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Network Working Group                                            S. KentRequest for Comments: 1422                                           BBNObsoletes:1114                                  IAB IRTF PSRG, IETF PEM                                                           February 1993Privacy Enhancement for Internet Electronic Mail:Part II: Certificate-Based Key ManagementStatus of this Memo   This RFC specifies an IAB standards track protocol for the Internet   community, and requests discussion and suggestions for improvements.   Please refer to the current edition of the "IAB Official Protocol   Standards" for the standardization state and status of this protocol.   Distribution of this memo is unlimited.Acknowledgements   This memo is the outgrowth of a series of meetings of the Privacy and   Security Research Group of the Internet Research Task Force (IRTF)   and the Privacy-Enhanced Electronic Mail Working Group of the   Internet Engineering Task Force (IETF).  I would like to thank the   members of the PSRG and the PEM WG for their comments and   contributions at the meetings which led to the preparation of this   document.  I also would like to thank contributors to the PEM-DEV   mailing list who have provided valuable input which is reflected in   this memo.1.  Executive Summary   This is one of a series of documents defining privacy enhancement   mechanisms for electronic mail transferred using Internet mail   protocols.RFC 1421 [6] prescribes protocol extensions and   processing procedures forRFC-822 mail messages, given that suitable   cryptographic keys are held by originators and recipients as a   necessary precondition.RFC 1423 [7] specifies algorithms, modes and   associated identifiers for use in processing privacy-enhanced   messages, as called for inRFC 1421 and this document.  This document   defines a supporting key management architecture and infrastructure,   based on public-key certificate techniques, to provide keying   information to message originators and recipients.RFC 1424 [8]   provides additional specifications for services in conjunction with   the key management infrastructure described herein.   The key management architecture described in this document is   compatible with the authentication framework described in CCITT 1988   X.509 [2].  This document goes beyond X.509 by establishingKent                                                            [Page 1]

RFC 1422           Certificate-Based Key Management        February 1993   procedures and conventions for a key management infrastructure for   use with Privacy Enhanced Mail (PEM) and with other protocols, from   both the TCP/IP and OSI suites, in the future.  There are several   motivations for establishing these procedures and conventions (as   opposed to relying only on the very general framework outlined in   X.509):       -It is important that a certificate management infrastructure           for use in the Internet community accommodate a range of           clearly-articulated certification policies for both users           and   organizations in a well-architected fashion.           Mechanisms must be provided to enable each user to be           aware of the policies governing any certificate which the           user may encounter.  This requires the introduction           and standardization of procedures and conventions that are           outside the scope of X.509.       -The procedures for authenticating originators and recipient in           the course of message submission and delivery should be           simple, automated and uniform despite the existence of           differing certificate management policies.  For example,           users should not have to engage in careful examination of a           complex set of certification relationships in order to           evaluate the credibility of a claimed identity.       -The authentication framework defined by X.509 is designed to           operate in the X.500 directory server environment.  However           X.500 directory servers are not expected to be ubiquitous           in the Internet in the near future, so some conventions           are adopted to facilitate operation of the key management           infrastructure in the near term.       -Public key cryptosystems are central to the authentication           technology of X.509 and those which enjoy the most           widespread use are patented in the U.S.  Although this           certification management scheme is compatible with           the use of different digital signature algorithms, it is           anticipated that the RSA cryptosystem will be used as           the primary signature algorithm in establishing the           Internet certification hierarchy.  Special license           arrangements have been made to facilitate the           use of this algorithm in the U.S. portion of Internet           environment.   The infrastructure specified in this document establishes a single   root for all certification within the Internet, the Internet Policy   Registration Authority (IPRA).  The IPRA establishes global policies,   described in this document, which apply to all certification effectedKent                                                            [Page 2]

RFC 1422           Certificate-Based Key Management        February 1993   under this hierarchy.  Beneath IPRA root are Policy Certification   Authorities (PCAs), each of which establishes and publishes (in the   form of an informational RFC) its policies for registration of users   or organizations.  Each PCA is certified by the IPRA. (It is   desirable that there be a relatively small number of PCAs, each with   a substantively different policy, to facilitate user familiarity with   the set of PCA policies.  However there is no explicit requirement   that the set of PCAs be limited in this fashion.)  Below PCAs,   Certification Authorities (CAs) will be established to certify users   and subordinate organizational entities (e.g., departments, offices,   subsidiaries, etc.).  Initially, we expect the majority of users will   be registered via organizational affiliation, consistent with current   practices for how most user mailboxes are provided.  In this sense   the registration is analogous to the issuance of a university or   company ID card.   Some CAs are expected to provide certification for residential users   in support of users who wish to register independent of any   organizational affiliation.  Over time, we anticipate that civil   government entities which  already provide analogous identification   services in other contexts, e.g.,  driver's licenses, may provide   this service.  For users who wish anonymity while taking advantage of   PEM privacy facilities, one or more PCAs will be established with   policies that allow for registration of users, under subordinate CAs,   who do not wish to disclose their identities.2.  Overview of Approach   This document defines a key management architecture based on the use   of public-key certificates, primarily in support of the message   encipherment and authentication procedures defined inRFC 1421.  The   concept of public-key certificates is defined in X.509 and this   architecture is a compliant subset of that envisioned in X.509.   Briefly, a (public-key) certificate is a data structure which   contains the name of a user (the "subject"), the public component   (This document adopts the terms "private component" and "public   component" to refer to the quantities which are, respectively, kept   secret and made publicly available in asymmetric cryptosystems.  This   convention is adopted to avoid possible confusion arising from use of   the term "secret key" to refer to either the former quantity or to a   key in a symmetric cryptosystem.)  of that user, and the name of an   entity (the "issuer") which vouches that the public component is   bound to the named user.  This data, along with a time interval over   which the binding is claimed to be valid, is cryptographically signed   by the issuer using the issuer's private component.  The subject and   issuer names in certificates are Distinguished Names (DNs) as defined   in the directory system (X.500).Kent                                                            [Page 3]

RFC 1422           Certificate-Based Key Management        February 1993   Once signed, certificates can be stored in directory servers,   transmitted via non-secure message exchanges, or distributed via any   other means that make certificates easily accessible to message   system users, without regard for the security of the transmission   medium.  Certificates are used in PEM to provide the originator of a   message with the (authenticated) public component of each recipient   and to provide each recipient with the (authenticated) public   component of the originator.  The following brief discussion   illustrates the procedures for both originator and recipients.   Prior to sending an encrypted message (using PEM), an originator must   acquire a certificate for each recipient and must validate these   certificates.  Briefly, validation is performed by checking the   digital signature in the certificate, using the public component of   the issuer whose private component was used to sign the certificate.   The issuer's public component is made available via some out of band   means (for the IPRA) or is itself distributed in a certificate to   which this validation procedure is applied recursively.  In the   latter case, the issuer of a user's certificate becomes the subject   in a certificate issued by another certifying authority (or a PCA),   thus giving rise to a certification hierarchy.  The validity interval   for each certificate is checked and Certificate Revocation Lists   (CRLs) are checked to ensure that none of the certificates employed   in the validation process has been revoked by an issuer.   Once a certificate for a recipient is validated, the public component   contained in the certificate is extracted and used to encrypt the   data encryption key (DEK), which, in turn, is used to encrypt the   message itself.  The resulting encrypted DEK is incorporated into the   Key-Info field of the message header.  Upon receipt of an encrypted   message, a recipient employs his private component to decrypt this   field, extracting the DEK, and then uses this DEK to decrypt the   message.   In order to provide message integrity and data origin authentication,   the originator generates a message integrity code (MIC), signs   (encrypts) the MIC using the private component of his public-key   pair, and includes the resulting value in the message header in the   MIC-Info field.  The certificate of the originator is (optionally)   included in the header in the Certificate field as described inRFC1421.  This is done in order to facilitate validation in the absence   of ubiquitous directory services.  Upon receipt of a privacy enhanced   message, a recipient validates the originator's certificate (using   the IPRA public component as the root of a certification path),   checks to ensure that it has not been revoked, extracts the public   component from the certificate, and uses that value to recover   (decrypt) the MIC.  The recovered MIC is compared against the locally   calculated MIC to verify the integrity and data origin authenticityKent                                                            [Page 4]

