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Kerberos Working Group                                        S. Hartman
Internet-Draft                                         Painless Security
Updates: 4120 (if approved)                                       L. Zhu
Intended status: Standards Track                   Microsoft Corporation
Expires: September 10, 2009                                March 9, 2009


        A Generalized Framework for Kerberos Pre-Authentication
                 draft-ietf-krb-wg-preauth-framework-10

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
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   This Internet-Draft will expire on September 10, 2009.

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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Abstract

   Kerberos is a protocol for verifying the identity of principals
   (e.g., a workstation user or a network server) on an open network.



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   The Kerberos protocol provides a mechanism called pre-authentication
   for proving the identity of a principal and for better protecting the
   long-term secrets of the principal.

   This document describes a model for Kerberos pre-authentication
   mechanisms.  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 KDC with a reply key delivery mechanism;
   this secure channel can be used to protect the authentication
   exchange 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.

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions and Terminology Used in This Document  . . . . . .  6
   3.  Model for Pre-Authentication . . . . . . . . . . . . . . . . .  6
     3.1.  Information Managed by the Pre-authentication Model  . . .  7
     3.2.  Initial Pre-authentication Required Error  . . . . . . . .  9
     3.3.  Client to KDC  . . . . . . . . . . . . . . . . . . . . . . 10
     3.4.  KDC to Client  . . . . . . . . . . . . . . . . . . . . . . 11
   4.  Pre-Authentication Facilities  . . . . . . . . . . . . . . . . 12
     4.1.  Client-authentication Facility . . . . . . . . . . . . . . 13
     4.2.  Strengthening-reply-key Facility . . . . . . . . . . . . . 14
     4.3.  Replacing-reply-key Facility . . . . . . . . . . . . . . . 15
     4.4.  KDC-authentication Facility  . . . . . . . . . . . . . . . 15
   5.  Requirements for Pre-Authentication Mechanisms . . . . . . . . 15
   6.  Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 16
     6.1.  Combining Keys . . . . . . . . . . . . . . . . . . . . . . 17
     6.2.  Protecting Requests/Responses  . . . . . . . . . . . . . . 18
     6.3.  Managing States for the KDC  . . . . . . . . . . . . . . . 19
     6.4.  Pre-authentication Set . . . . . . . . . . . . . . . . . . 20
     6.5.  Definition of Kerberos FAST Padata . . . . . . . . . . . . 23
       6.5.1.  FAST Armors  . . . . . . . . . . . . . . . . . . . . . 24
       6.5.2.  FAST Request . . . . . . . . . . . . . . . . . . . . . 26
       6.5.3.  FAST Response  . . . . . . . . . . . . . . . . . . . . 30
       6.5.4.  Authenticated Kerberos Error Messages using
               Kerberos FAST  . . . . . . . . . . . . . . . . . . . . 33
       6.5.5.  Outer and Inner Requests . . . . . . . . . . . . . . . 34
       6.5.6.  The Encrypted Challenge FAST Factor  . . . . . . . . . 34
     6.6.  Authentication Strength Indication . . . . . . . . . . . . 36
   7.  Assigned Constants . . . . . . . . . . . . . . . . . . . . . . 36
     7.1.  New Errors . . . . . . . . . . . . . . . . . . . . . . . . 37
     7.2.  Key Usage Numbers  . . . . . . . . . . . . . . . . . . . . 37
     7.3.  Authorization Data Elements  . . . . . . . . . . . . . . . 37
     7.4.  New PA-DATA Types  . . . . . . . . . . . . . . . . . . . . 37
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 37
     8.1.  Pre-authentication and Typed Data  . . . . . . . . . . . . 37
     8.2.  Fast Armor Types . . . . . . . . . . . . . . . . . . . . . 39
     8.3.  FAST Options . . . . . . . . . . . . . . . . . . . . . . . 40
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 40
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 41
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 41
     11.2. Informative References . . . . . . . . . . . . . . . . . . 41
   Appendix A.  Change History  . . . . . . . . . . . . . . . . . . . 42
     A.1.  Changes since 09 . . . . . . . . . . . . . . . . . . . . . 42
     A.2.  Changes since 08 . . . . . . . . . . . . . . . . . . . . . 42
     A.3.  Changes since 07 . . . . . . . . . . . . . . . . . . . . . 43
     A.4.  Changes since 06 . . . . . . . . . . . . . . . . . . . . . 43



