3.dec.com   General comments on this document should be sent to cat-ietf@mit.edu.   Minor corrections should be sent to the author.Kaufman                                                       [Page 119]