RFC 1422           Certificate-Based Key Management        February 1993   of the message.3.  Architecture   3.1  Scope and Restrictions   The architecture described below is intended to provide a basis for   managing public-key cryptosystem values in support of privacy   enhanced electronic mail in the Internet environment.  The   architecture describes procedures for registering certification   authorities and users, for generating and distributing certificates,   and for generating and distributing CRLs.RFC 1421 describes the   syntax and semantics of header fields used to transfer certificates   and to represent the DEK and MIC in this public-key context.   Definitions of the algorithms, modes of use and associated   identifiers are separated inRFC 1423 to facilitate the adoption of   additional algorithms in the future.  This document focuses on the   management aspects of certificate-based, public-key cryptography for   privacy enhanced mail.   The proposed architecture imposes conventions for the certification   hierarchy which are not strictly required by the X.509 recommendation   nor by the technology itself.  These conventions are motivated by   several factors, primarily the need for authentication semantics   compatible with automated validation and the automated determination   of the policies under which certificates are issued.   Specifically, the architecture proposes a system in which user (or   mailing list) certificates represent the leaves in a certification   hierarchy.  This certification hierarchy is largely isomorphic to the   X.500 directory naming hierarchy, with two exceptions: the IPRA forms   the root of the tree (the root of the X.500 DIT is not instantiated   as a node), and a number of Policy Certification Authorities (PCAs)   form the "roots" of subtrees, each of which represents a different   certification policy.   Not every level in the directory hierarchy need correspond to a   certification authority.  For example, the appearance of geographic   entities in a distinguished name (e.g., countries, states, provinces,   localities) does not require that various governments become   certifying authorities in order to instantiate this architecture.   However, it is anticipated that, over time, a number of such points   in the hierarchy will be instantiated as CAs in order to simplify   later transition of management to appropriate governmental   authorities.   These conventions minimize the complexity of validating user   certificates, e.g., by making explicit the relationship between aKent                                                            [Page 5]

RFC 1422           Certificate-Based Key Management        February 1993   certificate issuer and the user (via the naming hierarchy). Note that   in this architecture, only PCAs may be certified by the IPRA, and   every CA's certification path can be traced to a PCA, through zero or   more CAs.  If a CA is certified by more than one PCA, each   certificate issued by a PCA for the CA must contain a distinct public   component.  These conventions result in a certification hierarchy   which is a compatible subset of that permitted under X.509, with   respect to both syntax and semantics.   Although the key management architecture described in this document   has been designed primarily to support privacy enhanced mail, this   infrastructure also may, in principle, be used to support X.400 mail   security facilities (as per 1988 X.411) and X.500 directory   authentication facilities.  Thus, establishment of this   infrastructure paves the way for use of these and other OSI protocols   in the Internet in the future.  In the future, these certificates   also may be employed in the provision of security services in other   protocols in the TCP/IP and OSI suites as well.   3.2  Relation to X.509 Architecture   CCITT 1988 Recommendation X.509, "The Directory - Authentication   Framework", defines a framework for authentication of entities   involved in a distributed directory service.  Strong authentication,   as defined in X.509, is accomplished with the use of public-key   cryptosystems.  Unforgeable certificates are generated by   certification authorities; these authorities may be organized   hierarchically, though such organization is not required by X.509.   There is no implied mapping between a certification hierarchy and the   naming hierarchy imposed by directory system naming attributes.   This document interprets the X.509 certificate mechanism to serve the   needs of PEM in the Internet environment.  The certification   hierarchy proposed in this document in support of privacy enhanced   mail is intentionally a subset of that allowed under X.509.  This   certification hierarchy also embodies semantics which are not   explicitly addressed by X.509, but which are consistent with X.509   precepts.  An overview of the rationale for these semantics is   provided inSection 1.   3.3  Certificate Definition   Certificates are central to the key management architecture for X.509   and PEM.  This section provides an overview of the syntax and a   description of the semantics of certificates.Appendix A includes   the ASN.1 syntax for certificates.   A certificate includes the   following contents:Kent                                                            [Page 6]

RFC 1422           Certificate-Based Key Management        February 1993       1.  version       2.  serial number       3.  signature (algorithm ID and parameters)       4.  issuer name       5.  validity period       6.  subject name       7.  subject public key (and associated algorithm ID)   3.3.1  Version Number   The version number field is intended to facilitate orderly changes in   certificate formats over time.  The initial version number for   certificates used in PEM is the X.509 default which has a value of   zero (0), indicating the 1988 version.  PEM implementations are   encouraged to accept later versions as they are endorsed by   CCITT/ISO.   3.3.2  Serial Number   The serial number field provides a short form, unique identifier for   each certificate generated by an issuer.  An issuer must ensure that   no two distinct certificates with the same issuer DN contain the same   serial number.  (This requirement must be met even when the   certification function is effected on a distributed basis and/or when   the same issuer DN is certified under two different PCAs.  This is   especially critical for residential CAs certified under different   PCAs.) The serial number is used in CRLs to identify revoked   certificates, as described inSection 3.4.3.4.  Although this   attribute is an integer, PEM UA processing of this attribute need not   involve any arithmetic operations.  All PEM UA implementations must   be capable of processing serial numbers at least 128 bits in length,   and size-independent support serial numbers is encouraged.   3.3.3  Signature   This field specifies the algorithm used by the issuer to sign the   certificate, and any parameters associated with the algorithm. (The   certificate signature is appended to the data structure, as defined   by the signature macro in X.509.  This algorithm identification   information is replicated with the signature.)  The signature is   validated by the UA processing a certificate, in order to determine   that the integrity of its contents have not been modified subsequentKent                                                            [Page 7]

RFC 1422           Certificate-Based Key Management        February 1993   to signing by a CA (IPRA, or PCA).  In this context, a signature is   effected through the use of a Certificate Integrity Check (CIC)   algorithm and a public-key encryption algorithm.RFC 1423 contains   the definitions and algorithm IDs for signature algorithms employed   in this architecture.   3.3.4  Subject Name   A certificate provides a representation of its subject's identity in   the form of a Distinguished Name (DN).  The fundamental binding   ensured by the key management architecture is that between the public   component and the user's identity in this form.  A distinguished name   is an X.500 directory system concept and if a user is already   registered in an X.500 directory, his distinguished name is defined   via that registration.  Users who are not registered in a directory   should keep in mind likely directory naming structure (schema) when   selecting a distinguished name for inclusion in a certificate.   3.3.5  Issuer Name   A certificate provides a representation of its issuer's identity, in   the form of a Distinguished Name.  The issuer identification is used   to select the appropriate issuer public component to employ in   performing certificate validation.  (If an issuer (CA) is certified   by multiple PCAs, then the issuer DN does not uniquely identify the   public component used to sign the certificate.  In such circumstances   it may be necessary to attempt certificate validation using multiple   public components, from certificates held by the issuer under   different PCAs.  If the 1992 version of a certificate is employed,   the issuer may employ distinct issuer UIDs in the certificates it   issues, to further facilitate selection of the right issuer public   component.) The issuer is the certifying authority (IPRA, PCA or CA)   who vouches for the binding between the subject identity and the   public key contained in the certificate.   3.3.6  Validity Period   A certificate carries a pair of date and time indications, indicating   the start and end of the time period over which a certificate is   intended to be used.  The duration of the interval may be constant   for all user certificates issued by a given CA or it might differ   based on the nature of the user's affiliation.  For example, an   organization might issue certificates with shorter intervals to   temporary employees versus permanent employees.  It is recommended   that the UTCT (Coordinated Universal Time) values recorded here   specify granularity to no more than the minute, even though finer   granularity can be expressed in the format.  (Implementors are warned   that no DER is defined for UTCT in X.509, thus transformation betweenKent                                                            [Page 8]