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   Appendix B.  ASN.1 module  . . . . . . . . . . . . . . . . . . . . 44
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 46

















































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1.  Introduction

   The core Kerberos specification [RFC4120] treats pre-authentication
   data as an opaque typed hole in the messages to the 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, problems like 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 plugin 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 KDC, and it can optionally deliver a 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 in Section 6.5.  A pre-authentication type carried within
   FAST is called a FAST factor.  Creating a FAST factor is the easiest
   path to create a new pre-authentication mechanism.  FAST factors are
   significantly easier to analyze from a security standpoint than other



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   pre-authentication mechanisms.

   Mechanism designers should design FAST factors, instead of new pre-
   authentication mechanisms outside of FAST.


2.  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 Exchange 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.  See
   Section 6.3 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.


3.  Model for Pre-Authentication

   When a Kerberos client wishes to obtain a ticket using the
   authentication server, it sends an initial Authentication Service
   (AS) request.  If pre-authentication is required but not being used,
   then the KDC will respond with a KDC_ERR_PREAUTH_REQUIRED error.
   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



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   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 in
   Section 6.3.

   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 the
   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.

3.1.  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





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   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 contents of the KDC reply can be verified by the
      client principal


   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 in Section 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 are
   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 is 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.  This information is useful to keep track of 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 in Section 6.6.

   Initially the reply key has not been used.  A pre-authentication
   mechanism that uses the reply key to encrypt or checksum some data in



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   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 in Section 4.3 that the reply key SHOULD
   NOT be replaced after it is used.

   Initially the reply key has not been replaced.  If a mechanism
   implements the Replace Reply Key facility discussed in Section 4.3,
   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 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 principal'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 client
   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.

   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.

3.2.  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-



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   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_NEEDED
   (defined in Section 6.3) for pre-authentication configurations that
   use multi-round-trip mechanisms; see Section 3.4 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 or offered alone.  For each mechanism that is
   offered alone, the KDC includes the pre-authentication type ID of the
   mechanism in the padata sequence returned in the
   KDC_ERR_PREAUTH_REQUIRED error.

   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 pre-authentication is desirable.

3.3.  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



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   this specification MUST NOT ignore the padata defined in Section 6.3.
   Clients SHOULD process padata unrelated to this framework or other
   means of authenticating the user.  Clients SHOULD choose one
   authentication set or mechanism that could lead to authenticating the
   user and ignore the rest.  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 a padata it MUST NOT
   process the padata, allow the padata to affect the pre-authentication
   state, nor respond to the padata.

   For each padata the client chooses to process, the client processes
   the padata and modifies the pre-authentication state as required by
   that mechanism.  Padata are processed in the order received from the
   KDC.

   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.

3.4.  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; see Section 6.3 for a definition.  It then processes the
   padata in the request.  As mentioned in Section 3.3, the KDC MAY
   ignore padata that is 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_NEEDED 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



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   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 is
   generated, the error response is sent.  Typically the errors with the
   code KDC_ERR_MORE_PREAUTH_DATA_NEEDED in a conversation will include
   KDC state as discussed in Section 6.3.

   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.


4.  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.  By combining multiple facilities in a single mechanism,
   it is often easier to construct a secure, simple solution 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



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   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 4.2 discusses how to design mechanisms that modify the reply
   key to be split into a proposal and acceptance without requiring
   additional round trips to use the new reply key in subsequent pre-
   authentication.  Other changes in the state described in Section 3.1
   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.

4.1.  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 in Section 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 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.






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4.2.  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 in Section 6.1.

   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 and
   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 in Section 6.4) 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



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   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.

4.3.  Replacing-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.

   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 in Section 4.2 if appropriate.

