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Internet Engineering Task Force (IETF)                        S. HartmanRequest for Comments: 6113                             Painless SecurityUpdates:4120                                                     L. ZhuCategory: Standards Track                          Microsoft CorporationISSN: 2070-1721                                               April 2011A Generalized Framework for Kerberos Pre-AuthenticationAbstract   Kerberos is a protocol for verifying the identity of principals   (e.g., a workstation user or a network server) on an open network.   The Kerberos protocol provides a facility called pre-authentication.   Pre-authentication mechanisms can use this facility to extend the   Kerberos protocol and prove the identity of a principal.   This document describes a more formal model for this facility.  The   model describes what state in the Kerberos request a pre-   authentication mechanism is likely to change.  It also describes how   multiple pre-authentication mechanisms used in the same request will   interact.   This document also provides common tools needed by multiple pre-   authentication mechanisms.  One of these tools is a secure channel   between the client and the key distribution center with a reply key   strengthening mechanism; this secure channel can be used to protect   the authentication exchange and thus eliminate offline dictionary   attacks.  With these tools, it is relatively straightforward to chain   multiple authentication mechanisms, utilize a different key   management system, or support a new key agreement algorithm.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6113.Hartman & Zhu                Standards Track                    [Page 1]

RFC 6113               Kerberos Preauth Framework             April 2011Copyright Notice   Copyright (c) 2011 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Hartman & Zhu                Standards Track                    [Page 2]

RFC 6113               Kerberos Preauth Framework             April 2011Table of Contents1. Introduction ....................................................41.1. Conventions and Terminology Used in This Document ..........51.2. Conformance Requirements ...................................52. Model for Pre-Authentication ....................................62.1. Information Managed by the Pre-Authentication Model ........72.2. Initial Pre-Authentication Required Error ..................92.3. Client to KDC .............................................102.4. KDC to Client .............................................113. Pre-Authentication Facilities ..................................123.1. Client Authentication Facility ............................133.2. Strengthening Reply Key Facility ..........................133.3. Replace Reply Key Facility ................................143.4. KDC Authentication Facility ...............................154. Requirements for Pre-Authentication Mechanisms .................154.1. Protecting Requests/Responses .............................165. Tools for Use in Pre-Authentication Mechanisms .................175.1. Combining Keys ............................................175.2. Managing States for the KDC ...............................195.3. Pre-Authentication Set ....................................205.4. Definition of Kerberos FAST Padata ........................235.4.1. FAST Armors ........................................245.4.2. FAST Request .......................................265.4.3. FAST Response ......................................30           5.4.4. Authenticated Kerberos Error Messages Using                  Kerberos FAST ......................................335.4.5. Outer and Inner Requests ...........................345.4.6. The Encrypted Challenge FAST Factor ................345.5. Authentication Strength Indication ........................366. Assigned Constants .............................................376.1. New Errors ................................................376.2. Key Usage Numbers .........................................376.3. Authorization Data Elements ...............................376.4. New PA-DATA Types .........................................377. IANA Considerations ............................................387.1. Pre-Authentication and Typed Data .........................387.2. Fast Armor Types ..........................................407.3. FAST Options ..............................................408. Security Considerations ........................................419. Acknowledgements ...............................................4210. References ....................................................4310.1. Normative References .....................................4310.2. Informative References ...................................43Appendix A. Test Vectors for KRB-FX-CF2 ...........................45Appendix B. ASN.1 Module ..........................................46Hartman & Zhu                Standards Track                    [Page 3]

RFC 6113               Kerberos Preauth Framework             April 20111.  Introduction   The core Kerberos specification [RFC4120] treats pre-authentication   data (padata) as an opaque typed hole in the messages to the key   distribution center (KDC) that may influence the reply key used to   encrypt the KDC reply.  This generality has been useful: pre-   authentication data is used for a variety of extensions to the   protocol, many outside the expectations of the initial designers.   However, this generality makes designing more common types of pre-   authentication mechanisms difficult.  Each mechanism needs to specify   how it interacts with other mechanisms.  Also, tasks such as   combining a key with the long-term secrets or proving the identity of   the user are common to multiple mechanisms.  Where there are   generally well-accepted solutions to these problems, it is desirable   to standardize one of these solutions so mechanisms can avoid   duplication of work.  In other cases, a modular approach to these   problems is appropriate.  The modular approach will allow new and   better solutions to common pre-authentication problems to be used by   existing mechanisms as they are developed.   This document specifies a framework for Kerberos pre-authentication   mechanisms.  It defines the common set of functions that pre-   authentication mechanisms perform as well as how these functions   affect the state of the request and reply.  In addition, several   common tools needed by pre-authentication mechanisms are provided.   Unlike [RFC3961], this framework is not complete -- it does not   describe all the inputs and outputs for the pre-authentication   mechanisms.  Pre-authentication mechanism designers should try to be   consistent with this framework because doing so will make their   mechanisms easier to implement.  Kerberos implementations are likely   to have plug-in architectures for pre-authentication; such   architectures are likely to support mechanisms that follow this   framework plus commonly used extensions.  This framework also   facilitates combining multiple pre-authentication mechanisms, each of   which may represent an authentication factor, into a single multi-   factor pre-authentication mechanism.   One of these common tools is the flexible authentication secure   tunneling (FAST) padata type.  FAST provides a protected channel   between the client and the key distribution center (KDC), and it can   optionally deliver key material used to strengthen the reply key   within the protected channel.  Based on FAST, pre-authentication   mechanisms can extend Kerberos with ease, to support, for example,   password-authenticated key exchange (PAKE) protocols with zero-   knowledge password proof (ZKPP) [EKE] [IEEE1363.2].  Any pre-   authentication mechanism can be encapsulated in the FAST messages as   defined inSection 5.4.  A pre-authentication type carried within   FAST is called a "FAST factor".  Creating a FAST factor is theHartman & Zhu                Standards Track                    [Page 4]

RFC 6113               Kerberos Preauth Framework             April 2011   easiest path to create a new pre-authentication mechanism.  FAST   factors are significantly easier to analyze from a security   standpoint than other pre-authentication mechanisms.   Mechanism designers should design FAST factors, instead of new pre-   authentication mechanisms outside of FAST.1.1.  Conventions and Terminology Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].   This document should be read only after reading the documents   describing the Kerberos cryptography framework [RFC3961] and the core   Kerberos protocol [RFC4120].  This document may freely use   terminology and notation from these documents without reference or   further explanation.   The word padata is used as a shorthand for pre-authentication data.   A conversation is the set of all authentication messages exchanged   between the client and the client's Authentication Service (AS) in   order to authenticate the client principal.  A conversation as   defined here consists of all messages that are necessary to complete   the authentication between the client and the client's AS.  In the   Ticket Granting Service (TGS) exchange, a conversation consists of   the request message and the reply message.  The term conversation is   defined here for both AS and TGS for convenience of discussion.  SeeSection 5.2 for specific rules on the extent of a conversation in the   AS-REQ case.  Prior to this framework, implementations needed to use   implementation-specific heuristics to determine the extent of a   conversation.   If the KDC reply in an AS exchange is verified, the KDC is   authenticated by the client.  In this document, verification of the   KDC reply is used as a synonym of authentication of the KDC.1.2.  Conformance Requirements   This section summarizes the mandatory-to-implement subset of this   specification as a convenience to implementors.  The actual   requirements and their context are stated in the body of the   document.   Clients conforming to this specification MUST support the padata   defined inSection 5.2.Hartman & Zhu                Standards Track                    [Page 5]

RFC 6113               Kerberos Preauth Framework             April 2011   Conforming implementations MUST support Kerberos FAST padata   (Section 5.4).  Conforming implementations MUST implement the   FX_FAST_ARMOR_AP_REQUEST armor type.   Conforming implementations MUST support the encrypted challenge FAST   factor (Section 5.4.6).2.  Model for Pre-Authentication   When a Kerberos client wishes to obtain a ticket, it sends an initial   Authentication Service (AS) request to the KDC.  If pre-   authentication is required but not being used, then the KDC will   respond with a KDC_ERR_PREAUTH_REQUIRED error [RFC4120].   Alternatively, if the client knows what pre-authentication to use, it   MAY optimize away a round trip and send an initial request with   padata included in the initial request.  If the client includes the   padata computed using the wrong pre-authentication mechanism or   incorrect keys, the KDC MAY return KDC_ERR_PREAUTH_FAILED with no   indication of what padata should have been included.  In that case,   the client MUST retry with no padata and examine the error data of   the KDC_ERR_PREAUTH_REQUIRED error.  If the KDC includes pre-   authentication information in the accompanying error data of   KDC_ERR_PREAUTH_FAILED, the client SHOULD process the error data and   then retry.   The conventional KDC maintains no state between two requests;   subsequent requests may even be processed by a different KDC.  On the   other hand, the client treats a series of exchanges with KDCs as a   single conversation.  Each exchange accumulates state and hopefully   brings the client closer to a successful authentication.   These models for state management are in apparent conflict.  For many   of the simpler pre-authentication scenarios, the client uses one   round trip to find out what mechanisms the KDC supports.  Then, the   next request contains sufficient pre-authentication for the KDC to be   able to return a successful reply.  For these simple scenarios, the   client only sends one request with pre-authentication data and so the   conversation is trivial.  For more complex conversations, the KDC   needs to provide the client with a cookie to include in future   requests to capture the current state of the authentication session.   Handling of multiple round-trip mechanisms is discussed inSection 5.2.   This framework specifies the behavior of Kerberos pre-authentication   mechanisms used to identify users or to modify the reply key used to   encrypt the KDC reply.  The PA-DATA typed hole may be used to carry   extensions to Kerberos that have nothing to do with proving theHartman & Zhu                Standards Track                    [Page 6]

RFC 6113               Kerberos Preauth Framework             April 2011   identity of the user or establishing a reply key.  Such extensions   are outside the scope of this framework.  However, mechanisms that do   accomplish these goals should follow this framework.   This framework specifies the minimum state that a Kerberos   implementation needs to maintain while handling a request in order to   process pre-authentication.  It also specifies how Kerberos   implementations process the padata at each step of the AS request   process.2.1.  Information Managed by the Pre-Authentication Model   The following information is maintained by the client and KDC as each   request is being processed:   o  The reply key used to encrypt the KDC reply   o  How strongly the identity of the client has been authenticated   o  Whether the reply key has been used in this conversation   o  Whether the reply key has been replaced in this conversation   o  Whether the origin of the KDC reply can be verified by the client      (i.e., whether the KDC is authenticated to the client)   Conceptually, the reply key is initially the long-term key of the   principal.  However, principals can have multiple long-term keys   because of support for multiple encryption types, salts, and   string2key parameters.  As described inSection 5.2.7.5 of the   Kerberos protocol [RFC4120], the KDC sends PA-ETYPE-INFO2 to notify   the client what types of keys are available.  Thus, in full   generality, the reply key in the pre-authentication model is actually   a set of keys.  At the beginning of a request, it is initialized to   the set of long-term keys advertised in the PA-ETYPE-INFO2 element on   the KDC.  If multiple reply keys are available, the client chooses   which one to use.  Thus, the client does not need to treat the reply   key as a set.  At the beginning of a request, the client picks a key   to use.   KDC implementations MAY choose to offer only one key in the PA-ETYPE-   INFO2 element.  Since the KDC already knows the client's list of   supported enctypes from the request, no interoperability problems areHartman & Zhu                Standards Track                    [Page 7]