RFC 1422           Certificate-Based Key Management        February 1993   local and transfer syntax must be performed carefully, e.g., when   computing the hash value for a certificate.  For example, a UTCT   value which includes explict, zero values for seconds would not   produce the same hash value as one in which the seconds were   omitted.) It also recommended that all times be expressed as   Greenwich Mean Time (Zulu), to simplify comparisons and avoid   confusion relating to daylight savings time.  Note that UTCT   expresses the value of a year modulo 100 (with no indication of   century), hence comparisons involving dates in different centuries   must be performed with care.   The longer the interval, the greater the likelihood that compromise   of a private component or name change will render it invalid and thus   require that the certificate be revoked.  Once revoked, the   certificate must remain on the issuer's CRL (seeSection 3.4.3.4)   until the validity interval expires.  PCAs may impose restrictions on   the maximum validity interval that may be elected by CAs operating in   their certification domain (seeAppendix B).   3.3.7  Subject Public Key   A certificate carries the public component of its associated subject,   as well as an indication of the algorithm, and any algorithm   parameters, with which the public component is to be used.  This   algorithm identifier is independent of that which is specified in the   signature field described above.RFC 1423 specifies the algorithm   identifiers which may be used in this context.   3.4  Roles and Responsibilities   One way to explain the architecture proposed by this document is to   examine the roles which are defined for various entities in the   architecture and to describe what is required of each entity in order   for the proposed system to work properly.  The following sections   identify four types of entities within this architecture: users and   user agents, the Internet Policy Registration Authority, Policy   Certification Authorities, and other Certification Authorities.  For   each type of entity, this document specifies the procedures which the   entity must execute as part of the architecture and the   responsibilities the entity assumes as a function of its role in the   architecture.   3.4.1  Users and User Agents   The term User Agent (UA) is taken from CCITT X.400 Message Handling   Systems (MHS) Recommendations, which define it as follows: "In the   context of message handling, the functional object, a component of   MHS, by means of which a single direct user engages in messageKent                                                            [Page 9]

RFC 1422           Certificate-Based Key Management        February 1993   handling."   In the Internet environment, programs such as rand mh   and Gnu emacs rmail are UAs.  UAs exchange messages by calling on a   supporting Message Transfer Service (MTS), e.g., the SMTP mail relays   used in the Internet.   3.4.1.1  Generating and Protecting Component Pairs   A UA process supporting PEM must protect the private component of its   associated entity (e.g., a human user or a mailing list) from   disclosure, though the means by which this is effected is a local   matter.  It is essential that the user take all available precautions   to protect his private component as the secrecy of this value is   central to the security offered by PEM to that user.   For example,   the private component might be stored in encrypted form, protected   with a locally managed symmetric encryption key (e.g., using DES).   The user would supply a password or passphrase which would be   employed as a symmetric key to decrypt the private component when   required for PEM processing (either on a per message or per session   basis).  Alternatively, the private component might be stored on a   diskette which would be inserted by the user whenever he originated   or received PEM messages.  Explicit zeroing of memory locations where   this component transiently resides could provide further protection.   Other precautions, based on local operating system security   facilities, also should be employed.   It is recommended that each user employ ancillary software (not   otherwise associated with normal UA operation) or hardware to   generate his personal public-key component pair.  Software for   generating user component pairs will be available as part of the   reference implementation of PEM distributed freely in the U.S.   portion of the Internet.  It is critically important that the   component pair generation procedure be effected in as secure a   fashion as possible, to ensure that the resulting private component   is unpredictable.  Introduction of adequate randomness into the   component pair generation procedure is potentially the most difficult   aspect of this process and the user is advised to pay particular   attention to this aspect.  (Component pairs employed in public-key   cryptosystems tend to be large integers which must be "randomly"   selected subject to mathematical constraints imposed by the   cryptosystem.  Input(s) used to seed the component pair generation   process must be as unpredictable as possible.  An example of a poor   random number selection technique is one in which a pseudo-random   number generator is seeded solely with the current date and time.  An   attacker who could determine approximately when a component pair was   generated could easily regenerate candidate component pairs and   compare the public component to the user's public component to detect   when the corresponding private component had been found.)Kent                                                           [Page 10]

RFC 1422           Certificate-Based Key Management        February 1993   There is no requirement imposed by this architecture that anyone   other than the user, including any certification authority, have   access to the user's private component.  Thus a user may retain his   component pair even if his certificate changes, e.g., due to rollover   in the validity interval or because of a change of certifying   authority.  Even if a user is issued a certificate in the context of   his employment, there is generally no requirement that the employer   have access to the user's private component.  The rationale is that   any messages signed by the user are verifiable using his public   component.   In the event that the corresponding private component   becomes unavailable, any ENCRYPTED messages directed to the user   would be indecipherable and would require retransmission.   Note that if the user stores messages in ENCRYPTED form, these   messages also would become indecipherable in the event that the   private component is lost or changed.  To minimize the potential for   loss of data in such circumstances messages can be transformed into   MIC-ONLY or MIC-CLEAR form if cryptographically-enforced   confidentiality is not required for the messages stored within the   user's computer.  Alternatively, these transformed messages might be   forwarded in ENCRYPTED form to a (trivial) distribution list which   serves in a backup capacity and for which the user's employer holds   the private component.   A user may possess multiple certificates which may embody the same or   different public components.  For example, these certificates might   represent  a current and a former organizational user identity and a   residential user identity.  It is recommended that a PEM UA be   capable of supporting a user who possess multiple certificates,   irrespective of whether the certificates associated with the user   contain the same or different DNs or public components.   3.4.1.2  User Registration   Most details of user registration are a local matter, subject to   policies established by the user's CA and the PCA under which that CA   has been certified.  In general a user must provide, at a minimum,   his public component and distinguished name to a CA, or a   representative thereof, for inclusion in the user's certificate.   (The user also might provide a  complete certificate, minus the   signature, as described inRFC 1424.)  The CA will employ some means,   specified by the CA in accordance with the policy of its PCA, to   validate the user's claimed identity and to ensure that the public   component provided is associated with the user whose distinguished   name is to be bound into the certificate.  (In the case of PERSONA   certificates, described below, the procedure is a bit different.) The   certifying authority generates a certificate containing the user's   distinguished name and public component, the authority'sKent                                                           [Page 11]

RFC 1422           Certificate-Based Key Management        February 1993   distinguished name and other information (seeSection 3.3) and signs   the result using the private component of the authority.   3.4.1.3  CRL Management   Mechanisms for managing a UA certificate cache are, in typical   standards parlance, a local matter.  However, proper maintenance of   such a cache is critical to the correct, secure operation of a PEM UA   and provides a basis for improved performance.  Moreover, use of a   cache permits a PEM UA to operate in the absence of directories (and   in circumstances where directories are inaccessible).  The following   discussion  provides a paradigm for one aspect of cache management,   namely the processing of CRLs, the functional equivalent of which   must be embodied in any PEM UA implementation compliant with this   document.  The specifications for CRLs used with PEM are provided inSection 3.5.   X.500 makes provision for the storage of CRLs as directory attributes   associated with CA entries.  Thus, when X.500 directories become   widely available, UAs can retrieve CRLs from directories as required.   In the interim, the IPRA will coordinate with PCAs to provide a   robust database facility which will contain CRLs issued by the IPRA,   by PCAs, and by all CAs.  Access to this database will be provided   through mailboxes maintained by each PCA.  Every PEM UA must provide   a facility for requesting CRLs from this database using the   mechanisms defined inRFC 1424.  Thus the UA must include a   configuration parameter which specifies one or more mailbox addresses   from which CRLs may be retrieved.  Access to the CRL database may be   automated, e.g., as part of the certificate validation process (seeSection 3.6) or may be user directed.  Responses to CRL requests will   employ the PEM header format specified inRFC 1421 for CRL   propagation.  As noted inRFC 1421, every PEM UA must be capable of   processing CRLs distributed via such messages.  This message format   also may be employed to support a "push" (versus a "pull") model of   CRL distribution, i.e., to support unsolicited distribution of CRLs.   CRLs received by a PEM UA must be validated (A CRL is validated in   much the same manner as a certificate, i.e., the CIC (seeRFC 1113)   is calculated and compared against the decrypted signature value   obtained from the CRL.  SeeSection 3.6 for additional details   related to validation of certificates.) prior to being processed   against any cached certificate information.  Any cache entries which   match CRL entries should be marked as revoked, but it is not   necessary to delete cache entries marked as revoked nor to delete   subordinate entries.  In processing a CRL against the cache it is   important to recall that certificate serial numbers are unique only   for each issuer and that multiple, distinct CRLs may be issued under   the same CA DN (signed using different private components), so careKent                                                           [Page 12]