4.4.  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.


5.  Requirements for Pre-Authentication Mechanisms

   This section lists requirements for specifications of pre-
   authentication mechanisms.

   For each message in the pre-authentication mechanism, the



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   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 in
   Section 6.4).

   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 consideration section of
   the mechanism specification should show that the security
   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
   authentication.  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 in Section 6.3, 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.


6.  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.




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6.1.  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.
   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 pass-phrases.

       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.

   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 ) || ...



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   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.

   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.

6.2.  Protecting Requests/Responses

   Mechanism designers SHOULD protect clear text 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 that 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.

   If there is more than one round trip for an authentication exchange,
   mechanism designers need to allow either the client or the KDC to
   provide a checksum of all the messages exchanged on the wire in the
   conversation, and the checksum is then verified by the receiver.

   New mechanisms MUST NOT be hard-wired to use a specific algorithm.

   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 in Section 6.5) is
   encrypted in a protected channel, thus they do not share the un-
   authenticated-text issues with mechanisms designed as full-blown pre-



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   authentication mechanisms.

6.3.  Managing States for the KDC

   Kerberos KDCs are stateless.  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.  This padata is 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



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   implementation needs to consider expiration of tokens, key rollover
   and other security issues in token design.  The content of the cookie
   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.

   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_NEEDED error with a PA-FX-COOKIE to
   identify the conversation with the client according to Section 3.2.
   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_NEEDED   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
   cooking 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_NEEDED.  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.  Clients also need to deal with KDCs prior to this
   specification that do not include cookies; if 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.

6.4.  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              134

   A PA-AUTHENTICATION-SET padata element contains the ASN.1 DER
   encoding of the PA-AUTHENTICATION-SET structure:





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        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 which 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 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.  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 MAY 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



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         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 needs 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 interface 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 in Section 6.3 MUST be sent by the KDC so that the
   conversation can continue if the conversation involves multiple KDCs.
   The cookie may not be needed in the first message containing the PA-
   AUTHENTICATION-SET sequence as the KDC may be able to reconstruct the
   state from the PA-AUTHENTICATION-SET-SELECTED padata.  KDCs MUST
   support clients that do not include a cookie because they
   optimistically choose an authentication set, although they MAY always
   return 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_NEEDED as defined
   in Section 6.3 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 is 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



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   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.

6.5.  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 attack.  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, 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 in Section 5.

   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 [KRB-ANON] [RFC4556].

   The rest of this section describes the types of armors and the syntax
   of the messages used by FAST.  Conforming implementations MUST



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   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.

6.5.1.  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.

   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.

6.5.1.1.  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:



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       armor_key = KRB-FX-CF2( subkey, ticket_session_key,
                   "subkeyarmor", "ticketarmor" )

   The `ticket_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 on RFC4120, 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 [KRB-ANON] [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-
       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 (TBD)
   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-



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   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.

   As discussed previously, the server of an armor ticket MUST be the
   TGS of the realm from whom service is requested.  As a result, if
   this armor type is used when a ticket is being validated, proxied, or
   in other cases where a ticket other than a TGT is presented to the
   TGS, a TGT will be used as an armor ticket, while another ticket will
   be used in the pa-tgs-req authenticator.

6.5.2.  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 with an empty pa-value in a PREAUTH_REQUIRED
   error.  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.
















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       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.

   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



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      this case, the armor key is the same armor key that would be
      computed if the TGS-REQ authenticator was used in a
      FX_FAST_ARMOR_AP_REQUEST armor.  If a ticket other than a TGT is
      being presented to the TGS, a client SHOULD use some form of FAST
      armor such as a ticket-based armor with a TGT as an armor ticket.
      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 a
      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 a
      FX_FAST_ARMOR_AP_REQUEST, a non-TGT cannot be used as an armor
      ticket in FX_FAST_ARMOR_AP_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 an 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 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.
             ...
        }

   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),
            -- kdcfollow--referrals(16)




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     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  Requesting the KDC to follow 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 clear text, 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 [KRB-ANON] 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