RFC 6113               Kerberos Preauth Framework             April 2011   created by choosing a single possible reply key.  This way, the KDC   implementation avoids the complexity of treating the reply key as a   set.   When the padata in the request are verified by the KDC, then the   client is known to have that key; therefore, the KDC SHOULD pick the   same key as the reply key.   At the beginning of handling a message on both the client and the   KDC, the client's identity is not authenticated.  A mechanism may   indicate that it has successfully authenticated the client's   identity.  It is useful to keep track of this information on the   client in order to know what pre-authentication mechanisms should be   used.  The KDC needs to keep track of whether the client is   authenticated because the primary purpose of pre-authentication is to   authenticate the client identity before issuing a ticket.  The   handling of authentication strength using various authentication   mechanisms is discussed inSection 5.5.   Initially, the reply key is not used.  A pre-authentication mechanism   that uses the reply key to encrypt or checksum some data in the   generation of new keys MUST indicate that the reply key is used.   This state is maintained by the client and the KDC to enforce the   security requirement stated inSection 3.3 that the reply key SHOULD   NOT be replaced after it is used.   Initially, the reply key is not replaced.  If a mechanism implements   the Replace Reply Key facility discussed inSection 3.3, then the   state MUST be updated to indicate that the reply key has been   replaced.  Once the reply key has been replaced, knowledge of the   reply key is insufficient to authenticate the client.  The reply key   is marked as replaced in exactly the same situations as the KDC reply   is marked as not being verified to the client principal.  However,   while mechanisms can verify the KDC reply to the client, once the   reply key is replaced, then the reply key remains replaced for the   remainder of the conversation.   Without pre-authentication, the client knows that the KDC reply is   authentic and has not been modified because it is encrypted in a   long-term key of the client.  Only the KDC and the client know that   key.  So, at the start of a conversation, the KDC reply is presumed   to be verified using the client's long-term key.  It should be noted   that in this document, verifying the KDC reply means authenticating   the KDC, and these phrases are used interchangeably.  Any pre-   authentication mechanism that sets a new reply key not based on the   principal's long-term secret MUST either verify the KDC reply some   other way or indicate that the reply is not verified.  If a mechanism   indicates that the reply is not verified, then the clientHartman & Zhu                Standards Track                    [Page 8]

RFC 6113               Kerberos Preauth Framework             April 2011   implementation MUST return an error unless a subsequent mechanism   verifies the reply.  The KDC needs to track this state so it can   avoid generating a reply that is not verified.   In this specification, KDC verification/authentication refers to the   level of authentication of the KDC to the client provided byRFC4120.  There is a stronger form of KDC verification that, while   sometimes important in Kerberos deployments, is not addressed in this   specification: the typical Kerberos request does not provide a way   for the client machine to know that it is talking to the correct KDC.   Someone who can inject packets into the network between the client   machine and the KDC and who knows the password that the user will   give to the client machine can generate a KDC reply that will decrypt   properly.  So, if the client machine needs to authenticate that the   user is in fact the named principal, then the client machine needs to   do a TGS request for itself as a service.  Some pre-authentication   mechanisms may provide a way for the client machine to authenticate   the KDC.  Examples of this include signing the reply that can be   verified using a well-known public key or providing a ticket for the   client machine as a service.2.2.  Initial Pre-Authentication Required Error   Typically, a client starts a conversation by sending an initial   request with no pre-authentication.  If the KDC requires pre-   authentication, then it returns a KDC_ERR_PREAUTH_REQUIRED message.   After the first reply with the KDC_ERR_PREAUTH_REQUIRED error code,   the KDC returns the error code KDC_ERR_MORE_PREAUTH_DATA_REQUIRED   (defined inSection 5.2) for pre-authentication configurations that   use multi-round-trip mechanisms; seeSection 2.4 for details of that   case.   The KDC needs to choose which mechanisms to offer the client.  The   client needs to be able to choose what mechanisms to use from the   first message.  For example, consider the KDC that will accept   mechanism A followed by mechanism B or alternatively the single   mechanism C.  A client that supports A and C needs to know that it   should not bother trying A.   Mechanisms can either be sufficient on their own or can be part of an   authentication set -- a group of mechanisms that all need to   successfully complete in order to authenticate a client.  Some   mechanisms may only be useful in authentication sets; others may be   useful alone or in authentication sets.  For the second group of   mechanisms, KDC policy dictates whether the mechanism will be part of   an authentication set, offered alone, or both.  For each mechanism   that is offered alone (even if it is also offered in an   authentication set), the KDC includes the pre-authentication type IDHartman & Zhu                Standards Track                    [Page 9]

RFC 6113               Kerberos Preauth Framework             April 2011   of the mechanism in the padata sequence returned in the   KDC_ERR_PREAUTH_REQUIRED error.  Mechanisms that are only offered as   part of an authentication set are not directly represented in the   padata sequence returned in the KDC_ERR_PREAUTH_REQUIRED error,   although they are represented in the PA-AUTHENTICATION-SET sequence.   The KDC SHOULD NOT send data that is encrypted in the long-term   password-based key of the principal.  Doing so has the same security   exposures as the Kerberos protocol without pre-authentication.  There   are few situations where the KDC needs to expose cipher text   encrypted in a weak key before the client has proven knowledge of   that key, and where pre-authentication is desirable.2.3.  Client to KDC   This description assumes that a client has already received a   KDC_ERR_PREAUTH_REQUIRED from the KDC.  If the client performs   optimistic pre-authentication, then the client needs to guess values   for the information it would normally receive from that error   response or use cached information obtained in prior interactions   with the KDC.   The client starts by initializing the pre-authentication state as   specified.  It then processes the padata in the   KDC_ERR_PREAUTH_REQUIRED.   When processing the response to the KDC_ERR_PREAUTH_REQUIRED, the   client MAY ignore any padata it chooses unless doing so violates a   specification to which the client conforms.  Clients conforming to   this specification MUST NOT ignore the padata defined inSection 5.2.   Clients SHOULD choose one authentication set or mechanism that could   lead to authenticating the user and ignore other such mechanisms.   However, this rule does not affect the processing of padata unrelated   to this framework; clients SHOULD process such padata normally.   Since the list of mechanisms offered by the KDC is in the decreasing   preference order, clients typically choose the first mechanism or   authentication set that the client can usefully perform.  If a client   chooses to ignore padata, it MUST NOT process the padata, allow the   padata to affect the pre-authentication state, or respond to the   padata.   For each instance of padata the client chooses to process, the client   processes the padata and modifies the pre-authentication state as   required by that mechanism.Hartman & Zhu                Standards Track                   [Page 10]

RFC 6113               Kerberos Preauth Framework             April 2011   After processing the padata in the KDC error, the client generates a   new request.  It processes the pre-authentication mechanisms in the   order in which they will appear in the next request, updating the   state as appropriate.  The request is sent when it is complete.2.4.  KDC to Client   When a KDC receives an AS request from a client, it needs to   determine whether it will respond with an error or an AS reply.   There are many causes for an error to be generated that have nothing   to do with pre-authentication; they are discussed in the core   Kerberos specification.   From the standpoint of evaluating the pre-authentication, the KDC   first starts by initializing the pre-authentication state.  If a PA-   FX-COOKIE pre-authentication data item is present, it is processed   first; seeSection 5.2 for a definition.  It then processes the   padata in the request.  As mentioned inSection 2.3, the KDC MAY   ignore padata that are inappropriate for the configuration and MUST   ignore padata of an unknown type.  The KDC MUST NOT ignore padata of   types used in previous messages.  For example, if a KDC issues a   KDC_ERR_PREAUTH_REQUIRED error including padata of type x, then the   KDC cannot ignore padata of type x received in an AS-REQ message from   the client.   At this point, the KDC decides whether it will issue an error or a   reply.  Typically, a KDC will issue a reply if the client's identity   has been authenticated to a sufficient degree.   In the case of a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error, the KDC   first starts by initializing the pre-authentication state.  Then, it   processes any padata in the client's request in the order provided by   the client.  Mechanisms that are not understood by the KDC are   ignored.  Next, it generates padata for the error response, modifying   the pre-authentication state appropriately as each mechanism is   processed.  The KDC chooses the order in which it will generate   padata (and thus the order of padata in the response), but it needs   to modify the pre-authentication state consistently with the choice   of order.  For example, if some mechanism establishes an   authenticated client identity, then the subsequent mechanisms in the   generated response receive this state as input.  After the padata are   generated, the error response is sent.  Typically, the errors with   the code KDC_ERR_MORE_PREAUTH_DATA_REQUIRED in a conversation will   include KDC state, as discussed inSection 5.2.   To generate a final reply, the KDC generates the padata modifying the   pre-authentication state as necessary.  Then, it generates the final   response, encrypting it in the current pre-authentication reply key.Hartman & Zhu                Standards Track                   [Page 11]

RFC 6113               Kerberos Preauth Framework             April 20113.  Pre-Authentication Facilities   Pre-authentication mechanisms can be thought of as providing various   conceptual facilities.  This serves two useful purposes.  First,   mechanism authors can choose only to solve one specific small   problem.  It is often useful for a mechanism designed to offer key   management not to directly provide client authentication but instead   to allow one or more other mechanisms to handle this need.  Secondly,   thinking about the abstract services that a mechanism provides yields   a minimum set of security requirements that all mechanisms providing   that facility must meet.  These security requirements are not   complete; mechanisms will have additional security requirements based   on the specific protocol they employ.   A mechanism is not constrained to only offering one of these   facilities.  While such mechanisms can be designed and are sometimes   useful, many pre-authentication mechanisms implement several   facilities.  It is often easier to construct a secure, simple   solution by combining multiple facilities in a single mechanism than   by solving the problem in full generality.  Even when mechanisms   provide multiple facilities, they need to meet the security   requirements for all the facilities they provide.  If the FAST factor   approach is used, it is likely that one or a small number of   facilities can be provided by a single mechanism without complicating   the security analysis.   According to Kerberos extensibility rules (Section 1.5 of the   Kerberos specification [RFC4120]), an extension MUST NOT change the   semantics of a message unless a recipient is known to understand that   extension.  Because a client does not know that the KDC supports a   particular pre-authentication mechanism when it sends an initial   request, a pre-authentication mechanism MUST NOT change the semantics   of the request in a way that will break a KDC that does not   understand that mechanism.  Similarly, KDCs MUST NOT send messages to   clients that affect the core semantics unless the client has   indicated support for the message.   The only state in this model that would break the interpretation of a   message is changing the expected reply key.  If one mechanism changed   the reply key and a later mechanism used that reply key, then a KDC   that interpreted the second mechanism but not the first would fail to   interpret the request correctly.  In order to avoid this problem,   extensions that change core semantics are typically divided into two   parts.  The first part proposes a change to the core semantic -- for   example, proposes a new reply key.  The second part acknowledges that   the extension is understood and that the change takes effect.Section 3.2 discusses how to design mechanisms that modify the reply   key to be split into a proposal and acceptance without requiringHartman & Zhu                Standards Track                   [Page 12]

RFC 6113               Kerberos Preauth Framework             April 2011   additional round trips to use the new reply key in subsequent pre-   authentication.  Other changes in the state described inSection 2.1   can safely be ignored by a KDC that does not understand a mechanism.   Mechanisms that modify the behavior of the request outside the scope   of this framework need to carefully consider the Kerberos   extensibility rules to avoid similar problems.3.1.  Client Authentication Facility   The Client Authentication facility proves the identity of a user to   the KDC before a ticket is issued.  Examples of mechanisms   implementing this facility include the encrypted timestamp facility,   defined inSection 5.2.7.2 of the Kerberos specification [RFC4120].   Mechanisms that provide this facility are expected to mark the client   as authenticated.   Mechanisms implementing this facility SHOULD require the client to   prove knowledge of the reply key before transmitting a successful KDC   reply.  Otherwise, an attacker can intercept the pre-authentication   exchange and get a reply to attack.  One way of proving the client   knows the reply key is to implement the Replace Reply Key facility   along with this facility.  The Public Key Cryptography for Initial   Authentication in Kerberos (PKINIT) mechanism [RFC4556] implements   Client Authentication alongside Replace Reply Key.   If the reply key has been replaced, then mechanisms such as   encrypted-timestamp that rely on knowledge of the reply key to   authenticate the client MUST NOT be used.3.2.  Strengthening Reply Key Facility   Particularly when dealing with keys based on passwords, it is   desirable to increase the strength of the key by adding additional   secrets to it.  Examples of sources of additional secrets include the   results of a Diffie-Hellman key exchange or key bits from the output   of a smart card [KRB-WG.SAM].  Typically, these additional secrets   can be first combined with the existing reply key and then converted   to a protocol key using tools defined inSection 5.1.   Typically, a mechanism implementing this facility will know that the   other side of the exchange supports the facility before the reply key   is changed.  For example, a mechanism might need to learn the   certificate for a KDC before encrypting a new key in the public key   belonging to that certificate.  However, if a mechanism implementing   this facility wishes to modify the reply key before knowing that the   other party in the exchange supports the mechanism, it proposes   modifying the reply key.  The other party then includes a message   indicating that the proposal is accepted if it is understood andHartman & Zhu                Standards Track                   [Page 13]