RFC 1422           Certificate-Based Key Management        February 1993   must be exercised in effecting this cache search.  (This situation   may arise either because an organizational CA is certified by   multiple PCAs, or because multiple residential CAs are certified   under different PCAs.)   This procedure applies to cache entries associated with PCAs and CAs,   as well as user entries.  The UA also must retain each CRL to screen   incoming messages to detect use of revoked certificates carried in   PEM message headers.  Thus a UA must be capable of processing and   retaining CRLs issued by the IPRA (which will list revoked PCA   certificates), by any PCA (which will list revoked CA certificate   issued by that PCA), and by any CA (which will list revoked user or   subordinate CA certificates issued by that CA).   3.4.1.4  Facilitating Interoperation   In the absence of ubiquitous directory services or knowledge   (acquired through out-of-band means) that a recipient already   possesses the necessary issuer certificates, it is recommended that   an originating (PEM) UA include sufficient certificates to permit   validation of the user's public key.  To this end every PEM UA must   be capable of including a full (originator) certification path, i.e.,   including the user's certificate (using the "Originator-Certificate"   field) and every superior (CA/PCA) certificate (using "Issuer-   Certificate" fields) back to the IPRA, in a PEM message.  A PEM UA   may send less than a full certification path, e.g., based on analysis   of a recipient list, but a UA which provides this sort of   optimization must also provide the user with a capability to force   transmission of a full certification path.   Optimization for the transmitted originator certification path may be   effected by a UA as a side effect of the processing performed during   message submission.  When an originator submits an ENCRYPTED message   (as perRFC 1421, his UA must validate the certificates of the   recipients (seeSection 3.6).  In the course of performing this   validation the UA can determine the minimum set of certificates which   must be included to ensure that all recipients can process the   received message.  Submission of a MIC-ONLY or MIC-CLEAR message (as   perRFC 1421) does not entail validation of recipient certificates   and thus it may not be possible for the originator's UA to determine   the minimum certificate set as above.   3.4.2  The Internet Policy Registration Authority (IPRA)   The IPRA acts as the root of the certification hierarchy for the   Internet community.  The public component of the IPRA forms the   foundation for all certificate validation within this hierarchy.  The   IPRA will be operated under the auspices of the Internet Society, anKent                                                           [Page 13]

RFC 1422           Certificate-Based Key Management        February 1993   international, non-profit organization.  The IPRA certifies all PCAs,   ensuring that they agree to abide by the Internet-wide policy   established by the IPRA.  This policy, and the services provided by   the IPRA, are detailed below.   3.4.2.1  PCA Registration   The IPRA certifies only PCAs, not CAs or users.  Each PCA must file   with the IPRA a description of its proposed policy.  This document   will be published as an informational RFC.  A copy of the document,   signed by the IPRA (in the form of a PEM MIC-ONLY message) will be   made available via electronic mail access by the IPRA.  This   convention is adopted so that every Internet user has a reference   point for determining the policies associated with the issuance of   any certificate which he may encounter.  The existence of a digitally   signed copy of the document ensures the immutability of the document.   Authorization of a PCA to operate in the Internet hierarchy is   signified by the publication of the policy document, and the issuance   of a certificate to the PCA, signed by the IPRA.  An outline for PCA   policy statements is contained inSection 3.4.3 of this document.   As part of registration, each PCA will be required to execute a legal   agreement with the IPRA, and to pay a fee to defray the costs of   operating the IPRA.  Each a PCA must specify its distinguished name.   The IPRA will take reasonable precautions to ensure that the   distinguished name claimed by a PCA is legitimate, e.g., requiring   the PCA to provide documentation supporting its claim to a DN.   However, the certification of a PCA by the IPRA does not constitute a   endorsement of the PCA's claim to this DN outside of the context of   this certification system.   3.4.2.2  Ensuring the Uniqueness of Distinguished Names   A fundamental requirement of this certification scheme is that   certificates are not issued to distinct entities under the same   distinguished name.  This requirement is important to the success of   distributed management for the certification hierarchy.  The IPRA   will not certify two PCAs with the same distinguished name and no PCA   may certify two CAs with the same DN.  However, since PCAs are   expected to certify organizational CAs in widely disjoint portions of   the directory namespace, and since X.500 directories are not   ubiquitous, a facility is required for coordination among PCAs to   ensure the uniqueness of CA DNs.  (This architecture allows multiple   PCAs to certify residential CAs and thus multiple, distinct   residential CAs with identical DNs may come into existence, at least   until such time as civil authorities assume responsibilities for such   certification.  Thus, on an interim basis, the architecture   explicitly accommodates the potential for duplicate residential CAKent                                                           [Page 14]

RFC 1422           Certificate-Based Key Management        February 1993   DNs.)   In support of the uniqueness requirement, the IPRA will establish and   maintain a database to detect potential, unintended duplicate   certification of CA distinguished names.  This database will be made   accessible to all PCAs via an email interface.  Each entry in this   database will consist of a 4-tuple.  The first element in each entry   is a hash value, computed on a canonical, ASN.1 encoded   representation of a CA distinguished name.  The second element   contains the subjectPublicKey that appears in the CA's certificate.   The third element is the distinguished name of the PCA which   registered the entry.  The fourth element consists of the date and   time at which the entry was made, as established by the IPRA.  This   database structure provides a degree of privacy for CAs registered by   PCAs, while providing a facility for ensuring global uniqueness of CA   DNs certified in this scheme.   In order to avoid conflicts, a PCA should query the database using a   CA DN hash value as a search key, prior to certifying a CA.  The   database will return any entries which match the query, i.e., which   have the same CA DN.  The PCA can use the information contained in   any returned entries to determine if any PCAs should be contacted to   resolve possible DN conflicts.  If no potential conflicts appear, a   PCA can then submit a candidate entry, consisting of the first three   element values, plus any entries returned by the query.  The database   will register this entry, supplying the time and date stamp, only if   two conditions are met: (1) the first two elements (the CA DN hash   and the CA subjectPublicKey) of the candidate entry together must be   unique and, (2) any other entries included in the submission must   match what the current database would return if the query   corresponding to the candidate entry were submitted.   If the database detects a conflicting entry (failure of case 1   above), or if the submission indicates that the PCA's perception of   possible conflicting entries is not current (failure of case 2), the   submission is rejected and the database will return the potential   conflicting entry (entries).  If the submission is successful, the   database will return the timestamped new entry.  The database does   not, in itself, guarantee uniqueness of CA DNs as it allows for two   DNs associated with different public components to be registered.   Rather, it is the responsibility of PCAs to coordinate with one   another whenever the database indicates a potential DN conflict and   to resolve such conflicts prior to certification of CAs.  Details of   the protocol used to access the database will be provided in another   document.   As noted earlier, a CA may be certified under more than one PCA,   e.g., because the CA wants to issue certificates under two differentKent                                                           [Page 15]