      The Kerberos client described in [RFC4120] has to request referral
      TGTs along the authentication path in order to get a service
      ticket for the target service.  The Kerberos client described in
      the [REFERRALS] needs to contact the AS specified in the error
      response in order to complete client referrals.  The kdc-follow-
      referrals option is designed to minimize the number of messages
      that need to be processed by the client.  This option is useful
      when, for example, the client may contact the KDC via a satellite
      link that has high network latency, or the client has limited
      computational capabilities.  If the kdc-follow-referrals option is
      set, the KDC MAY act as the client to follow TGS referrals
      [REFERRALS], and return the service ticket to the named server
      principal in the client request using the reply key expected by



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      the client.  That is, rather than returning a referral, the KDC
      follows that referral by contacting a remote KDC and processing
      the referral.  The kdc-referrals option can be implemented when
      the KDC knows the reply key.  The KDC can ignore kdc-referrals
      option when it does not understand it or it does not allow this
      option based on local policy.  The client SHOULD be capable of
      processing the KDC responses when this option is not honored by
      the KDC.  Clients SHOULD use TCP to contact a KDC if this option
      is going to be used to avoid problems when the client's UDP
      retransmit algorithm has timeouts insufficient to allow the KDC to
      interact with remote KDCs.

   The padata field contains a list of PA-DATA structures as described
   in Section 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.  However pre-
   authentication data methods such as [RFC4556] that include a checksum
   of the KDC-REQ-BODY should checksum the outer KDC-REQ-BODY.  These
   methods will already be bound to the inner body through the integrity
   protection in the FAST request.

6.5.3.  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 to
   Section 6.5.4.

   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.








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      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                 52

   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 who does not receive a PA-FX-FAST-REPLY in the
   KDC response MUST support a local policy that rejects 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.
            rep-key     [1] EncryptionKey OPTIONAL,
                -- This, if present, replaces the reply key for AS and
                -- TGS.
                -- MUST be absent in KRB-ERROR.
            finished    [2] KrbFastFinished OPTIONAL,
                -- Present in AS or TGS reply; absent otherwise.
            ...
        }

   The padata field in the KrbFastResponse structure contains a list of
   PA-DATA structures as described in Section 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 is otherwise in the outer KDC-
   REP structure into this field.  The padata field in the KDC reply
   structure outside of the PA-FX-FAST-REPLY structure typically



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   includes only the PA-FX- FAST-REPLY padata and optionally the PA-FX-
   COOKIE padata.

   The rep-key field, if present, contains the reply key that is used to
   encrypted the KDC reply.  The rep-key field MUST be absent in the
   case where an error occurs.  The enctype of the rep-key is the
   strongest mutually supported by the KDC and the client.

   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.
          checksum    [4] Checksum,
              -- Checksum performed over all the messages in the
              -- conversation, except the containing message.
              -- The checksum key is the armor key as defined in
              -- Section 6.5.1, and the checksum type is the required
              -- checksum type of the armor key.
          ticket-checksum [5] 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 in Section 5.6.1 of
   [RFC4120].  The client MUST use the KDC's time in these fields
   thereafter when using the returned ticket.  Note that the KDC's time
   in AS-REP may not match the authtime in the reply ticket if the kdc-
   follow-referrals option is requested and honored by the KDC.  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 time stamp returned.



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   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 checksum field contains a checksum of all the messages in the
   conversation prior to the containing message (the containing message
   is excluded).  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.  The ticket-checksum
   is a checksum of the issued ticket using the same key and key usage.

   When FAST padata is included, the PA-FX-COOKIE padata as defined in
   Section 6.3 MUST also be included if the KDC expects at least one
   more message from the client in order to complete the authentication.

6.5.4.  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.  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, then that
   information should be transmitted in a pa-data element within the
   KRBFastResponse structure.  The padata-type is the same as the data-
   type would be in the typed data element and the padata-value is the



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   same as the data-value.  As discussed in Section 8, 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].