RFC 6113               Kerberos Preauth Framework             April 2011   meets policy.  In many cases, it is desirable to use the new reply   key for client authentication and for other facilities.  Waiting for   the other party to accept the proposal and actually modify the reply   key state would add an additional round trip to the exchange.   Instead, mechanism designers are encouraged to include a typed hole   for additional padata in the message that proposes the reply key   change.  The padata included in the typed hole are generated assuming   the new reply key.  If the other party accepts the proposal, then   these padata are considered as an inner level.  As with the outer   level, one authentication set or mechanism is typically chosen for   client authentication, along with auxiliary mechanisms such as KDC   cookies, and other mechanisms are ignored.  When mechanisms include   such a container, the hint provided for use in authentication sets   (as defined inSection 5.3) MUST contain a sequence of inner   mechanisms along with hints for those mechanisms.  The party   generating the proposal can determine whether the padata were   processed based on whether the proposal for the reply key is   accepted.   The specific formats of the proposal message, including where padata   are included, is a matter for the mechanism specification.   Similarly, the format of the message accepting the proposal is   mechanism specific.   Mechanisms implementing this facility and including a typed hole for   additional padata MUST checksum that padata using a keyed checksum or   encrypt the padata.  This requirement protects against modification   of the contents of the typed hole.  By modifying these contents, an   attacker might be able to choose which mechanism is used to   authenticate the client, or to convince a party to provide text   encrypted in a key that the attacker had manipulated.  It is   important that mechanisms strengthen the reply key enough that using   it to checksum padata is appropriate.3.3.  Replace Reply Key Facility   The Replace Reply Key facility replaces the key in which a successful   AS reply will be encrypted.  This facility can only be used in cases   where knowledge of the reply key is not used to authenticate the   client.  The new reply key MUST be communicated to the client and the   KDC in a secure manner.  This facility MUST NOT be used if there can   be a man-in-the-middle between the client and the KDC.  Mechanisms   implementing this facility MUST mark the reply key as replaced in the   pre-authentication state.  Mechanisms implementing this facility MUST   either provide a mechanism to verify the KDC reply to the client or   mark the reply as unverified in the pre-authentication state.   Mechanisms implementing this facility SHOULD NOT be used if a   previous mechanism has used the reply key.Hartman & Zhu                Standards Track                   [Page 14]

RFC 6113               Kerberos Preauth Framework             April 2011   As with the Strengthening Reply Key facility, Kerberos extensibility   rules require that the reply key not be changed unless both sides of   the exchange understand the extension.  In the case of this facility,   it will likely be the case for both sides to know that the facility   is available by the time that the new key is available to be used.   However, mechanism designers can use a container for padata in a   proposal message, as discussed inSection 3.2, if appropriate.3.4.  KDC Authentication Facility   This facility verifies that the reply comes from the expected KDC.   In traditional Kerberos, the KDC and the client share a key, so if   the KDC reply can be decrypted, then the client knows that a trusted   KDC responded.  Note that the client machine cannot trust the client   unless the machine is presented with a service ticket for it   (typically, the machine can retrieve this ticket by itself).   However, if the reply key is replaced, some mechanism is required to   verify the KDC.  Pre-authentication mechanisms providing this   facility allow a client to determine that the expected KDC has   responded even after the reply key is replaced.  They mark the pre-   authentication state as having been verified.4.  Requirements for Pre-Authentication Mechanisms   This section lists requirements for specifications of pre-   authentication mechanisms.   For each message in the pre-authentication mechanism, the   specification describes the pa-type value to be used and the contents   of the message.  The processing of the message by the sender and   recipient is also specified.  This specification needs to include all   modifications to the pre-authentication state.   Generally, mechanisms have a message that can be sent in the error   data of the KDC_ERR_PREAUTH_REQUIRED error message or in an   authentication set.  If the client needs information, such as trusted   certificate authorities, in order to determine if it can use the   mechanism, then this information should be in that message.  In   addition, such mechanisms should also define a pa-hint to be included   in authentication sets.  Often, the same information included in the   padata-value is appropriate to include in the pa-hint (as defined inSection 5.3).   In order to ease security analysis, the mechanism specification   should describe what facilities from this document are offered by the   mechanism.  For each facility, the security considerations section of   the mechanism specification should show that the securityHartman & Zhu                Standards Track                   [Page 15]

RFC 6113               Kerberos Preauth Framework             April 2011   requirements of that facility are met.  This requirement is   applicable to any FAST factor that provides authentication   information.   Significant problems have resulted in the specification of Kerberos   protocols because much of the KDC exchange is not protected against   alteration.  The security considerations section should discuss   unauthenticated plaintext attacks.  It should either show that   plaintext is protected or discuss what harm an attacker could do by   modifying the plaintext.  It is generally acceptable for an attacker   to be able to cause the protocol negotiation to fail by modifying   plaintext.  More significant attacks should be evaluated carefully.   As discussed inSection 5.2, there is no guarantee that a client will   use the same KDCs for all messages in a conversation.  The mechanism   specification needs to show why the mechanism is secure in this   situation.  The hardest problem to deal with, especially for   challenge/response mechanisms is to make sure that the same response   cannot be replayed against two KDCs while allowing the client to talk   to any KDC.4.1.  Protecting Requests/Responses   Mechanism designers SHOULD protect cleartext portions of pre-   authentication data.  Various denial-of-service attacks and downgrade   attacks against Kerberos are possible unless plaintexts are somehow   protected against modification.  An early design goal of Kerberos   Version 5 [RFC4120] was to avoid encrypting more of the   authentication exchange than was required.  (Version 4 doubly-   encrypted the encrypted part of a ticket in a KDC reply, for   example).  This minimization of encryption reduces the load on the   KDC and busy servers.  Also, during the initial design of Version 5,   the existence of legal restrictions on the export of cryptography   made it desirable to minimize of the number of uses of encryption in   the protocol.  Unfortunately, performing this minimization created   numerous instances of unauthenticated security-relevant plaintext   fields.   Mechanisms MUST guarantee that by the end of a successful   authentication exchange, both the client and the KDC have verified   all the plaintext sent by the other party.  If there is more than one   round trip in the exchange, mechanisms MUST additionally guarantee   that no individual messages were reordered or replayed from a   previous exchange.  Strategies for accomplishing this include using   message authentication codes (MACs) to protect the plaintext as it is   sent including some form of nonce or cookie to allow for the chaining   of state from one message to the next or exchanging a MAC of the   entire conversation after a key is established.Hartman & Zhu                Standards Track                   [Page 16]

RFC 6113               Kerberos Preauth Framework             April 2011   Mechanism designers need to provide a strategy for updating   cryptographic algorithms, such as defining a new pre-authentication   type for each algorithm or taking advantage of the client's list of   supportedRFC 3961 encryption types to indicate the client's support   for cryptographic algorithms.   Primitives defined in [RFC3961] are RECOMMENDED for integrity   protection and confidentiality.  Mechanisms based on these primitives   are crypto-agile as the result of using [RFC3961] along with   [RFC4120].  The advantage afforded by crypto-agility is the ability   to incrementally deploy a fix specific to a particular algorithm thus   avoid a multi-year standardization and deployment cycle, when real   attacks do arise against that algorithm.   Note that data used by FAST factors (defined inSection 5.4) is   encrypted in a protected channel; thus, they do not share the un-   authenticated-text issues with mechanisms designed as full-blown pre-   authentication mechanisms.5.  Tools for Use in Pre-Authentication Mechanisms   This section describes common tools needed by multiple pre-   authentication mechanisms.  By using these tools, mechanism designers   can use a modular approach to specify mechanism details and ease   security analysis.5.1.  Combining Keys   Frequently, a weak key needs to be combined with a stronger key   before use.  For example, passwords are typically limited in size and   insufficiently random: therefore, it is desirable to increase the   strength of the keys based on passwords by adding additional secrets.   An additional source of secrecy may come from hardware tokens.   This section provides standard ways to combine two keys into one.   KRB-FX-CF1() is defined to combine two passphrases.       KRB-FX-CF1(UTF-8 string, UTF-8 string) -> (UTF-8 string)       KRB-FX-CF1(x, y) := x || y   Where || denotes concatenation.  The strength of the final key is   roughly the total strength of the individual keys being combined,   assuming that the string_to_key() function [RFC3961] uses all its   input evenly.Hartman & Zhu                Standards Track                   [Page 17]

RFC 6113               Kerberos Preauth Framework             April 2011   An example usage of KRB-FX-CF1() is when a device provides random but   short passwords, the password is often combined with a personal   identification number (PIN).  The password and the PIN can be   combined using KRB-FX-CF1().   KRB-FX-CF2() combines two protocol keys based on the pseudo-random()   function defined in [RFC3961].   Given two input keys, K1 and K2, where K1 and K2 can be of two   different enctypes, the output key of KRB-FX-CF2(), K3, is derived as   follows:       KRB-FX-CF2(protocol key, protocol key, octet string,                 octet string)  ->  (protocol key)       PRF+(K1, pepper1) -> octet-string-1       PRF+(K2, pepper2) -> octet-string-2       KRB-FX-CF2(K1, K2, pepper1, pepper2) :=              random-to-key(octet-string-1 ^ octet-string-2)   Where ^ denotes the exclusive-OR operation.  PRF+() is defined as   follows:    PRF+(protocol key, octet string) -> (octet string)    PRF+(key, shared-info) := pseudo-random( key,  1 || shared-info ) ||                  pseudo-random( key, 2 || shared-info ) ||                  pseudo-random( key, 3 || shared-info ) || ...   Here the counter value 1, 2, 3, and so on are encoded as a one-octet   integer.  The pseudo-random() operation is specified by the enctype   of the protocol key.  PRF+() uses the counter to generate enough bits   as needed by the random-to-key() [RFC3961] function for the   encryption type specified for the resulting key; unneeded bits are   removed from the tail.  Unless otherwise specified, the resulting   enctype of KRB-FX-CF2 is the enctype of k1.  The pseudo-random()   operation is theRFC 3961 pseudo-random() operation for the   corresponding input key; the random-to-key() operation is theRFC3961 random-to-key operation for the resulting key.   Mechanism designers MUST specify the values for the input parameter   pepper1 and pepper2 when combining two keys using KRB-FX-CF2().  The   pepper1 and pepper2 MUST be distinct so that if the two keys being   combined are the same, the resulting key is not a trivial key.Hartman & Zhu                Standards Track                   [Page 18]

RFC 6113               Kerberos Preauth Framework             April 20115.2.  Managing States for the KDC   Kerberos KDCs are stateless in that there is no requirement that   clients will choose the same KDC for the second request in a   conversation.  Proxies or other intermediate nodes may also influence   KDC selection.  So, each request from a client to a KDC must include   sufficient information that the KDC can regenerate any needed state.   This is accomplished by giving the client a potentially long opaque   cookie in responses to include in future requests in the same   conversation.  The KDC MAY respond that a conversation is too old and   needs to restart by responding with a KDC_ERR_PREAUTH_EXPIRED error.       KDC_ERR_PREAUTH_EXPIRED            90   When a client receives this error, the client SHOULD abort the   existing conversation, and restart a new one.   An example, where more than one message from the client is needed, is   when the client is authenticated based on a challenge/response   scheme.  In that case, the KDC needs to keep track of the challenge   issued for a client authentication request.   The PA-FX-COOKIE padata type is defined in this section to facilitate   state management in the AS exchange.  These padata are sent by the   KDC when the KDC requires state for a future transaction.  The client   includes this opaque token in the next message in the conversation.   The token may be relatively large; clients MUST be prepared for   tokens somewhat larger than the size of all messages in a   conversation.       PA-FX-COOKIE                       133           -- Stateless cookie that is not tied to a specific KDC.   The corresponding padata-value field [RFC4120] contains an opaque   token that will be echoed by the client in its response to an error   from the KDC.   The cookie token is generated by the KDC and transmitted in a PA-FX-   COOKIE pre-authentication data item of a KRB-ERROR message.  The   client MUST copy the exact cookie encapsulated in a PA-FX-COOKIE data   element into the next message of the same conversation.  The content   of the cookie field is a local matter of the KDC.  As a result, it is   not generally possible to mix KDC implementations from different   vendors in the same realm.  However, the KDC MUST construct the   cookie token in such a manner that a malicious client cannot subvert   the authentication process by manipulating the token.  The KDC   implementation needs to consider expiration of tokens, key rollover,   and other security issues in token design.  The content of the cookieHartman & Zhu                Standards Track                   [Page 19]