RFC 1422           Certificate-Based Key Management        February 1993   policies.  If a CA is certified by multiple different PCAs, the CA   must employ a different public key pair for each PCA.  In such   circumstances the certificate issued to the CA by each PCA will   contain a different subjectPublicKey and thus will represent a   different entry in this database.  The same situation may arise if   multiple, equivalent residential CAs are certified by different PCAs.   To complete the strategy for ensuring uniqueness of DNs, there is a   DN subordination requirement levied on CAs.  In general, CAs are   expected to sign certificates only if the subject DN in the   certificate is subordinate to the issuer (CA) DN.  This ensures that   certificates issued by a CA are syntactically constrained to refer to   subordinate entities in the X.500 directory information tree (DIT),   and this further limits the possibility of duplicate DN registration.   CAs may sign certificates which do not comply with this requirement   if the certificates are "cross-certificates" or "reverse   certificates" (see X.509) used with applications other than PEM.   The IPRA also will establish and maintain a separate database to   detect potential duplicate certification of (residential) user   distinguished names.  Each entry in this database will consist of 4-   tuple as above, but the first components is the hash of a residential   user DN and the third component is the DN of the residential CA DN   which registered the user.  This structure provides a degree of   privacy for users registered by CAs which service residential users   while providing a facility for ensuring global uniqueness of user DNs   certified under this scheme.  The same database access facilities are   provided as described above for the CA database.  Here it is the   responsibility of the CAs to coordinate whenever the database   indicates a potential conflict and to resolve the conflict prior to   (residential) user certification.   3.4.2.3  Accuracy of Distinguished Names   As noted above, the IPRA will make a reasonable effort to ensure that   PCA DNs are accurate.  The procedures employed to ensure the accuracy   of a CA distinguished name, i.e., the confidence attached to the   DN/public component binding implied by a certificate, will vary   according to PCA policy.  However, it is expected that every PCA will   make a good faith effort to ensure the legitimacy of each CA DN   certified by the PCA.  Part of this effort should include a check   that the purported CA DN is consistent with any applicable national   standards for DN assignment, e.g., NADF recommendations within North   America [5,9].Kent                                                           [Page 16]

RFC 1422           Certificate-Based Key Management        February 1993   3.4.2.4  Distinguished Name Conventions   A few basic DN conventions are included in the IPRA policy.  The IPRA   will certify PCAs, but not CAs nor users.  PCAs will certify CAs, but   not users.  These conventions are required to allow simple   certificate validation within PEM, as described later.  Certificates   issued by CAs (for use with PEM) will be for users or for other CAs,   either of which must have DNs subordinate to that of the issuing CA.   The attributes employed in constructing DNs will be specified in a   list maintained by the IANA, to provide a coordinated basis for   attribute identification for all applications employing DNs.  This   list will initially be populated with attributes taken from X.520.   This document does not impose detailed restrictions on the attributes   used to identify different entities to which certificates are issued,   but PCAs may impose such restrictions as part of their policies.   PCAs, CAs and users are urged to employ only those DN attributes   which have printable representations, to facilitate display and   entry.   3.4.2.5  CRL Management   Among the procedures articulated by each PCA in its policy statement   are procedures for the maintenance and distribution of CRLs by the   PCA itself and by its subordinate CAs.  The frequency of issue of   CRLs may vary according to PCA-specific policy, but every PCA and CA   must issue a CRL upon inception to provide a basis for uniform   certificate validation procedures throughout the Internet hierarchy.   The IPRA will maintain a CRL for all the PCAs it certifies and this   CRL will be updated monthly.  Each PCA will maintain a CRL for all of   the CAs which it certifies and these CRLs will be updated in   accordance with each PCA's policy.   The format for these CRLs is   that specified inSection 3.5.2 of the document.   In the absence of ubiquitous X.500 directory services, the IPRA will   require each PCA to provide, for its users, robust database access to   CRLs for the Internet hierarchy, i.e., the IPRA CRL, PCA CRLs, and   CRLs from all CAs.  The means by which this database is implemented   is to be coordinated between the IPRA and PCAs.  This database will   be accessible via email as specified inRFC 1424, both for retrieval   of (current) CRLs by any user, and for submission of new CRLs by CAs,   PCAs and the IPRA.  Individual PCAs also may elect to maintain CRL   archives for their CAs, but this is not required by this policy.   3.4.2.6  Public Key Algorithm Licensing Issues   This certification hierarchy is architecturally independent of any   specific digital signature (public key) algorithm.  Some algorithms,Kent                                                           [Page 17]

RFC 1422           Certificate-Based Key Management        February 1993   employed for signing certificates and validating certificate   signatures, are patented in some countries.  The IPRA will not grant   a license to any PCA for the use of any signature algorithm in   conjunction with the management of this certification hierarchy.  The   IPRA will acquire, for itself, any licenses needed for it to sign   certificates and CRLs for PCAs, for all algorithms which the IPRA   supports.  Every PCA will be required to represent to the IPRA that   the PCA has obtained any licenses required to issue (sign)   certificates and CRLs in the environment(s) which the PCA will serve.   For example, the RSA cryptosystem is patented in the United States   and thus any PCA operating in the U.S. and using RSA to sign   certificates and CRLs must represent that it has a valid license to   employ the RSA algorithm in this fashion.  In contrast, a PCA   employing RSA and operating outside of the U.S. would represent that   it is exempt from these licensing constraints.   3.4.3  Policy Certification Authorities   The policy statement submitted by a prospective PCA must address the   topics in the following outline.  Additional policy information may   be contained in the statement, but PCAs are requested not to use   these statements as advertising vehicles.   1. PCA Identity-  The DN of the PCA must be specified.  A postal   address, an Internet mail address, and telephone (and optional fax)   numbers must be provided for (human) contact with the PCA.  The date   on which this statement is effective, and its scheduled duration must   be specified.   2. PCA Scope- Each PCA must describe the community which the PCA   plans to serve.  A PCA should indicate if it will certify   organizational, residential, and/or PERSONA CAs.   There is not a   requirement that a single PCA serve only one type of CA, but if a PCA   serves multiple types of CAs, the policy statement must specify   clearly how a user can distinguish among these classes.  If the PCA   will operate CAs to directly serve residential or PERSONA users, it   must so state.   3. PCA Security & Privacy- Each PCA must specify the technical and   procedural security measures it will employ in the generation and   protection of its component pair.  If any security requirements are   imposed on CAs certified by the PCA these must be specified as well.   A PCA also must specify what measures it will take to protect the   privacy of any information collected in the course of certifying CAs.   If the PCA operates one or more CAs directly, to serve residential or   PERSONA users, then this statement on privacy measures applies to   these CAs as well.Kent                                                           [Page 18]

RFC 1422           Certificate-Based Key Management        February 1993   4. Certification Policy-  Each PCA must specify the policy and   procedures which govern its certification of CAs and how this policy   applies transitively to entities (users or subordinate CAs) certified   by these CAs.  For example, a PCA must state what procedure is   employed to verify the claimed identity of a CA, and the CA's right   to use a DN.  Similarly, if any requirements are imposed on CAs to   validate the identity of users, these requirements must be specified.   Since all PCAs are required to cooperate in the resolution of   potential DN conflicts, each PCA is required to specify the procedure   it will employ to resolve such conflicts.  If the PCA imposes a   maximum validity interval for the CA certificates it issues, and/or   for user (or subordinate CA) certificates issued by the CAs it   certifies, then these restrictions must be specified.   5. CRL Management-  Each PCA must specify the frequency with which it   will issue scheduled CRLs.  It also must specify any constraints it   imposes on the frequency of scheduled issue of CRLs by the CAs it   certifies, and by subordinate CAs.  Both maximum and minimum   constraints should be specified.  Since the IPRA policy calls for   each CRL issued by a CA to be forwarded to the cognizant PCA, each   PCA must specify a mailbox address to which CRLs are to be   transmitted.  The PCA also must specify a mailbox address for CRL   queries.  If the PCA offers any additional CRL management services,   e.g., archiving of old CRLs, then procedures for invoking these   services must be specified.  If the PCA requires CAs to provide any   additional CRL management services, such services must be specified   here.   6. Naming Conventions- If the PCA imposes any conventions on DNs used   by the CAs it certifies, or by entities certified by these CAs, these   conventions must be specified.  If any semantics are associated with   such conventions, these semantics must be specified.   7. Business Issues- If a legal agreement must be executed between a   PCA and the CAs it certifies, reference to that agreement must be   noted, but the agreement itself ought not be a part of the policy   statement.  Similarly, if any fees are charged by the PCA this should   be noted, but the fee structure per se ought not be part of this   policy statement.   8. Other- Any other topics the PCA deems relevant to a statement of   its policy can be included.  However, the PCA should be aware that a   policy statement is considered to be an immutable, long lived   document and thus considerable care should be exercised in deciding   what material is to be included in the statement.Kent                                                           [Page 19]