6.5.5.  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 two pa-data items: PA-FX-FAST and PA-FX-
   COOKIE.  The client MAY include additional pa-data, but the KDC MUST
   ignore the outer request body and any padata besides PA-FX-FAST and
   PA-FX-COOKIE 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 and
   PA-FX-COOKIE should be included in PA-FX-FAST.  The client MUST
   ignore other padata outside of PA-FX-FAST.

6.5.6.  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
   time stamp pre-authentication option described in [RFC4120].  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 to
   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



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   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 time stamp reasonably close to the KDC's
   current time and encrypts it in the challenge key.  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 time stamps are the
   same; to be a replay, the ciphertext MUST be identical.  Allowing
   clients to re-use time stamps avoids requiring that clients maintain
   state about which time stamps 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 replace the reply key before
   issuing a ticket.  The client MUST check that the timestamp decrypts
   properly.  The client MAY check that the timestamp is winthin 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.

   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 replace the reply
   key.  It does not provide the strengthening-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



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   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.

6.6.  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 in Section 6.4.  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.

   The AD-authentication-strength element MUST be included in the AD-IF-
   RELEVANT, thus it can be ignored if it is unknown to the receiver.


7.  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 8 creates IANA registries for a different set of constants
   used by the extensions described in this document.




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7.1.  New Errors

           KDC_ERR_PREAUTH_EXPIRED                 90
           KDC_ERR_MORE_PREAUTH_DATA_NEEDED        91
           KDC_ERR_PREAUTH_BAD_AUTHENTICATION_SET  92
           KDC_ERR_UNKNOWN_CRITICAL_FAST_OPTIONS            93

7.2.  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             55

7.3.  Authorization Data Elements

           AD-authentication-strength              70
           AD-fx-fast-armor                        71

7.4.  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                  138


8.  IANA Considerations

   This document creates a number of IANA registries.  These registries
   should all be located under
   http://www.iana.org/assignments/kerberos-parameters.

8.1.  Pre-authentication and Typed Data

   RFC 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 registration



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   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.

      Type                Value    Reference
  ----------------------------------------------------------------------
  PA-TGS-REQ                 1    RFC 4120
  PA-ENC-TIMESTAMP           2    RFC 4120
  PA-PW-SALT                 3    RFC 4120
  [reserved]                 4
  PA-ENC-UNIX-TIME           5    (deprecated)
  PA-SANDIA-SECUREID         6
  PA-SESAME                  7
  PA-OSF-DCE                 8
  PA-CYBERSAFE-SECUREID      9
  PA-AFS3-SALT               10
  PA-ETYPE-INFO              11   RFC 4120
  PA-SAM-CHALLENGE           12   (sam/otp)
  PA-SAM-RESPONSE            13   (sam/otp)
  PA-PK-AS-REQ_OLD           14   draft-ietf-cat-kerberos-pk-init-09
  PA-PK-AS-REP_OLD           15   draft-ietf-cat-kerberos-pk-init-09



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  PA-PK-AS-REQ               16   RFC 4556
  PA-PK-AS-REP               17   RFC 4556
  PA-ETYPE-INFO2             19   RFC 4120
  PA-USE-SPECIFIED-KVNO      20
  PA-SAM-REDIRECT            21   (sam/otp)
  PA-GET-FROM-TYPED-DATA     22   (embedded in typed data)
  TD-PADATA                  22   (embeds padata)
  PA-SAM-ETYPE-INFO          23   (sam/otp)
  PA-ALT-PRINC               24   (crawdad@fnal.gov)
  PA-SAM-CHALLENGE2          30   (kenh@pobox.com)
  PA-SAM-RESPONSE2           31   (kenh@pobox.com)
  PA-EXTRA-TGT               41   Reserved extra TGT
  TD-PKINIT-CMS-CERTIFICATES 101  CertificateSet from CMS
  TD-KRB-PRINCIPAL           102  PrincipalName
  TD-KRB-REALM               103  Realm
  TD-TRUSTED-CERTIFIERS      104  PKINIT
  TD-CERTIFICATE-INDEX       105  PKINIT
  TD-APP-DEFINED-ERROR       106  Application specific
  TD-REQ-NONCE               107  INTEGER
  TD-REQ-SEQ                 108  INTEGER
  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  draft-ietf-krb-wg-preauth-framework
  PA-AUTHENTICATION-SET      134  draft-ietf-krb-wg-preauth-framework
  PA-AUTH-SET-SELECTED       135  draft-ietf-krb-wg-preauth-framework
  PA-FX-FAST                 136  draft-ietf-krb-wg-preauth-framework
  PA-FX-ERROR                137  draft-ietf-krb-wg-preauth-framework
  PA-ENCRYPTED-CHALLENGE     138  draft-ietf-krb-wg-preauth-framework
  PA-OTP-CHALLENGE           141  (gareth.richards@rsa.com)
  PA-OTP-REQUEST             142  (gareth.richards@rsa.com)
  PA-OTP-CONFIRM             143  (gareth.richards@rsa.com)
  PA-SUPPORTED-ETYPES        165  MS-KILE