RFC 6113               Kerberos Preauth Framework             April 2011   field is likely specific to the pre-authentication mechanisms used to   authenticate the client.  If a client authentication response can be   replayed to multiple KDCs via the PA-FX-COOKIE mechanism, an   expiration in the cookie is RECOMMENDED to prevent the response being   presented indefinitely.  Implementations need to consider replay both   of an entire conversation and of messages within a conversation when   designing what information is stored in a cookie and how pre-   authentication mechanisms are implemented.   If at least one more message for a mechanism or a mechanism set is   expected by the KDC, the KDC returns a   KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error with a PA-FX-COOKIE to   identify the conversation with the client, according toSection 2.2.   The cookie is not expected to stay constant for a conversation: the   KDC is expected to generate a new cookie for each message.        KDC_ERR_MORE_PREAUTH_DATA_REQUIRED   91   A client MAY throw away the state associated with a conversation and   begin a new conversation by discarding its state and not including a   cookie in the first message of a conversation.  KDCs that comply with   this specification MUST include a cookie in a response when the   client can continue the conversation.  In particular, a KDC MUST   include a cookie in a KDC_ERR_PREAUTH_REQUIRED or   KDC_ERR_MORE_PREAUTH_DATA_REQUIRED.  KDCs SHOULD include a cookie in   errors containing additional information allowing a client to retry.   One reasonable strategy for meeting these requirements is to always   include a cookie in KDC errors.   A KDC MAY indicate that it is terminating a conversation by not   including a cookie in a response.  When FAST is used, clients can   assume that the absence of a cookie means that the KDC is ending the   conversation.  Similarly, if a cookie is seen at all during a   conversation, clients MAY assume that the absence of a cookie in a   future message means that the KDC is ending the conversation.   Clients also need to deal with KDCs, prior to this specification,   that do not include cookies; if neither cookies nor FAST are used in   a conversation, the absence of a cookie is not a strong indication   that the KDC is terminating the conversation.5.3.  Pre-Authentication Set   If all mechanisms in a group need to successfully complete in order   to authenticate a client, the client and the KDC SHOULD use the PA-   AUTHENTICATION-SET padata element.        PA-AUTHENTICATION-SET              134Hartman & Zhu                Standards Track                   [Page 20]

RFC 6113               Kerberos Preauth Framework             April 2011   A PA-AUTHENTICATION-SET padata element contains the ASN.1 DER   encoding of the PA-AUTHENTICATION-SET structure:        PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM        PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {            pa-type      [0] Int32,                -- same as padata-type.            pa-hint      [1] OCTET STRING OPTIONAL,            pa-value     [2] OCTET STRING OPTIONAL,            ...        }   The pa-type field of the PA-AUTHENTICATION-SET-ELEM structure   contains the corresponding value of padata-type in PA-DATA [RFC4120].   Associated with the pa-type is a pa-hint, which is an octet string   specified by the pre-authentication mechanism.  This hint may provide   information for the client that helps it determine whether the   mechanism can be used.  For example, a public-key mechanism might   include the certificate authorities it trusts in the hint info.  Most   mechanisms today do not specify hint info; if a mechanism does not   specify hint info, the KDC MUST NOT send a hint for that mechanism.   To allow future revisions of mechanism specifications to add hint   info, clients MUST ignore hint info received for mechanisms that the   client believes do not support hint info.  The pa-value element of   the PA-AUTHENTICATION-SET-ELEM sequence is included to carry the   first padata-value from the KDC to the client.  If the client chooses   this authentication set, then the client MUST process this pa-value.   The pa-value element MUST be absent for all but the first entry in   the authentication set.  Clients MUST ignore the pa-value for the   second and following entries in the authentication set.   If the client chooses an authentication set, then its first AS-REQ   message MUST contain a PA-AUTH-SET-SELECTED padata element.  This   element contains the encoding of the PA-AUTHENTICATION-SET sequence   received from the KDC corresponding to the authentication set that is   chosen.  The client MUST use the same octet values received from the   KDC; it cannot re-encode the sequence.  This allows KDCs to use bit-   wise comparison to identify the selected authentication set.   Permitting bit-wise comparison may limit the ability to use certain   pre-authentication mechanisms that generate a dynamic challenge in an   authentication set with optimistic selection of an authentication   set.  As with other optimistic pre-authentication failures, the KDC   MAY return KDC_ERR_PREAUTH_FAILED with a new list of pre-   authentication mechanisms (including authentication sets) if   optimistic pre-authentication fails.  The PA-AUTH-SET-SELECTED padata   element MUST come before any padata elements from the authentication   set in the padata sequence in the AS-REQ message.  The client MAYHartman & Zhu                Standards Track                   [Page 21]

RFC 6113               Kerberos Preauth Framework             April 2011   cache authentication sets from prior messages and use them to   construct an optimistic initial AS-REQ.  If the KDC receives a PA-   AUTH-SET-SELECTED padata element that does not correspond to an   authentication set that it would offer, then the KDC returns the   KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET error.  The e-data in this   error contains a sequence of padata just as for the   KDC_ERR_PREAUTH_REQUIRED error.         PA-AUTH-SET-SELECTED                   135         KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET 92   The PA-AUTHENTICATION-SET appears only in the first message from the   KDC to the client.  In particular, the client MAY fail if the   authentication mechanism sets change as the conversation progresses.   Clients MAY assume that the hints provided in the authentication set   contain enough information that the client knows what user interface   elements need to be displayed during the entire authentication   conversation.  Exceptional circumstances, such as expired passwords   or expired accounts, may require that additional user interface be   displayed.  Mechanism designers need to carefully consider the design   of their hints so that the client has this information.  This way,   clients can construct necessary dialogue boxes or wizards based on   the authentication set and can present a coherent user interface.   Current standards for user interfaces do not provide an acceptable   experience when the client has to ask additional questions later in   the conversation.   When indicating which sets of pre-authentication mechanisms are   supported, the KDC includes a PA-AUTHENTICATION-SET padata element   for each pre-authentication mechanism set.   The client sends the padata-value for the first mechanism it picks in   the pre-authentication set, when the first mechanism completes, the   client and the KDC will proceed with the second mechanism, and so on   until all mechanisms complete successfully.  The PA-FX-COOKIE, as   defined inSection 5.2, MUST be sent by the KDC.  One reason for this   requirement is so that the conversation can continue if the   conversation involves multiple KDCs.  KDCs MUST support clients that   do not include a cookie because they optimistically choose an   authentication set, although they MAY always return a   KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET and include a cookie in that   message.  Clients that support PA-AUTHENTICATION-SET MUST support PA-   FX-COOKIE.   Before the authentication succeeds and a ticket is returned, the   message that the client sends is an AS-REQ and the message that the   KDC sends is a KRB-ERROR message.  The error code in the KRB-ERROR   message from the KDC is KDC_ERR_MORE_PREAUTH_DATA_REQUIRED as definedHartman & Zhu                Standards Track                   [Page 22]

RFC 6113               Kerberos Preauth Framework             April 2011   inSection 5.2 and the accompanying e-data contains the DER encoding   of ASN.1 type METHOD-DATA.  The KDC includes the padata elements in   the METHOD-DATA.  If there are no padata, the e-data field is absent   in the KRB-ERROR message.   If the client sends the last message for a given mechanism, then the   KDC sends the first message for the next mechanism.  If the next   mechanism does not start with a KDC-side challenge, then the KDC   includes a padata item with the appropriate pa-type and an empty pa-   data.   If the KDC sends the last message for a particular mechanism, the KDC   also includes the first padata for the next mechanism.5.4.  Definition of Kerberos FAST Padata   As described in [RFC4120], Kerberos is vulnerable to offline   dictionary attacks.  An attacker can request an AS-REP and try   various passwords to see if they can decrypt the resulting ticket.RFC 4120 provides the encrypted timestamp pre-authentication method   that ameliorates the situation somewhat by requiring that an attacker   observe a successful authentication.  However, stronger security is   desired in many environments.  The Kerberos FAST pre-authentication   padata defined in this section provides a tool to significantly   reduce vulnerability to offline dictionary attacks.  When combined   with encrypted challenge, FAST requires an attacker to mount a   successful man-in-the-middle attack to observe ciphertext.  When   combined with host keys, FAST can even protect against active   attacks.  FAST also provides solutions to common problems for pre-   authentication mechanisms such as binding of the request and the   reply and freshness guarantee of the authentication.  FAST itself,   however, does not authenticate the client or the KDC; instead, it   provides a typed hole to allow pre-authentication data be tunneled.   A pre-authentication data element used within FAST is called a "FAST   factor".  A FAST factor captures the minimal work required for   extending Kerberos to support a new pre-authentication scheme.   A FAST factor MUST NOT be used outside of FAST unless its   specification explicitly allows so.  The typed holes in FAST messages   can also be used as generic holes for other padata that are not   intended to prove the client's identity, or establish the reply key.   New pre-authentication mechanisms SHOULD be designed as FAST factors,   instead of full-blown pre-authentication mechanisms.   FAST factors that are pre-authentication mechanisms MUST meet the   requirements inSection 4.Hartman & Zhu                Standards Track                   [Page 23]

RFC 6113               Kerberos Preauth Framework             April 2011   FAST employs an armoring scheme.  The armor can be a Ticket Granting   Ticket (TGT) obtained by the client's machine using the host keys to   pre-authenticate with the KDC, or an anonymous TGT obtained based on   anonymous PKINIT [RFC6112] [RFC4556].   The rest of this section describes the types of armors and the syntax   of the messages used by FAST.  Conforming implementations MUST   support Kerberos FAST padata.   Any FAST armor scheme MUST provide a fresh armor key for each   conversation.  Clients and KDCs can assume that if a message is   encrypted and integrity protected with a given armor key, then it is   part of the conversation using that armor key.   All KDCs in a realm MUST support FAST if FAST is offered by any KDC   as a pre-authentication mechanism.5.4.1.  FAST Armors   An armor key is used to encrypt pre-authentication data in the FAST   request and the response.  The KrbFastArmor structure is defined to   identify the armor key.  This structure contains the following two   fields: the armor-type identifies the type of armors and the armor-   value is an OCTET STRING that contains the description of the armor   scheme and the armor key.        KrbFastArmor ::= SEQUENCE {            armor-type   [0] Int32,                -- Type of the armor.            armor-value  [1] OCTET STRING,                -- Value of the armor.            ...        }   The value of the armor key is a matter of the armor type   specification.  Only one armor type is defined in this document.        FX_FAST_ARMOR_AP_REQUEST           1   The FX_FAST_ARMOR_AP_REQUEST armor is based on Kerberos tickets.   Conforming implementations MUST implement the   FX_FAST_ARMOR_AP_REQUEST armor type.  If a FAST KDC receives an   unknown armor type it MUST respond with KDC_ERR_PREAUTH_FAILED.   An armor type may be appropriate for use in armoring AS requests,   armoring TGS requests, or both.  TGS armor types MUST authenticate   the client to the KDC, typically by binding the TGT sub-session keyHartman & Zhu                Standards Track                   [Page 24]