RFC 1422           Certificate-Based Key Management        February 1993   3.4.4  Certification Authorities   In X.509 the term "certification authority" is defined as "an   authority trusted by one or more users to create and assign   certificates".  X.509 imposes few constraints on CAs, but practical   implementation of a worldwide certification system requires   establishment of technical and procedural conventions by which all   CAs are expected to abide.  Such conventions are established   throughout this document.  All CAs are required to maintain a   database of the DNs which they have certified and to take measures to   ensure that they do not certify duplicate DNs, either for users or   for subordinate CAs.   It is critical that the private component of a CA be afforded a high   level of security, otherwise the authenticity guarantee implied by   certificates signed by the CA is voided.  Some PCAs may impose   stringent requirements on CAs within their purview to ensure that a   high level of security is afforded the certificate signing process,   but not all PCAs are expected to impose such constraints.   3.4.4.1  Organizational CAs   Many of the CAs certified by PCAs are expected to represent   organizations.  A wide range of organizations are encompassed by this   model: commercial, governmental, educational, non-profit,   professional societies, etc.  The common thread is that the entities   certified by these CAs have some form of affiliation with the   organization.  The object classes for organizations, organizational   units, organizational persons, organizational roles, etc., as defined   in X.521, form the models for entities certified by such CAs.  The   affiliation implied by organizational certification motivates the DN   subordination requirement cited inSection 3.4.2.4.   As an example, an organizational user certificate might contain a   subject DN of the form: C = "US" SP = "Massachusetts" L = "Cambridge"   O = "Bolt Beranek and Newman" OU = "Communications Division" CN =   "Steve Kent".  The issuer of this certificate might have a DN of the   form: C = "US" SP = "Massachusetts" L = "Cambridge" O= "Bolt Beranek   and Newman".  Note that the organizational unit attribute is omitted   from the issuer DN, implying that there is no CA dedicated to the   "Communications Division".   3.4.4.2  Residential CAs   Users may wish to obtain certificates which do not imply any   organizational affiliation but which do purport to accurately and   uniquely identify them.  Such users can be registered as residential   persons and the DN of such a user should be consistent with theKent                                                           [Page 20]

RFC 1422           Certificate-Based Key Management        February 1993   attributes of the corresponding X.521 object class.  Over time we   anticipate that such users will be accommodated by civil government   entities who will assume electronic certification responsibility at   geographically designated points in the naming hierarchy.  Until   civil authorities are prepared to issue certificates of this form,   residential user CAs will accommodate such users.   Because residential CAs may be operated under the auspices of   multiple PCAs, there is a potential for the same residential CA DN to   be assumed by several distinct entities.  This represents the one   exception to the rule articulated throughout this document that no   two entities may have the same DN.  This conflict is tolerated so as   to allow residential CAs to be established offering different   policies.  Two requirements are levied upon residential CAs as a   result: (1) residential CAs must employ the residential DN conflict   detection database maintained by the IPRA, and (2) residential CAs   must coordinate to ensure that they do not assign duplicate   certificate serial numbers.   As an example, a residential user certificate might include a subject   name of the form: C = "US" SP = "Massachusetts" L = "Boston" PA = "19   North Square" CN = "Paul Revere."  The issuer of that certificate   might have a DN of the form: C = "US"  SP = "Massachusetts" L =   "Boston".  Note that the issuer DN is superior to the subject DN, as   required by the IPRA policy described earlier.   3.4.4.3  PERSONA CAs   One or more CAs will be established to accommodate users who wish to   conceal their identities while making use of PEM security features,   e.g., to preserve the anonymity offered by "arbitrary" mailbox names   in the current mail environment.  In this case the certifying   authority is explicitly NOT vouching for the identity of the user.   All such certificates are issued under a PERSONA CA, subordinate to a   PCA with a PERSONA policy, to warn users explicitly that the subject   DN is NOT a validated user identity.  To minimize the possibility of   syntactic confusion with certificates which do purport to specify an   authenticated user identity, a PERSONA certificate is issued as a   form of organizational user certificate, not a residential user   certificate.  There are no explicit, reserved words used to identify   PERSONA user certificates.   A CA issuing PERSONA certificates must institute procedures to ensure   that it does not issue the same subject DN to multiple users (a   constraint required for all certificates of any type issued by any   CA).  There are no requirements on an issuer of PERSONA certificates   to maintain any other records that might bind the true identity of   the subject to his certificate.  However, a CA issuing suchKent                                                           [Page 21]

RFC 1422           Certificate-Based Key Management        February 1993   certificates must establish procedures (not specified in this   document) in order to allow the holder of a PERSONA certificate to   request that his certificate be revoked (i.e., listed on a CRL).   As an example, a PERSONA user certificate might include a subject DN   of the form:  C = "US" SP = "Massachusetts" L = "Boston" O =   "Pseudonyms R US" CN = "Paul Revere."  The issuer of this certificate   might have a DN of the form: C = "US"  SP = "Massachusetts" L =   "Boston" O = "Pseudonyms R US".  Note the differences between this   PERSONA user certificate for "Paul Revere" and the corresponding   residential user certificate for the same common name.   3.4.4.4  CA Responsibilities for CRL Management   As X.500 directory servers become available, CRLs should be   maintained and accessed via these servers.  However, prior to   widespread deployment of X.500 directories, this document adopts some   additional requirements for CRL management by CAs and PCAs.  As per   X.509, each CA is required to maintain a CRL (in the format specified   by this document inAppendix A) which contains entries for all   certificates issued and later revoked by the CA.  Once a certificate   is entered on a CRL it remains there until the validity interval   expires.  Each PCA is required to maintain a CRL for revoked CA   certificates within its domain.  The interval at which a CA issues a   CRL is not fixed by this document, but the PCAs may establish minimum   and maximum intervals for such issuance.   As noted earlier, each PCA will provide access to a database   containing CRLs issued by the IPRA, PCAs, and all CAs.  In support of   this requirement, each CA must supply its current CRL to its PCA in a   fashion consistent with CRL issuance rules imposed by the PCA and   with the next scheduled issue date specified by the CA (seeSection3.5.1).  CAs may distribute CRLs to subordinate UAs using the CRL   processing type available in PEM messages (seeRFC 1421).  CAs also   may provide access to CRLs via the database mechanism described inRFC 1424 and alluded to immediately above.   3.5  Certificate Revocation   3.5.1  X.509 CRLs   X.509 states that it is a CA's responsibility to maintain: "a time-   stamped list of the certificates it issued which have been revoked."   There are two primary reasons for a CA to revoke a certificate, i.e.,   suspected compromise of a private component (invalidating the   corresponding public component) or change of user affiliation   (invalidating the DN).  The use of Certificate Revocation Lists   (CRLs) as defined in X.509 is one means of propagating informationKent                                                           [Page 22]