8.2.  Fast Armor Types

   FAST armor types are defined in Section 6.5.1.  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.





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8.3.  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.

      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   Requesting the KDC to follow
                                   referrals


9.  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 denied of service using this option, KDCs MAY
   restrict the number of simultaneous active requests with this option
   for any given client principal.

   With regarding to 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
   clear text introduced by the Encrypted Challenge FAST factor.


10.  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 [ID.CROSS].



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   Joel Webber had a proposal for a mechanism similar to FAST that
   created a protected tunnel for Kerberos pre-authentication.


11.  References

11.1.  Normative References

   [KRB-ANON]
              Zhu, L. and P. Leach, "Kerberos Anonymity Support",
              draft-ietf-krb-wg-anon-04.txt (work in progress), 2007.

   [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.

11.2.  Informative References

   [ID.CROSS]
              Sakane, S., Zrelli, S., and M. Ishiyama , "Problem
              Statement on the Operation of Kerberos in a Specific
              System", draft-sakane-krb-cross-problem-statement-02.txt
              (work in progress), April 2007.

   [KRB-WG.SAM]
              Hornstein, K., Renard, K., Neuman, C., and G. Zorn,
              "Integrating Single-use Authentication Mechanisms with
              Kerberos", draft-ietf-krb-wg-kerberos-sam-02.txt (work in
              progress), October 2003.

   [REFERRALS]
              Raeburn, K. and L. Zhu, "Generating KDC Referrals to
              Locate Kerberos Realms",
              draft-ietf-krb-wg-kerberos-referrals-10.txt (work in
              progress), 2007.







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Appendix A.  Change History

   RFC editor, please remove this section before publication.

A.1.  Changes since 09

      Clarify conversations by defining for TGS and by describing how
      cookies form conversation boundaries.
      Simplify text surrounding when finish is included: always for AS
      and TGS reply, never for error.
      Fill in IANA and constants

A.2.  Changes since 08

      Fix a number of typos
      Rename anonymous flag to hide-client-name; rename kdc-referals to
      kdc-follow-referrals
      Clarify how anonymous pkinit interacts with KDC verified.
      Introduce AD-fx-fast-armor authorization data to deal with
      unprivileged processes constructing KDC requests.  Note that a TGT
      is always used for armor tickets if the armor field is present; if
      you proxy or validate you'll end up with a TGT armor ticket and
      another ticket in the pa-tgs-req.  Alternatively you can simply
      use the other ticket in the PA-TGS-REQ; weak consensus within WG.
      All KDCs in a realm MUST support FAST if it is to be offered.
      The cookie message is always generated by the KDC.
      Note that the client can trust and need not verify the time stamp
      in the finish message.  This can seed the client's idea of KDC
      time.
      Note that the client name in the finish message may differ from
      the name in the request if referrals are used.
      Note that KDCs should advertize fast in preauth_required errors.
      Armor key is constructed using KRB-FX-CF2.  This is true even in
      the TGS case; there is no security reason to do this.  Using the
      subkey as done in draft 08 would be fine, but the current text
      uses the same procedure both in the TGS and AS case.
      Use a different challenge key in each direction in the encrypted
      challenge option.
      Note that the KDC should process PA-FX-COOKIE before other padata.
      KRB-FX-CF2 uses k1's enctype for the result; change around calling
      order so we pass in subkeys and armor keys as k1 in preference to
      long-term keys or ticket session keys.
      Clarify the relationship between authentication sets and cookies.
      A cookie may not be needed in the first message.  Clarify how this
      interacts with optimistic clients.
      Remove text raising a concern that RFC 3961 may permit ciphertext
      transformations that do not change plaintext: discussion on the
      list came to the conclusion that RFC 3961 does not permit this.