RFC 6113               Kerberos Preauth Framework             April 2011   to the armor key.  As discussed below, it is desirable for AS armor   types to authenticate the KDC to the client, but this is not   required.   FAST implementations MUST maintain state about whether the armor   mechanism authenticates the KDC.  If it does not, then a FAST factor   that authenticates the KDC MUST be used if the reply key is replaced.5.4.1.1.  Ticket-Based Armors   This is a ticket-based armoring scheme.  The armor-type is   FX_FAST_ARMOR_AP_REQUEST, the armor-value contains an ASN.1 DER   encoded AP-REQ.  The ticket in the AP-REQ is called an armor ticket   or an armor TGT.  The subkey field in the AP-REQ MUST be present.   The armor key is defined by the following function:       armor_key = KRB-FX-CF2( subkey, ticket_session_key,                   "subkeyarmor", "ticketarmor" )   The 'ticket_session_key' is the session key from the ticket in the   ap-req.  The 'subkey' is the ap-req subkey.  This construction   guarantees that both the KDC (through the session key) and the client   (through the subkey) contribute to the armor key.   The server name field of the armor ticket MUST identify the TGS of   the target realm.  Here are three common ways in the decreasing   preference order how an armor TGT SHOULD be obtained:   1.  If the client is authenticating from a host machine whose       Kerberos realm has an authentication path to the client's realm,       the host machine obtains a TGT by using the host keys.  If the       client's realm is different than the realm of the local host, the       machine then obtains a cross-realm TGT to the client's realm as       the armor ticket.  Otherwise, the host's primary TGT is the armor       ticket.   2.  If the client's host machine cannot obtain a host ticket strictly       based onRFC 4120, but the KDC has an asymmetric signing key       whose binding with the expected KDC can be verified by the       client, the client can use anonymous PKINIT [RFC6112] [RFC4556]       to authenticate the KDC and obtain an anonymous TGT as the armor       ticket.  The armor ticket can also be a cross-realm TGT obtained       based on the initial primary TGT obtained using anonymous PKINIT       with KDC authentication.   3.  Otherwise, the client uses anonymous PKINIT to get an anonymous       TGT without KDC authentication and that TGT is the armor ticket.       Note that this mode of operation is vulnerable to man-in-the-Hartman & Zhu                Standards Track                   [Page 25]

RFC 6113               Kerberos Preauth Framework             April 2011       middle attacks at the time of obtaining the initial anonymous       armor TGT.   If anonymous PKINIT is used to obtain the armor ticket, the KDC   cannot know whether its signing key can be verified by the client;   hence, the KDC MUST be marked as unverified from the KDC's point of   view while the client could be able to authenticate the KDC by   verifying the KDC's signing key is bound with the expected KDC.  The   client needs to carefully consider the risk and benefit tradeoffs   associated with active attacks before exposing cipher text encrypted   using the user's long-term secrets when the armor does not   authenticate the KDC.   The TGS MUST reject a request if there is an AD-fx-fast-armor (71)   element in the authenticator of the pa-tgs-req padata or if the   ticket in the authenticator of a pa-tgs-req contains the AD-fx-fast-   armor authorization data element.  These tickets and authenticators   MAY be used as FAST armor tickets but not to obtain a ticket via the   TGS.  This authorization data is used in a system where the   encryption of the user's pre-authentication data is performed in an   unprivileged user process.  A privileged process can provide to the   user process a host ticket, an authenticator for use with that   ticket, and the sub-session key contained in the authenticator.  In   order for the host process to ensure that the host ticket is not   accidentally or intentionally misused, (i.e., the user process might   use the host ticket to authenticate as the host), it MUST include a   critical authorization data element of the type AD-fx-fast-armor when   providing the authenticator or in the enc-authorization-data field of   the TGS request used to obtain the TGT.  The corresponding ad-data   field of the AD-fx-fast-armor element is empty.   This armor type is only valid for AS requests; implicit armor,   described below in TGS processing, is the only supported way to   establish an armor key for the TGS at this time.5.4.2.  FAST Request   A padata type PA-FX-FAST is defined for the Kerberos FAST pre-   authentication padata.  The corresponding padata-value field   [RFC4120] contains the DER encoding of the ASN.1 type PA-FX-FAST-   REQUEST.  As with all pre-authentication types, the KDC SHOULD   advertise PA-FX-FAST in a PREAUTH_REQUIRED error.  KDCs MUST send the   advertisement of PA-FX-FAST with an empty pa-value.  Clients MUST   ignore the pa-value of PA-FX-FAST in an initial PREAUTH_REQUIRED   error.  FAST is not expected to be used in an authentication set:   clients will typically use FAST padata if available and this decision   should not depend on what other pre-authentication methods are   available.  As such, no pa-hint is defined for FAST at this time.Hartman & Zhu                Standards Track                   [Page 26]

RFC 6113               Kerberos Preauth Framework             April 2011       PA-FX-FAST                         136           -- Padata type for Kerberos FAST       PA-FX-FAST-REQUEST ::= CHOICE {           armored-data [0] KrbFastArmoredReq,           ...       }       KrbFastArmoredReq ::= SEQUENCE {           armor        [0] KrbFastArmor OPTIONAL,               -- Contains the armor that identifies the armor key.               -- MUST be present in AS-REQ.           req-checksum [1] Checksum,               -- For AS, contains the checksum performed over the type               -- KDC-REQ-BODY for the req-body field of the KDC-REQ               -- structure;               -- For TGS, contains the checksum performed over the type               -- AP-REQ in the PA-TGS-REQ padata.               -- The checksum key is the armor key, the checksum               -- type is the required checksum type for the enctype of               -- the armor key, and the key usage number is               -- KEY_USAGE_FAST_REQ_CHKSUM.           enc-fast-req [2] EncryptedData, -- KrbFastReq --               -- The encryption key is the armor key, and the key usage               -- number is KEY_USAGE_FAST_ENC.           ...       }       KEY_USAGE_FAST_REQ_CHKSUM          50       KEY_USAGE_FAST_ENC                 51   The PA-FX-FAST-REQUEST structure contains a KrbFastArmoredReq type.   The KrbFastArmoredReq encapsulates the encrypted padata.   The enc-fast-req field contains an encrypted KrbFastReq structure.   The armor key is used to encrypt the KrbFastReq structure, and the   key usage number for that encryption is KEY_USAGE_FAST_ENC.   The armor key is selected as follows:   o  In an AS request, the armor field in the KrbFastArmoredReq      structure MUST be present and the armor key is identified      according to the specification of the armor type.Hartman & Zhu                Standards Track                   [Page 27]

RFC 6113               Kerberos Preauth Framework             April 2011   o  There are two possibilities for armor for a TGS request.  If the      ticket presented in the PA-TGS-REQ authenticator is a TGT, then      the client SHOULD NOT include the armor field in the Krbfastreq      and a subkey MUST be included in the PA-TGS-REQ authenticator.  In      this case, the armor key is the same armor key that would be      computed if the TGS-REQ authenticator was used in an      FX_FAST_ARMOR_AP_REQUEST armor.  Clients MAY present a non-TGT in      the PA-TGS-REQ authenticator and omit the armor field, in which      case the armor key is the same that would be computed if the      authenticator were used in an FX_FAST_ARMOR_AP_REQUEST armor.      This is the only case where a ticket other than a TGT can be used      to establish an armor key; even though the armor key is computed      the same as an FX_FAST_ARMOR_AP_REQUEST, a non-TGT cannot be used      as an armor ticket in FX_FAST_ARMOR_AP_REQUEST.  Alternatively, a      client MAY use an armor type defined in the future for use with      the TGS request.   The req-checksum field contains a checksum computed differently for   AS and TGS.  For an AS-REQ, it is performed over the type KDC-REQ-   BODY for the req-body field of the KDC-REQ structure of the   containing message; for a TGS-REQ, it is performed over the type AP-   REQ in the PA-TGS-REQ padata of the TGS request.  The checksum key is   the armor key, and the checksum type is the required checksum type   for the enctype of the armor key per [RFC3961].  This checksum MUST   be a keyed checksum and it is included in order to bind the FAST   padata to the outer request.  A KDC that implements FAST will ignore   the outer request, but including a checksum is relatively cheap and   may prevent confusing behavior.   The KrbFastReq structure contains the following information:        KrbFastReq ::= SEQUENCE {            fast-options [0] FastOptions,                -- Additional options.            padata       [1] SEQUENCE OF PA-DATA,                -- padata typed holes.            req-body     [2] KDC-REQ-BODY,                -- Contains the KDC request body as defined in Section                -- 5.4.1 of [RFC4120].                -- This req-body field is preferred over the outer field                -- in the KDC request.             ...        }Hartman & Zhu                Standards Track                   [Page 28]

RFC 6113               Kerberos Preauth Framework             April 2011   The fast-options field indicates various options that are to modify   the behavior of the KDC.  The following options are defined:        FastOptions ::= KerberosFlags            -- reserved(0),            -- hide-client-names(1),       Bits    Name                    Description      -----------------------------------------------------------------       0     RESERVED              Reserved for future expansion of this                                   field.       1     hide-client-names     Requesting the KDC to hide client                                   names in the KDC response, as                                   described next in this section.       16    kdc-follow-referrals  reserved [REFERRALS].   Bits 1 through 15 inclusive (with bit 1 and bit 15 included) are   critical options.  If the KDC does not support a critical option, it   MUST fail the request with KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS, and   there is no accompanying e-data defined in this document for this   error code.  Bit 16 and onward (with bit 16 included) are non-   critical options.  KDCs conforming to this specification ignore   unknown non-critical options.        KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS   93   The hide-client-names Option      The Kerberos response defined in [RFC4120] contains the client      identity in cleartext.  This makes traffic analysis      straightforward.  The hide-client-names option is designed to      complicate traffic analysis.  If the hide-client-names option is      set, the KDC implementing PA-FX-FAST MUST identify the client as      the anonymous principal [RFC6112] in the KDC reply and the error      response.  Hence, this option is set by the client if it wishes to      conceal the client identity in the KDC response.  A conforming KDC      ignores the client principal name in the outer KDC-REQ-BODY field,      and identifies the client using the cname and crealm fields in the      req-body field of the KrbFastReq structure.   The kdc-follow-referrals Option      This option is reserved for [REFERRALS].Hartman & Zhu                Standards Track                   [Page 29]

RFC 6113               Kerberos Preauth Framework             April 2011   The padata field contains a list of PA-DATA structures as described   inSection 5.2.7 of [RFC4120].  These PA-DATA structures can contain   FAST factors.  They can also be used as generic typed-holes to   contain data not intended for proving the client's identity or   establishing a reply key, but for protocol extensibility.  If the KDC   supports the PA-FX-FAST-REQUEST padata, unless otherwise specified,   the client MUST place any padata that is otherwise in the outer KDC   request body into this field.  In a TGS request, PA-TGS-REQ padata is   not included in this field and it is present in the outer KDC request   body.   The KDC-REQ-BODY in the FAST structure is used in preference to the   KDC-REQ-BODY outside of the FAST pre-authentication.  The outer KDC-   REQ-BODY structure SHOULD be filled in for backwards compatibility   with KDCs that do not support FAST.  A conforming KDC ignores the   outer KDC-REQ-BODY field in the KDC request.  Pre-authentication data   methods such as [RFC4556] that include a checksum of the KDC-REQ-BODY   should checksum the KDC-REQ-BODY in the FAST structure.   In a TGS request, a client MAY include the AD-fx-fast-used authdata   either in the pa-tgs-req authenticator or in the authorization data   in the pa-tgs-req ticket.  If the KDC receives this authorization   data but does not find a FAST padata, then it MUST return   KRB_APP_ERR_MODIFIED.5.4.3.  FAST Response   The KDC that supports the PA-FX-FAST padata MUST include a PA-FX-FAST   padata element in the KDC reply.  In the case of an error, the PA-FX-   FAST padata is included in the KDC responses according toSection 5.4.4.   The corresponding padata-value field [RFC4120] for the PA-FX-FAST in   the KDC response contains the DER encoding of the ASN.1 type PA-FX-   FAST-REPLY.      PA-FX-FAST-REPLY ::= CHOICE {          armored-data [0] KrbFastArmoredRep,          ...      }      KrbFastArmoredRep ::= SEQUENCE {          enc-fast-rep      [0] EncryptedData, -- KrbFastResponse --              -- The encryption key is the armor key in the request, and              -- the key usage number is KEY_USAGE_FAST_REP.          ...      }      KEY_USAGE_FAST_REP                 52Hartman & Zhu                Standards Track                   [Page 30]