RFC 1422           Certificate-Based Key Management        February 1993   relative to certificate revocation, though it is not a perfect   mechanism.  In particular, an X.509 CRL indicates only the age of the   information contained in it; it does not provide any basis for   determining if the list is the most current CRL available from a   given CA.   The proposed architecture establishes a format for a CRL in which not   only the date of issue, but also the next scheduled date of issue is   specified.  Adopting this convention, when the next scheduled issue   date arrives a CA (Throughout this section, when the term "CA" is   employed, it should be interpreted broadly, to include the IPRA and   PCAs as well as organizational, residential, and PERSONA CAs.) will   issue a new CRL, even if there are no changes in the list of entries.   In this fashion each CA can independently establish and advertise the   frequency with which CRLs are issued by that CA.  Note that this does   not preclude CRL issuance on a more frequent basis, e.g., in case of   some emergency, but no system-wide mechanisms are architected for   alerting users that such an unscheduled issuance has taken place.   This scheduled CRL issuance convention allows users (UAs) to   determine whether a given CRL is "out of date," a facility not   available from the (1988) X.509 CRL format.   The description of CRL management in the text and the format for CRLs   specified in X.509 (1988) are inconsistent.  For example, the latter   associates an issuer distinguished name with each revoked certificate   even though the text states that a CRL contains entries for only a   single issuer (which is separately specified in the CRL format).  The   CRL format adopted for PEM is a (simplified) format consistent with   the text of X.509, but not identical to the accompanying format. The   ASN.1 format for CRLs used with PEM is provided inAppendix A.   X.509 also defines a syntax for the "time-stamped list of revoked   certificates representing other CAs."  This syntax, the   "AuthorityRevocationList" (ARL) allows the list to include references   to certificates issued by CAs other than the list maintainer.  There   is no syntactic difference between these two lists except as they are   stored in directories.  Since PEM is expected to be used prior to   widespread directory deployment, this distinction between ARLs and   CRLs is not syntactically significant.  As a simplification, this   document specifies the use the CRL format defined below for   revocation both of user and of CA certificates.   3.5.2  PEM CRL FormatAppendix A contains the ASN.1 description of CRLs specified by this   document.  This section provides an informal description of CRL   components analogous to that provided for certificates inSection3.3.Kent                                                           [Page 23]

RFC 1422           Certificate-Based Key Management        February 1993       1. signature (signature algorithm ID and parameters)       2. issuer       3. last update       4. next update       5. revoked certificates   The "signature" is a data item completely analogous to the signature   data item in a certificate. Similarly, the "issuer" is the DN of the   CA which signed the CRL.  The "last update" and "next update" fields   contain time and date values (UTCT format) which specify,   respectively, when this CRL was issued and when the next CRL is   scheduled to be issued.  Finally, "revoked certificates" is a   sequence of ordered pairs, in which the first element is the serial   number of the revoked certificate and the second element is the time   and date of the revocation for that certificate.   The semantics for this second element are not made clear in X.509.   For example, the time and date specified might indicate when a   private component was thought to have been compromised or it may   reflect when the report of such compromise was reported to the CA.   For uniformity, this document adopts the latter convention, i.e., the   revocation date specifies the time and date at which a CA formally   acknowledges a report of a compromise or a change or DN attributes.   As with certificates, it is recommended that the UTCT values be of no   finer granularity than minutes and that all values be stated in terms   of Zulu.   3.6  Certificate Validation   3.6.1  Validation Basics   Every UA must contain the public component of the IPRA as the root   for its certificate validation database.  Public components   associated with PCAs must be identified as such, so that the   certificate validation process described below can operate correctly.   Whenever a certificate for a PCA is entered into a UA cache, e.g., if   encountered in a PEM message encapsulated header, the certificate   must NOT be entered into the cache automatically.  Rather, the user   must be notified and must explicitly direct the UA to enter any PCA   certificate data into the cache.  This precaution is essential   because introduction of a PCA certificate into the cache implies user   recognition of the policy associated with the PCA.Kent                                                           [Page 24]

RFC 1422           Certificate-Based Key Management        February 1993   Validating a certificate begins with verifying that the signature   affixed to the certificate is valid, i.e., that the hash value   computed on the certificate contents matches the value that results   from decrypting the signature field using the public component of the   issuer.  In order to perform this operation the user must possess the   public component of the issuer, either via some integrity-assured   channel, or by extracting it from another (validated) certificate.   In order to rapidly terminate this recursive validation process, we   recommend each PCA sign certificates for all CAs within its domain,   even CAs which are certified by other, superior CAs in the   certification hierarchy.   The public component needed to validate certificates signed by the   IPRA is made available to each user as part of the registration or   via the PEM installation process.  Thus a user will be able to   validate any PCA certificate immediately.  CAs are certified by PCAs,   so validation of a CA certificate requires processing a validation   path of length two.  User certificates are issued by CAs (either   immediately subordinate to PCAs or subordinate to other CAs), thus   validation of a user certificate may require three or more steps.   Local caching of validated certificates by a UA can be used to speed   up this process significantly.   Consider the situation in which a user receives a privacy enhanced   message from an originator with whom the recipient has never   previously corresponded, and assume that the message originator   includes a full certification path in the PEM message header.  First   the recipient can use the IPRA's public component to validate a PCA   certificate contained in an Issuer-Certificate field.  Using the   PCA's public component extracted from this certificate, the CA   certificate in an Issuer-Certificate field also can be validated.   This process cam be repeated until the certificate for the   originator, from the Originator-Certificate field, is validated.   Having performed this certificate validation process, the recipient   can extract the originator's public component and use it to decrypt   the content of the MIC-Info field.  By comparing the decrypted   contents of this field against the MIC computed locally on the   message the user verifies the data origin authenticity and integrity   of the message.  It is recommended that implementations of privacy   enhanced mail cache validated public components (acquired from   incoming mail) to speed up this process.  If a message arrives from   an originator whose public component is held in the recipient's cache   (and if the cache is maintained in a fashion that ensures timely   incorporation of received CRLs), the recipient can immediately employ   that public component without the need for the certificate validation   process described here. (For some digital signature algorithms, the   processing required for certificate validation is considerably fasterKent                                                           [Page 25]

RFC 1422           Certificate-Based Key Management        February 1993   than that involved in signing a certificate.  Use of such algorithms   serves to minimize the computational burden on UAs.)   3.6.2  Display of Certificate Validation Data   PEM provides authenticated identities for message recipients and   originators expressed in the form of distinguished names.  Mail   systems in which PEM is employed may employ identifiers other than   DNs as the primary means of identifying recipients or originators.   Thus, in order to benefit from these authentication facilities, each   PEM implementation must employ some means of binding native mail   system identifiers to distinguished names in a fashion which does not   undermine this basic PEM functionality.   For example, if a human user interacts directly with PEM, then the   full DN of the originator of any message received using PEM should be   displayed for the user.  Merely displaying the PEM-protected message   content, containing an originator name from the native mail system,   does not provide equivalent security functionality and could allow   spoofing.  If the recipient of a message is a forwarding agent such   as a list exploder or mail relay, display of the originator's DN is   not a relevant requirement.  In all cases the essential requirement   is that the ultimate recipient of a PEM message be able to ascertain   the identity of the originator based on the PEM certification system,   not on unauthenticated identification information, e.g., extracted   from the native message system.   Conversely, for the originator of an ENCRYPTED message, it is   important that recipient identities be linked to the DNs as expressed   in PEM certificates.  This can be effected in a variety of ways by   the PEM implementation, e.g., by display of recipient DNs upon   message submission or by a tightly controlled binding between local   aliases and the DNs.  Here too, if the originator is a forwarding   process this linkage might be effected via various mechanisms not   applicable to direct human interaction.  Again, the essential   requirement is to avoid procedures which might undermine the   authentication services provided by PEM.   As described above, it is a local matter how and what certification   information is displayed for a human user in the course of submission   or delivery of a PEM message.  Nonetheless all PEM implementations   must provide a user with the ability to display a full certification   path for any certificate employed in PEM upon demand.  Implementors   are urged to not overwhelm the user with certification path   information which might confuse him or distract him from the critical   information cited above.Kent                                                           [Page 26]