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      Remove binding key concept; use the armor key instead.  The cookie
      becomes just an octet string.
      Include PA-FX-ERROR to protect the error information per Dublin.
      Returning preauth_failed in the failed to decrypt encrypted
      challenge seems fine; remove the issue marker
      Add a section describing what goes in the inner and outer request.
      I believe it is redundant but found it useful while putting
      together an implementation proposal.
      Use hyphen rather than underscore in the constants for pre-
      authentication data to be consistent with RFC 4120.
      Add a ticket-checksum to the finished message
      Remove redundant KEY_USAGE_FAST_ARMOR.
      Add protocol constants section for non-IANA registrations and
      flesh out IANA section.
      Clarify that kdc-req-body checksums should always use the outer
      body even for mechanisms like PKINIT that include their own (now
      redundant) checksum.
      Remove mechanism for encapsulating typed data in padata; just
      reflect the value.

A.3.  Changes since 07

      Propose replacement of authenticated timestamp with encrypted
      challenge.  The desire to avoid clients needing time
      synchronization and to simply the factor.
      Add a requirement that any FAST armor scheme must provide a fresh
      key for each conversation.  This allows us to assume that anything
      encrypted/integrity protected in the right key is fresh and not
      subject to cross-conversation cut and paste.
      Removed heartbeat padata.  The KDC will double up messages if it
      needs to; the client simply sends its message and waits for the
      next response.
      Define PA-AUTH-SET-SELECTED
      Clarify a KDC cannot ignore padata is has claimed to support

A.4.  Changes since 06

      Note that even for replace reply key it is likely that the side
      using the mechanism will know that the other side supports it.
      Since it is reasonably unlikely we'll need a container mechanism
      other than FAST itself, we don't need to optimize for that case.
      So, we want to optimize for implementation simplicity.  Thus if
      you do have such a container mechanism interacting with
      authentication sets we'll assume that the hint need to describe
      hints for all contained mechanisms.  This closes out a long-
      standing issue.





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      Write up what Sam believes is the consensus on UI and prompts in
      the authentication set: clients MAY assume that they have all the
      UI information they need.


Appendix 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
                FROM KerberosV5Spec2 { iso(1) identified-organization(3)
                  dod(6) internet(1) security(5) kerberosV5(2)
                  modules(4) krb5spec2(2) };
                  -- as defined in RFC 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.



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              -- 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.
          ...
      }

      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),
          -- anonymous(1),
          -- kdc-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 {



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          padata      [0] SEQUENCE OF PA-DATA,
              -- padata typed holes.
          rep-key     [1] EncryptionKey OPTIONAL,
              -- This, if present, replaces the reply key for AS and
              -- TGS.
              -- MUST be absent in KRB-ERROR.
          finished    [2] KrbFastFinished OPTIONAL,
              -- Present in AS or TGS reply; absent otherwise.
          ...
      }

      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.
          checksum    [4] Checksum,
              -- Checksum performed over all the messages in the
              -- conversation, except the containing message.
              -- The checksum key is the armor key as defined in
              -- Section 6.5.1, and the checksum type is the required
              -- checksum type of the armor key.
          ticket-checksum [5] 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.
      END


Authors' Addresses

   Sam hartman
   Painless Security

   Email: hartmans-ietf@mit.edu





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   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: lzhu@microsoft.com












































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