RFC 6113               Kerberos Preauth Framework             April 2011   The PA-FX-FAST-REPLY structure contains a KrbFastArmoredRep   structure.  The KrbFastArmoredRep structure encapsulates the padata   in the KDC reply in the encrypted form.  The KrbFastResponse is   encrypted with the armor key used in the corresponding request, and   the key usage number is KEY_USAGE_FAST_REP.   The Kerberos client MUST support a local policy that rejects the   response if PA-FX-FAST-REPLY is not included in the response.   Clients MAY also support policies that fall back to other mechanisms   or that do not use pre-authentication when FAST is unavailable.  It   is important to consider the potential downgrade attacks when   deploying such a policy.   The KrbFastResponse structure contains the following information:       KrbFastResponse ::= SEQUENCE {           padata         [0] SEQUENCE OF PA-DATA,               -- padata typed holes.           strengthen-key [1] EncryptionKey OPTIONAL,               -- This, if present, strengthens the reply key for AS and               -- TGS. MUST be present for TGS.               -- MUST be absent in KRB-ERROR.           finished       [2] KrbFastFinished OPTIONAL,               -- Present in AS or TGS reply; absent otherwise.           nonce          [3] UInt32,               -- Nonce from the client request.           ...  }   The padata field in the KrbFastResponse structure contains a list of   PA-DATA structures as described inSection 5.2.7 of [RFC4120].  These   PA-DATA structures are used to carry data advancing the exchange   specific for the FAST factors.  They can also be used as generic   typed-holes for protocol extensibility.  Unless otherwise specified,   the KDC MUST include any padata that are otherwise in the outer KDC-   REP or KDC-ERROR structure into this field.  The padata field in the   KDC reply structure outside of the PA-FX-FAST-REPLY structure   typically includes only the PA-FX-FAST-REPLY padata.   The strengthen-key field provides a mechanism for the KDC to   strengthen the reply key.  If set, the strengthen-key value MUST be   randomly generated to have the same etype as that of the reply key   before being strengthened, and then the reply key is strengthened   after all padata items are processed.  Let padata-reply-key be the   reply key after padata processing.   reply-key = KRB-FX-CF2(strengthen-key, padata-reply-key,                         "strengthenkey", "replykey")Hartman & Zhu                Standards Track                   [Page 31]

RFC 6113               Kerberos Preauth Framework             April 2011   The strengthen-key field MAY be set in an AS reply; it MUST be set in   a TGS reply; it must be absent in an error reply.  The strengthen key   is required in a TGS reply so that an attacker cannot remove the FAST   PADATA from a TGS reply, causing the KDC to appear not to support   FAST.   The finished field contains a KrbFastFinished structure.  It is   filled by the KDC in the final message in the conversation.  This   field is present in an AS-REP or a TGS-REP when a ticket is returned,   and it is not present in an error reply.   The KrbFastFinished structure contains the following information:        KrbFastFinished ::= SEQUENCE {            timestamp       [0] KerberosTime,            usec            [1] Microseconds,                -- timestamp and usec represent the time on the KDC when                -- the reply was generated.            crealm          [2] Realm,            cname           [3] PrincipalName,                -- Contains the client realm and the client name.            ticket-checksum [4] Checksum,                -- checksum of the ticket in the KDC-REP using the armor                -- and the key usage is KEY_USAGE_FAST_FINISH.                -- The checksum type is the required checksum type                -- of the armor key.            ...        }        KEY_USAGE_FAST_FINISHED            53   The timestamp and usec fields represent the time on the KDC when the   reply ticket was generated, these fields have the same semantics as   the corresponding identically named fields inSection 5.6.1 of   [RFC4120].  The client MUST use the KDC's time in these fields   thereafter when using the returned ticket.  The client need not   confirm that the timestamp returned is within allowable clock skew:   the armor key guarantees that the reply is fresh.  The client MAY   trust the timestamp returned.   The cname and crealm fields identify the authenticated client.  If   facilities described in [REFERRALS] are used, the authenticated   client may differ from the client in the FAST request.   The ticket-checksum is a checksum of the issued ticket.  The checksum   key is the armor key, and the checksum type is the required checksum   type of the enctype of that key, and the key usage number is   KEY_USAGE_FAST_FINISHED.Hartman & Zhu                Standards Track                   [Page 32]

RFC 6113               Kerberos Preauth Framework             April 2011   When FAST padata is included, the PA-FX-COOKIE padata as defined inSection 5.2 MUST be included in the padata sequence in the   KrbFastResponse sequence if the KDC expects at least one more message   from the client in order to complete the authentication.   The nonce field in the KrbFastResponse contains the value of the   nonce field in the KDC-REQ of the corresponding client request and it   binds the KDC response with the client request.  The client MUST   verify that this nonce value in the reply matches with that of the   request and reject the KDC reply otherwise.  To prevent the response   from one message in a conversation from being replayed to a request   in another message, clients SHOULD use a new nonce for each message   in a conversation.5.4.4.  Authenticated Kerberos Error Messages Using Kerberos FAST   If the Kerberos FAST padata was included in the request, unless   otherwise specified, the e-data field of the KRB-ERROR message   [RFC4120] contains the ASN.1 DER encoding of the type METHOD-DATA   [RFC4120] and a PA-FX-FAST is included in the METHOD-DATA.  The KDC   MUST include all the padata elements such as PA-ETYPE-INFO2 and   padata elements that indicate acceptable pre-authentication   mechanisms [RFC4120] in the KrbFastResponse structure.   The KDC MUST also include a PA-FX-ERROR padata item in the   KRBFastResponse structure.  The padata-value element of this sequence   is the ASN.1 DER encoding of the type KRB-ERROR.  The e-data field   MUST be absent in the PA-FX-ERROR padata.  All other fields should be   the same as the outer KRB-ERROR.  The client ignores the outer error   and uses the combination of the padata in the KRBFastResponse and the   error information in the PA-FX-ERROR.              PA-FX-ERROR                        137   If the Kerberos FAST padata is included in the request but not   included in the error reply, it is a matter of the local policy on   the client to accept the information in the error message without   integrity protection.  However, the client SHOULD process the KDC   errors as the result of the KDC's inability to accept the AP_REQ   armor and potentially retry another request with a different armor   when applicable.  The Kerberos client MAY process an error message   without a PA-FX-FAST-REPLY, if that is only intended to return better   error information to the application, typically for trouble-shooting   purposes.   In the cases where the e-data field of the KRB-ERROR message is   expected to carry a TYPED-DATA [RFC4120] element, that information   should be transmitted in a pa-data element within the KRBFastResponseHartman & Zhu                Standards Track                   [Page 33]

RFC 6113               Kerberos Preauth Framework             April 2011   structure.  The padata-type is the same as the data-type would be in   the typed data element and the padata-value is the same as the data-   value.  As discussed inSection 7, data-types and padata-types are   drawn from the same namespace.  For example, the   TD_TRUSTED_CERTIFIERS structure is expected to be in the KRB-ERROR   message when the error code is KDC_ERR_CANT_VERIFY_CERTIFICATE   [RFC4556].5.4.5.  Outer and Inner Requests   Typically, a client will know that FAST is being used before a   request containing PA-FX-FAST is sent.  So, the outer AS request   typically only includes one pa-data item: PA-FX-FAST.  The client MAY   include additional pa-data, but the KDC MUST ignore the outer request   body and any padata besides PA-FX-FAST if and only if PA-FX-FAST is   processed.  In the case of the TGS request, the outer request should   include PA-FX-FAST and PA-TGS-REQ.   When an AS generates a response, all padata besides PA-FX-FAST should   be included in PA-FX-FAST.  The client MUST ignore other padata   outside of PA-FX-FAST.5.4.6.  The Encrypted Challenge FAST Factor   The encrypted challenge FAST factor authenticates a client using the   client's long-term key.  This factor works similarly to the encrypted   timestamp pre-authentication option described in [RFC4120].  The word   "challenge" is used instead of "timestamp" because while the   timestamp is used as an initial challenge, if the KDC and client do   not have synchronized time, then the KDC can provide updated time to   the client to use as a challenge.  The client encrypts a structure   containing a timestamp in the challenge key.  The challenge key used   by the client to send a message to the KDC is KRB-FX-   CF2(armor_key,long_term_key, "clientchallengearmor",   "challengelongterm").  The challenge key used by the KDC encrypting   to the client is KRB-FX-CF2(armor_key, long_term_key,   "kdcchallengearmor", "challengelongterm").  Because the armor key is   fresh and random, the challenge key is fresh and random.  The only   purpose of the timestamp is to limit the validity of the   authentication so that a request cannot be replayed.  A client MAY   base the timestamp on the KDC time in a KDC error and need not   maintain accurate time synchronization itself.  If a client bases its   time on an untrusted source, an attacker may trick the client into   producing an authentication request that is valid at some future   time.  The attacker may be able to use this authentication request to   make it appear that a client has authenticated at that future time.   If ticket-based armor is used, then the lifetime of the ticket will   limit the window in which an attacker can make the client appear toHartman & Zhu                Standards Track                   [Page 34]

RFC 6113               Kerberos Preauth Framework             April 2011   have authenticated.  For many situations, the ability of an attacker   to cause a client to appear to have authenticated is not a   significant concern; the ability to avoid requiring time   synchronization on clients is more valuable.   The client sends a padata of type PA-ENCRYPTED-CHALLENGE.  The   corresponding padata-value contains the DER encoding of ASN.1 type   EncryptedChallenge.      EncryptedChallenge ::= EncryptedData              -- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key              -- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the              -- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.      PA-ENCRYPTED-CHALLENGE          138      KEY_USAGE_ENC_CHALLENGE_CLIENT  54      KEY_USAGE_ENC_CHALLENGE_KDC     55   The client includes some timestamp reasonably close to the KDC's   current time and encrypts it in the challenge key in a PA-ENC-TS-ENC   structure (seeSection 5.2.7.2 in RFC 4120).  Clients MAY use the   current time; doing so prevents the exposure where an attacker can   cause a client to appear to authenticate in the future.  The client   sends the request including this factor.   On receiving an AS-REQ containing the PA-ENCRYPTED-CHALLENGE FAST   factor, the KDC decrypts the timestamp.  If the decryption fails the   KDC SHOULD return KDC_ERR_PREAUTH_FAILED, including PA-ETYPE-INFO2 in   the KRBFastResponse in the error.  The KDC confirms that the   timestamp falls within its current clock skew returning   KRB_APP_ERR_SKEW if not.  The KDC then SHOULD check to see if the   encrypted challenge is a replay.  The KDC MUST NOT consider two   encrypted challenges replays simply because the timestamps are the   same; to be a replay, the ciphertext MUST be identical.  Allowing   clients to reuse timestamps avoids requiring that clients maintain   state about which timestamps have been used.   If the KDC accepts the encrypted challenge, it MUST include a padata   element of type PA-ENCRYPTED-CHALLENGE.  The KDC encrypts its current   time in the challenge key.  The KDC MUST strengthen the reply key   before issuing a ticket.  The client MUST check that the timestamp   decrypts properly.  The client MAY check that the timestamp is within   the window of acceptable clock skew for the client.  The client MUST   NOT require that the timestamp be identical to the timestamp in the   issued credentials or the returned message.Hartman & Zhu                Standards Track                   [Page 35]

RFC 6113               Kerberos Preauth Framework             April 2011   The encrypted challenge FAST factor provides the following   facilities: Client Authentication and KDC Authentication.  This FAST   factor also takes advantage of the FAST facility to strengthen the   reply key.  It does not provide the Replace Reply Key facility.  The   Security Considerations section of this document provides an   explanation why the security requirements are met.   The encrypted challenge FAST factor can be useful in an   authentication set.  No pa-hint is defined because the only   information needed by this mechanism is information contained in the   PA-ETYPE-INFO2 pre-authentication data.  KDCs are already required to   send PA-ETYPE-INFO2.  If KDCs were not required to send PA-ETYPE-   INFO2 then that information would need to be part of a hint for   encrypted challenge.   Conforming implementations MUST support the encrypted challenge FAST   factor.5.5.  Authentication Strength Indication   Implementations that have pre-authentication mechanisms offering   significantly different strengths of client authentication MAY choose   to keep track of the strength of the authentication used as an input   into policy decisions.  For example, some principals might require   strong pre-authentication, while less sensitive principals can use   relatively weak forms of pre-authentication like encrypted timestamp.   An AuthorizationData data type AD-Authentication-Strength is defined   for this purpose.        AD-authentication-strength         70   The corresponding ad-data field contains the DER encoding of the pre-   authentication data set as defined inSection 5.3.  This set contains   all the pre-authentication mechanisms that were used to authenticate   the client.  If only one pre-authentication mechanism was used to   authenticate the client, the pre-authentication set contains one   element.  Unless otherwise specified, the hint and value fields of   the members of this sequence MUST be empty.  In order to permit   mechanisms to carry additional information about strength in these   fields in the future, clients and application servers MUST ignore   non-empty hint and value fields for mechanisms unless the   implementation is updated with the interpretation of these fields for   a given pre-authentication mechanism in this authorization element.   The AD-authentication-strength element MUST be included in the AD-   KDC-ISSUED container so that the KDC integrity protects its contents.   This element can be ignored if it is unknown to the receiver.Hartman & Zhu                Standards Track                   [Page 36]