RFC 1422           Certificate-Based Key Management        February 1993   3.6.3  Validation Procedure Details   Every PEM implementation is required to perform the following   validation steps for every public component employed in the   submission of an ENCRYPTED PEM message or the delivery of an   ENCRYPTED, MIC-ONLY, or MIC-CLEAR PEM message.  Each public component   may be acquired from an internal source, e.g., from a (secure) cache   at the originator/recipient or it may be obtained from an external   source, e.g., the PEM header of an incoming message or a directory.   The following procedures applies to the validation of certificates   from either type of source.   Validation of a public component involves constructing a   certification path between the component and the public component of   the IPRA.  The validity interval for every certificate in this path   must be checked.  PEM software must, at a minimum, warn the user if   any certificate in the path fails the validity interval check, though   the form of this warning is a local matter.  For example, the warning   might indicate which certificate in the path had expired.  Local   security policy may prohibit use of expired certificates.   Each certificate also must be checked against the current CRL from   the certificate's issuer to ensure that revoked certificates are not   employed.  If the UA does not have access to the current CRL for any   certificate in the path, the user must be warned.  Again, the form of   the warning is a local matter.  For example, the warning might   indicate whether the CRL is unavailable or, if available but not   current, the CRL issue date should be displayed. Local policy may   prohibit use of a public component which cannot be checked against a   current CRL, and in such cases the user should receive the same   information provided by the warning indications described above.   If any revoked certificates are encountered in the construction of a   certification path, the user must be warned.  The form of the warning   is a local matter, but it is recommended that this warning be more   stringent than those previously alluded to above.  For example, this   warning might display the issuer and subject DNs from the revoked   certificate and the date of revocation, and then require the user to   provide a positive response before the submission or delivery process   may proceed.  In the case of message submission, the warning might   display the identity of the recipient affected by this validation   failure and the user might be provided with the option to specify   that this recipient be dropped from recipient list processing without   affecting PEM processing for the remaining recipients.  Local policy   may prohibit PEM processing if a revoked certificate is encountered   in the course of constructing a certification path.   Note that in order to comply with these validation procedures, aKent                                                           [Page 27]

RFC 1422           Certificate-Based Key Management        February 1993   certificate cache must maintain all of the information contained in a   certificate, not just the DNs and the public component.  For example   the serial number and validity interval must be associated with the   cache entry to comply with the checks described above.  Also note   that these procedures apply to human interaction in message   submission and delivery and are not directly applicable to forwarding   processes.  When non human interaction is involved, a compliant PEM   implementation must provide parameters to enable a process to specify   whether certificate validation will succeed or fail if any of the   conditions arise which would result in warnings to a human user.   Finally, in the course of validating certificates as described above,   one additional check must be performed: the subject DN of every   certificate must be subordinate to the certificate issuer DN, except   if the issuer is the IPRA or a PCA (hence another reason to   distinguish the IPRA and PCA entries in a certificate cache).  This   requirement is levied upon all PEM implementations as part of   maintaining the certification hierarchy constraints defined in this   document.  Any certificate which does not comply with these   requirements is considered invalid and must be rejected in PEM   submission or delivery processing.  The user  must be notified of the   nature of this fatal error.Kent                                                           [Page 28]

RFC 1422           Certificate-Based Key Management        February 1993A.Appendix A: ASN.1 Syntax for Certificates and CRLsA.1  Certificate Syntax   The X.509 certificate format is defined by the following ASN.1   syntax:   Certificate ::= SIGNED SEQUENCE{           version [0]     Version DEFAULT v1988,           serialNumber    CertificateSerialNumber,           signature       AlgorithmIdentifier,           issuer          Name,           validity        Validity,           subject         Name,           subjectPublicKeyInfo    SubjectPublicKeyInfo}   Version ::=     INTEGER {v1988(0)}   CertificateSerialNumber ::=     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}   The components of this structure are defined by ASN.1 syntax defined   in the X.500 Series Recommendations.RFC 1423 provides references   for and the values of AlgorithmIdentifiers used by PEM in the   subjectPublicKeyInfo and the signature data items.  It also describes   how a signature is generated and the results represented.  Because   the certificate is a signed data object, the distinguished encoding   rules (see X.509,section 8.7) must be applied prior to signing.Kent                                                           [Page 29]

RFC 1422           Certificate-Based Key Management        February 1993A.2  Certificate Revocation List Syntax   The following ASN.1 syntax, derived from X.509 and aligned with the   suggested format in recently submitted defect reports, defines the   format of CRLs for use in the PEM environment.   CertificateRevocationList ::= SIGNED SEQUENCE{           signature       AlgorithmIdentifier,           issuer          Name,           lastUpdate      UTCTime,           nextUpdate      UTCTime,           revokedCertificates                           SEQUENCE OF CRLEntry OPTIONAL}   CRLEntry ::= SEQUENCE{           userCertificate SerialNumber,           revocationDate UTCTime}References   [1] CCITT Recommendation X.411 (1988), "Message Handling Systems:       Message Transfer System: Abstract Service Definition and       Procedures".   [2] CCITT Recommendation X.509 (1988), "The Directory -       Authentication Framework".   [3] CCITT Recommendation X.520 (1988), "The Directory - Selected       Attribute Types".   [4] NIST Special Publication 500-183, "Stable Agreements for Open       Systems Interconnection Protocols," Version 4, Edition 1,       December 1990.   [5] North American Directory Forum, "A Naming Scheme for c=US",RFC1255, NADF, September 1991.   [6] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part       I: Message Encryption and Authentication Procedures",RFC 1421,       DEC, February 1993.   [7] Balenson, D., "Privacy Enhancement for Internet Electronic Mail:       Part III: Algorithms, Modes, and Identifiers",RFC 1423, TIS,       February 1993.   [8] Balaski, B., "Privacy Enhancement for Internet Electronic Mail:       Part IV: Notary, Co-Issuer, CRL-Storing and CRL-Retrieving       Services",RFC 1424, RSA Laboratories, February 1993.Kent                                                           [Page 30]

RFC 1422           Certificate-Based Key Management        February 1993   [9] North American Directory Forum, "NADF Standing Documents: A Brief       Overview",RFC 1417, NADF, February 1993.Patent Statement   This version of Privacy Enhanced Mail (PEM) relies on the use of   patented public key encryption technology for authentication and   encryption.  The Internet Standards Process as defined inRFC 1310   requires a written statement from the Patent holder that a license   will be made available to applicants under reasonable terms and   conditions prior to approving a specification as a Proposed, Draft or   Internet Standard.   The Massachusetts Institute of Technology and the Board of Trustees   of the Leland Stanford Junior University have granted Public Key   Partners (PKP) exclusive sub-licensing rights to the following   patents issued in the United States, and all of their corresponding   foreign patents:      Cryptographic Apparatus and Method      ("Diffie-Hellman")............................... No. 4,200,770      Public Key Cryptographic Apparatus      and Method ("Hellman-Merkle").................... No. 4,218,582      Cryptographic Communications System and      Method ("RSA")................................... No. 4,405,829      Exponential Cryptographic Apparatus      and Method ("Hellman-Pohlig").................... No. 4,424,414   These patents are stated by PKP to cover all known methods of   practicing the art of Public Key encryption, including the variations   collectively known as El Gamal.   Public Key Partners has provided written assurance to the Internet   Society that parties will be able to obtain, under reasonable,   nondiscriminatory terms, the right to use the technology covered by   these patents.  This assurance is documented inRFC 1170 titled   "Public Key Standards and Licenses".  A copy of the written assurance   dated April 20, 1990, may be obtained from the Internet Assigned   Number Authority (IANA).   The Internet Society, Internet Architecture Board, Internet   Engineering Steering Group and the Corporation for National Research   Initiatives take no position on the validity or scope of the patents   and patent applications, nor on the appropriateness of the terms of   the assurance.  The Internet Society and other groups mentioned aboveKent                                                           [Page 31]

RFC 1422           Certificate-Based Key Management        February 1993   have not made any determination as to any other intellectual property   rights which may apply to the practice of this standard. Any further   consideration of these matters is the user's own responsibility.Security Considerations   This entire document is about security.Author's Address   Steve Kent   BBN Communications   50 Moulton Street   Cambridge, MA 02138   Phone: (617) 873-3988   EMail: kent@BBN.COMKent                                                           [Page 32]