RFC 6113               Kerberos Preauth Framework             April 20116.  Assigned Constants   The pre-authentication framework and FAST involve using a number of   Kerberos protocol constants.  This section lists protocol constants   first introduced in this specification drawn from registries not   managed by IANA.  Many of these registries would best be managed by   IANA; that is a known issue that is out of scope for this document.   The constants described in this section have been accounted for and   will appear in the next revision of the Kerberos core specification   or in a document creating IANA registries.Section 7 creates IANA registries for a different set of constants   used by the extensions described in this document.6.1.  New Errors           KDC_ERR_PREAUTH_EXPIRED                 90           KDC_ERR_MORE_PREAUTH_DATA_REQUIRED      91           KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET  92           KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS   936.2.  Key Usage Numbers           KEY_USAGE_FAST_REQ_CHKSUM               50           KEY_USAGE_FAST_ENC                      51           KEY_USAGE_FAST_REP                      52           KEY_USAGE_FAST_FINISHED                 53           KEY_USAGE_ENC_CHALLENGE_CLIENT          54           KEY_USAGE_ENC_CHALLENGE_KDC             556.3.  Authorization Data Elements           AD-authentication-strength              70           AD-fx-fast-armor                        71           AD-fx-fast-used                         726.4.  New PA-DATA Types           PA-FX-COOKIE                            133           PA-AUTHENTICATION-SET                   134           PA-AUTH-SET-SELECTED                    135           PA-FX-FAST                              136           PA-FX-ERROR                             137           PA-ENCRYPTED-CHALLENGE                  138Hartman & Zhu                Standards Track                   [Page 37]

RFC 6113               Kerberos Preauth Framework             April 20117.  IANA Considerations   This document creates a number of IANA registries.  These registries   are all located under Kerberos Parameters onhttp://www.iana.org.   See [RFC5226] for descriptions of the registration policies used in   this section.7.1.  Pre-Authentication and Typed DataRFC 4120 defines pre-authentication data, which can be included in a   KDC request or response in order to authenticate the client or extend   the protocol.  In addition, it defines Typed-Data, which is an   extension mechanism for errors.  Both pre-authentication data and   typed data are carried as a 32-bit signed integer along with an octet   string.  The encoding of typed data and pre-authentication data is   slightly different.  However, the types for pre-authentication data   and typed-data are drawn from the same namespace.  By convention,   registrations starting with TD- are typed data and registrations   starting with PA- are pre-authentication data.  It is important that   these data types be drawn from the same namespace, because some   errors where it would be desirable to include typed data require the   e-data field to be formatted as pre-authentication data.   When Kerberos FAST is used, pre-authentication data encoding is   always used.   There is one apparently conflicting registration between typed data   and pre-authentication data.  PA-GET-FROM-TYPED-DATA and TD-PADATA   are both assigned the value 22.  However, this registration is simply   a mechanism to include an element of the other encoding.  The use of   both should be deprecated.   This document creates a registry for pre-authentication and typed   data.  The registration procedures are as follows.  Expert review for   pre-authentication mechanisms designed to authenticate users, KDCs,   or establish the reply key.  The expert first determines that the   purpose of the method is to authenticate clients, KDCs, or to   establish the reply key.  If so, expert review is appropriate.  The   expert evaluates the security and interoperability of the   specification.   IETF review is required if the expert believes that the pre-   authentication method is broader than these three areas.  Pre-   authentication methods that change the Kerberos state machine or   otherwise make significant changes to the Kerberos protocol should be   Standards Track RFCs.  A concern that a particular method needs to be   a Standards Track RFC may be raised as an objection during IETF   review.Hartman & Zhu                Standards Track                   [Page 38]

RFC 6113               Kerberos Preauth Framework             April 2011   Several of the registrations indicated below were made at a time when   the Kerberos protocol was less mature and do not meet the current   requirements for this registry.  These registrations are included in   order to accurately document what is known about the use of these   protocol code points and to avoid conflicts.     Type                Value    Reference ---------------------------------------------------------------------- PA-TGS-REQ                 1    [RFC4120] PA-ENC-TIMESTAMP           2    [RFC4120] PA-PW-SALT                 3    [RFC4120] [reserved]                 4    [RFC6113] PA-ENC-UNIX-TIME           5    (deprecated) [RFC4120] PA-SANDIA-SECUREID         6    [RFC4120] PA-SESAME                  7    [RFC4120] PA-OSF-DCE                 8    [RFC4120] PA-CYBERSAFE-SECUREID      9    [RFC4120] PA-AFS3-SALT               10   [RFC4120] [RFC3961] PA-ETYPE-INFO              11   [RFC4120] PA-SAM-CHALLENGE           12   [KRB-WG.SAM] PA-SAM-RESPONSE            13   [KRB-WG.SAM] PA-PK-AS-REQ_OLD           14   [PK-INIT-1999] PA-PK-AS-REP_OLD           15   [PK-INIT-1999] PA-PK-AS-REQ               16   [RFC4556] PA-PK-AS-REP               17   [RFC4556] PA-PK-OCSP-RESPONSE        18   [RFC4557] PA-ETYPE-INFO2             19   [RFC4120] PA-USE-SPECIFIED-KVNO      20   [RFC4120] PA-SVR-REFERRAL-INFO       20   [REFERRALS] PA-SAM-REDIRECT            21   [KRB-WG.SAM] PA-GET-FROM-TYPED-DATA     22   (embedded in typed data) [RFC4120] TD-PADATA                  22   (embeds padata) [RFC4120] PA-SAM-ETYPE-INFO          23   (sam/otp) [KRB-WG.SAM] PA-ALT-PRINC               24   (crawdad@fnal.gov) [HW-AUTH] PA-SERVER-REFERRAL         25   [REFERRALS] PA-SAM-CHALLENGE2          30   (kenh@pobox.com) [KRB-WG.SAM] PA-SAM-RESPONSE2           31   (kenh@pobox.com) [KRB-WG.SAM] PA-EXTRA-TGT               41   Reserved extra TGT [RFC6113] TD-PKINIT-CMS-CERTIFICATES 101  CertificateSet from CMS TD-KRB-PRINCIPAL           102  PrincipalName TD-KRB-REALM               103  Realm TD-TRUSTED-CERTIFIERS      104  [RFC4556] TD-CERTIFICATE-INDEX       105  [RFC4556] TD-APP-DEFINED-ERROR       106  Application specific [RFC6113] TD-REQ-NONCE               107  INTEGER [RFC6113] TD-REQ-SEQ                 108  INTEGER [RFC6113] TD_DH_PARAMETERS           109  [RFC4556] TD-CMS-DIGEST-ALGORITHMS   111  [ALG-AGILITY]Hartman & Zhu                Standards Track                   [Page 39]

RFC 6113               Kerberos Preauth Framework             April 2011 TD-CERT-DIGEST-ALGORITHMS  112  [ALG-AGILITY] PA-PAC-REQUEST             128  [MS-KILE] PA-FOR_USER                129  [MS-KILE] PA-FOR-X509-USER           130  [MS-KILE] PA-FOR-CHECK_DUPS          131  [MS-KILE] PA-AS-CHECKSUM             132  [MS-KILE] PA-FX-COOKIE               133  [RFC6113] PA-AUTHENTICATION-SET      134  [RFC6113] PA-AUTH-SET-SELECTED       135  [RFC6113] PA-FX-FAST                 136  [RFC6113] PA-FX-ERROR                137  [RFC6113] PA-ENCRYPTED-CHALLENGE     138  [RFC6113] PA-OTP-CHALLENGE           141  (gareth.richards@rsa.com) [OTP-PREAUTH] PA-OTP-REQUEST             142  (gareth.richards@rsa.com) [OTP-PREAUTH] PA-OTP-CONFIRM             143  (gareth.richards@rsa.com) [OTP-PREAUTH] PA-OTP-PIN-CHANGE          144  (gareth.richards@rsa.com) [OTP-PREAUTH] PA-EPAK-AS-REQ             145  (sshock@gmail.com) [RFC6113] PA-EPAK-AS-REP             146  (sshock@gmail.com) [RFC6113] PA_PKINIT_KX               147  [RFC6112] PA_PKU2U_NAME              148  [PKU2U] PA-SUPPORTED-ETYPES        165  [MS-KILE] PA-EXTENDED_ERROR          166  [MS-KILE]7.2.  Fast Armor Types   FAST armor types are defined inSection 5.4.1.  A FAST armor type is   a signed 32-bit integer.  FAST armor types are assigned by standards   action.          Type    Name                   Description        ------------------------------------------------------------          0                              Reserved.          1   FX_FAST_ARMOR_AP_REQUEST   Ticket armor using an ap-req.7.3.  FAST Options   A FAST request includes a set of bit flags to indicate additional   options.  Bits 0-15 are critical; other bits are non-critical.   Assigning bits greater than 31 may require special support in   implementations.  Assignment of FAST options requires standards   action.Hartman & Zhu                Standards Track                   [Page 40]

RFC 6113               Kerberos Preauth Framework             April 2011      Type    Name                   Description     -------------------------------------------------------------------      0     RESERVED               Reserved for future expansion of this                                   field.      1     hide-client-names      Requesting the KDC to hide client                                   names in the KDC response      16    kdc-follow-referrals   Reserved.8.  Security Considerations   The kdc-referrals option in the Kerberos FAST padata requests the KDC   to act as the client to follow referrals.  This can overload the KDC.   To limit the damages of denial of service using this option, KDCs MAY   restrict the number of simultaneous active requests with this option   for any given client principal.   Regarding the facilities provided by the Encrypted Challenge FAST   factor, the challenge key is derived from the client secrets and   because the client secrets are known only to the client and the KDC,   the verification of the EncryptedChallenge structure proves the   client's identity, the verification of the EncryptedChallenge   structure in the KDC reply proves that the expected KDC responded.   Therefore, the Encrypted Challenge FAST factor as a pre-   authentication mechanism offers the following facilities: Client   Authentication and KDC Authentication.  There is no un-authenticated   cleartext introduced by the Encrypted Challenge FAST factor.   FAST provides an encrypted tunnel over which pre-authentication   conversations can take place.  In addition, FAST optionally   authenticates the KDC to the client.  It is the responsibility of   FAST factors to authenticate the client to the KDC.  Care MUST be   taken to design FAST factors such that they are bound to the   conversation.  If this is not done, a man-in-the-middle may be able   to cut&paste a FAST factor from one conversation to another.  The   easiest way to do this is to bind each FAST factor to the armor key   that is guaranteed to be unique for each conversation.   The anonymous PKINIT mode for obtaining an armor ticket does not   always authenticate the KDC to the client before the conversation   begins.  Tracking the KDC verified state guarantees that by the end   of the conversation, the client has authenticated the KDC.  However,   FAST factor designers need to consider the implications of using   their factor when the KDC has not yet been authenticated.  If this   proves problematic in an environment, then the particular FAST factor   should not be used with anonymous PKINIT.   Existing pre-authentication mechanisms are believed to be at least as   secure when used with FAST as they are when used outside of FAST.Hartman & Zhu                Standards Track                   [Page 41]

RFC 6113               Kerberos Preauth Framework             April 2011   One part of this security is making sure that when pre-authentication   methods checksum the request, they checksum the inner request rather   than the outer request.  If the mechanism checksummed the outer   request, a man-in-the-middle could observe it outside a FAST tunnel   and then cut&paste it into a FAST exchange where the inner rather   than outer request would be used to select attributes of the issued   ticket.  Such attacks would typically invalidate auditing information   or create a situation where the client and KDC disagree about what   ticket is issued.  However, such attacks are unlikely to allow an   attacker who would not be able to authenticate as a principal to do   so.  Even so, FAST is believed to defend against these attacks in   existing legacy mechanism.  However, since there is no standard for   how legacy mechanisms bind the request to the pre-authentication or   provide integrity protection, security analysis can be difficult.  In   some cases, FAST may significantly improve the integrity protection   of legacy mechanisms.   The security of the TGS exchange depends on authenticating the client   to the KDC.  In the AS exchange, this is done using pre-   authentication data or FAST factors.  In the TGS exchange, this is   done by presenting a TGT and by using the session (or sub-session)   key in constructing the request.  Because FAST uses a request body in   the inner request, encrypted in the armor key, rather than the   request body in the outer request, it is critical that establishing   the armor key be tied to the authentication of the client to the KDC.   If this is not done, an attacker could manipulate the options   requested in the TGS request, for example, requesting a ticket with   different validity or addresses.  The easiest way to bind the armor   key to the authentication of the client to the KDC is for the armor   key to depend on the sub-session key of the TGT.  This is done with   the implicit TGS armor supported by this specification.  Future armor   types designed for use with the TGS MUST either bind their armor keys   to the TGT or provide another mechanism to authenticate the client to   the KDC.9.  Acknowledgements   Sam Hartman would like to thank the MIT Kerberos Consortium for its   funding of his time on this project.   Several suggestions from Jeffrey Hutzelman based on early revisions   of this documents led to significant improvements of this document.   The proposal to ask one KDC to chase down the referrals and return   the final ticket is based on requirements in [CROSS].   Joel Weber had a proposal for a mechanism similar to FAST that   created a protected tunnel for Kerberos pre-authentication.Hartman & Zhu                Standards Track                   [Page 42]

RFC 6113               Kerberos Preauth Framework             April 2011   Srinivas Cheruku and Greg Hudson provided valuable review comments.10.  References10.1.  Normative References   [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate                   Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3961]       Raeburn, K., "Encryption and Checksum Specifications                   for Kerberos 5",RFC 3961, February 2005.   [RFC4120]       Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The                   Kerberos Network Authentication Service (V5)",RFC 4120, July 2005.   [RFC4556]       Zhu, L. and B. Tung, "Public Key Cryptography for                   Initial Authentication in Kerberos (PKINIT)",RFC 4556, June 2006.   [RFC5226]       Narten, T. and H. Alvestrand, "Guidelines for Writing                   an IANA Considerations Section in RFCs",BCP 26,RFC 5226, May 2008.   [RFC6112]       Zhu, L., Leach, P., and S. Hartman "Anonymity Support                   for Kerberos",RFC 6112, April 2011.10.2.  Informative References   [ALG-AGILITY]   Astrand, L. and L. Zhu,"PK-INIT algorithm agility",                   Work in Progress, August 2008.   [CROSS]         Sakane, S., Zrelli, S., and M. Ishiyama , "Problem                   statement on the cross-realm operation of Kerberos in                   a specific system", Work in Progress, July 2007.   [EKE]           Bellovin, S. and M. Merritt, "Augmented Encrypted Key                   Exchange: A Password-Based Protocol Secure Against                   Dictionary Attacks and Password File Compromise,                   Proceedings of the 1st ACM Conference on Computer and                   Communications Security, ACM Press.", November 1993.   [HW-AUTH]       Crawford, M., "Passwordless Initial Authentication to                   Kerberos by Hardware  Preauthentication", Work                   in Progress, October 2006.   [IEEE1363.2]    IEEE, "IEEE P1363.2: Password-Based Public-Key                   Cryptography", 2004.Hartman & Zhu                Standards Track                   [Page 43]

RFC 6113               Kerberos Preauth Framework             April 2011   [KRB-WG.SAM]    Hornstein, K., Renard, K., Neuman, C., and G. Zorn,                   "Integrating Single-use Authentication Mechanisms                   with Kerberos", Work in Progress, July 2004.   [MS-KILE]       Microsoft, "Kerberos Protocol Extensions", <http://msdn.microsoft.com/en-us/library/cc206927.aspx>.   [OTP-PREAUTH]   Richards, G.,"OTP Pre-authentication", Work                   in Progress, February 2011.   [PK-INIT-1999]  Tung, B., Neuman, C., Hur, M., Medvinsky, A.,                   Medvinsky, S., Wray, J., and J. Trostle, "Public Key                   Cryptography for Initial Authentication in Kerberos",                   Work in Progress, July 1999.   [PKU2U]         Zhu, L., Altman, J., and N. Williams, "Public Key                   Cryptography Based User-to-User Authentication -                   (PKU2U)", Work in Progress, November 2008.   [REFERRALS]     Hartman, S., Ed., Raeburn, K., and L. Zhu, "Kerberos                   Principal Name Canonicalization and KDC-Generated                   Cross-Realm Referrals", Work in Progress, March 2011.   [RFC4557]       Zhu, L., Jaganathan, K., and N. Williams, "Online                   Certificate Status Protocol (OCSP) Support for Public                   Key Cryptography for Initial Authentication in                   Kerberos (PKINIT)",RFC 4557, June 2006.Hartman & Zhu                Standards Track                   [Page 44]

RFC 6113               Kerberos Preauth Framework             April 2011Appendix A.  Test Vectors for KRB-FX-CF2   This informative appendix presents test vectors for the KRB-FX-CF2   function.  Test vectors are presented for several encryption types.   In all cases, the first key (k1) is the result of string-to-   key("key1", "key1", default_parameters) and the second key (k2) is   the result of string-to-key("key2", "key2", default_parameters).   Both keys are of the same enctype.  The presented test vector is the   hexadecimal encoding of the key produced by KRB-FX-CF2(k1, k2, "a",   "b").  The peppers are one-octet ASCII strings.   In performing interoperability testing, there was significant   ambiguity surrounding [RFC3961] pseudo-random operations.  These test   vectors assume that the AES pseudo-random operation is   aes-ecb(trunc128(sha-1(input))) where trunc128 truncates its input to   128 bits.  The 3DES pseudo-random operation is assumed to be   des3-cbc(trunc128(sha-1(input))).  The DES pseudo-random operation is   assumed to be des-cbc(md5(input)).  As specified inRFC 4757, the RC4   pseudo-random operation is hmac-sha1(input).   Interoperability testing also demonstrated ambiguity surrounding the   DES random-to-key operation.  The random-to-key operation is assumed   to be distribute 56 bits into high-7-bits of 8 octets and generate   parity.   These test vectors were produced with revision 22359 of the MIT   Kerberos sources.  The AES 256 and AES 128 test vectors have been   confirmed by multiple other implementors.  The RC4 test vectors have   been confirmed by one other implementor.  The DES and triple DES test   vectors have not been confirmed.   aes 128 (enctype 17): 97df97e4b798b29eb31ed7280287a92a   AES256 (enctype 18): 4d6ca4e629785c1f01baf55e2e548566                        b9617ae3a96868c337cb93b5e72b1c7b   DES (enctype 1): 43bae3738c9467e6   3DES (enctype 16): e58f9eb643862c13ad38e529313462a7f73e62834fe54a01   RC4 (enctype 23): 24d7f6b6bae4e5c00d2082c5ebab3672Hartman & Zhu                Standards Track                   [Page 45]

RFC 6113               Kerberos Preauth Framework             April 2011Appendix B.  ASN.1 Module      KerberosPreauthFramework {            iso(1) identified-organization(3) dod(6) internet(1)            security(5) kerberosV5(2) modules(4) preauth-framework(3)      } DEFINITIONS EXPLICIT TAGS ::= BEGIN      IMPORTS           KerberosTime, PrincipalName, Realm, EncryptionKey, Checksum,           Int32, EncryptedData, PA-ENC-TS-ENC, PA-DATA, KDC-REQ-BODY,           Microseconds, KerberosFlags, UInt32                FROM KerberosV5Spec2 { iso(1) identified-organization(3)                  dod(6) internet(1) security(5) kerberosV5(2)                  modules(4) krb5spec2(2) };                  -- as defined inRFC 4120.      PA-AUTHENTICATION-SET ::= SEQUENCE OF PA-AUTHENTICATION-SET-ELEM      PA-AUTHENTICATION-SET-ELEM ::= SEQUENCE {          pa-type      [0] Int32,              -- same as padata-type.          pa-hint      [1] OCTET STRING OPTIONAL,          pa-value     [2] OCTET STRING OPTIONAL,          ...      }      KrbFastArmor ::= SEQUENCE {          armor-type   [0] Int32,              -- Type of the armor.          armor-value  [1] OCTET STRING,              -- Value of the armor.          ...      }      PA-FX-FAST-REQUEST ::= CHOICE {          armored-data [0] KrbFastArmoredReq,          ...      }      KrbFastArmoredReq ::= SEQUENCE {          armor        [0] KrbFastArmor OPTIONAL,              -- Contains the armor that identifies the armor key.              -- MUST be present in AS-REQ.          req-checksum [1] Checksum,              -- For AS, contains the checksum performed over the type              -- KDC-REQ-BODY for the req-body field of the KDC-REQ              -- structure;              -- For TGS, contains the checksum performed over the typeHartman & Zhu                Standards Track                   [Page 46]

RFC 6113               Kerberos Preauth Framework             April 2011              -- AP-REQ in the PA-TGS-REQ padata.              -- The checksum key is the armor key, the checksum              -- type is the required checksum type for the enctype of              -- the armor key, and the key usage number is              -- KEY_USAGE_FAST_REQ_CHKSUM.          enc-fast-req [2] EncryptedData, -- KrbFastReq --              -- The encryption key is the armor key, and the key usage              -- number is KEY_USAGE_FAST_ENC.          ...      }      KrbFastReq ::= SEQUENCE {          fast-options [0] FastOptions,              -- Additional options.          padata       [1] SEQUENCE OF PA-DATA,              -- padata typed holes.          req-body     [2] KDC-REQ-BODY,              -- Contains the KDC request body as defined in Section              -- 5.4.1 of [RFC4120].              -- This req-body field is preferred over the outer field              -- in the KDC request.           ...      }      FastOptions ::= KerberosFlags          -- reserved(0),          -- hide-client-names(1),          -- kdc-follow-referrals(16)      PA-FX-FAST-REPLY ::= CHOICE {          armored-data [0] KrbFastArmoredRep,          ...      }      KrbFastArmoredRep ::= SEQUENCE {          enc-fast-rep      [0] EncryptedData, -- KrbFastResponse --              -- The encryption key is the armor key in the request, and              -- the key usage number is KEY_USAGE_FAST_REP.          ...      }      KrbFastResponse ::= SEQUENCE {          padata         [0] SEQUENCE OF PA-DATA,              -- padata typed holes.          strengthen-key [1] EncryptionKey OPTIONAL,              -- This, if present, strengthens the reply key for AS and              -- TGS.  MUST be present for TGS              -- MUST be absent in KRB-ERROR.Hartman & Zhu                Standards Track                   [Page 47]

RFC 6113               Kerberos Preauth Framework             April 2011          finished       [2] KrbFastFinished OPTIONAL,              -- Present in AS or TGS reply; absent otherwise.          nonce          [3] UInt32,              -- Nonce from the client request.          ...      }      KrbFastFinished ::= SEQUENCE {          timestamp       [0] KerberosTime,          usec            [1] Microseconds,              -- timestamp and usec represent the time on the KDC when              -- the reply was generated.          crealm          [2] Realm,          cname           [3] PrincipalName,              -- Contains the client realm and the client name.          ticket-checksum [4] Checksum,              -- checksum of the ticket in the KDC-REP  using the armor              -- and the key usage is KEY_USAGE_FAST_FINISH.              -- The checksum type is the required checksum type              -- of the armor key.          ...      }      EncryptedChallenge ::= EncryptedData              -- Encrypted PA-ENC-TS-ENC, encrypted in the challenge key              -- using key usage KEY_USAGE_ENC_CHALLENGE_CLIENT for the              -- client and KEY_USAGE_ENC_CHALLENGE_KDC for the KDC.      ENDAuthors' Addresses   Sam Hartman   Painless Security   EMail: hartmans-ietf@mit.edu   Larry Zhu   Microsoft Corporation   One Microsoft Way   Redmond, WA  98052   US   EMail: larry.zhu@microsoft.comHartman & Zhu                Standards Track                   [Page 48]