draft-ietf-krb-wg-preauth-framework-01.txt   [plain text]

Kerberos Working Group                                        S. Hartman
Internet-Draft                                                       MIT
Expires: January 17, 2005                                  July 19, 2004

        A Generalized Framework for Kerberos Pre-Authentication

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   Copyright (C) The Internet Society (2004). All Rights Reserved.


   Kerberos is a protocol for verifying the identity of principals
   (e.g., a workstation user or a network server) on an open network.
   The Kerberos protocol provides a mechanism called pre-authentication
   for proving the identity  of a principal and for better protecting
   the long-term secret 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.

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   This document also provides common tools needed by multiple
   pre-authentication mechanisms.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [1].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Model for Pre-Authentication . . . . . . . . . . . . . . . . .  4
     2.1   Information Managed by Model . . . . . . . . . . . . . . .  5
     2.2   The Preauth_Required Error . . . . . . . . . . . . . . . .  7
     2.3   Client to KDC  . . . . . . . . . . . . . . . . . . . . . .  7
     2.4   KDC to Client  . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Pre-Authentication Facilities  . . . . . . . . . . . . . . . .  9
     3.1   Client Authentication  . . . . . . . . . . . . . . . . . . 10
     3.2   Strengthen Reply Key . . . . . . . . . . . . . . . . . . . 10
     3.3   Replace Reply Key  . . . . . . . . . . . . . . . . . . . . 11
     3.4   Verify Response  . . . . . . . . . . . . . . . . . . . . . 11
   4.  Requirements for Pre-Authentication Mechanisms . . . . . . . . 12
   5.  Tools for Use in Pre-Authentication Mechanisms . . . . . . . . 13
     5.1   Combine Keys . . . . . . . . . . . . . . . . . . . . . . . 13
     5.2   Signing Requests/Responses . . . . . . . . . . . . . . . . 13
     5.3   Managing State for the KDC . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 17
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 17
   A.  Todo List  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Introduction

   The core Kerberos specification 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 response.  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 the 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 secret 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
   pre-authentication mechanisms perform as well as how these functions
   affect the state of the request and response.  In addition several
   common tools needed by pre-authentication mechanisms are provided.
   Unlike [3], this framework is not complete--it does not describe all
   the inputs and outputs for the pre-authentication mechanisms.
   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 document should be read only after reading the documents
   describing the Kerberos cryptography framework [3] and the core
   Kerberos protocol [2].  This document freely uses terminology and
   notation from these documents without reference or further

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2.  Model for Pre-Authentication

   when a Kerberos client wishes to obtain a ticket using the
   authentication server, it sends an initial AS request.  If
   pre-authentication is 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 a round-trip and send
   an initial request with padata included.  If the client includes the
   wrong padata, the server MAY return KDC_ERR_PREAUTH_FAILED with no
   indication of what padata should have been included.  For
   interoperability reasons, clients that include optimistic
   pre-authentication MUST retry with no padata and examine the
   response to their initial optimistic request.

   The 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
   authentication session.  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 response.  For these simple scenarios,
   the client only sends one request with pre-authentication data and so
   the authentication session is trivial.  For more complex
   authentication sessions, 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 5.3.

   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 response.  The padata 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.  These 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 pre-authentication data at each step of
   the AS request process.

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2.1  Information Managed by 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 response
   o  How strongly the identity of the client has been authenticated
   o  Whether the reply key has been used in this authentication session
   o  Whether the reply key has been replaced in this authentication
   o  Whether the contents of the KDC response can be  verified by the
      client principal
   o  Whether the contents of the KDC response can be  verified by the
      client machine

   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 of the
   Kerberos protocol [2], 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 handling a request, the client picks a reply
   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.

   At the beginning of handling a message on both the client and 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.  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

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   relatively weak forms of pre-authentication like encrypted timestamp.

   Initially the reply key has not been used.  A pre-authentication
   mechanism that uses the reply key either directly to encrypt or
   checksum some data or indirectly in the generation of new keys MUST
   indicate that the reply key is used.  This state is maintained by the
   client and KDC to enforce the security requirement stated in Section
   3.3 that the reply key cannot 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 3.3,
   then the state MUST be updated to indicate that the reply key has
   been replaced. Once the reply key has been replaced, knowledge of the
   reply key is insufficient to authenticate the client. The reply key
   is marked 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 request to the client, once the
   reply key is replaced, then the reply key remains replaced for the
   remainder of the authentication session.

   Without pre-authentication, the client knows that the KDC request is
   authentic and has not been modified because it is encrypted in the
   long-term key of the client.  Only the KDC and client know that key.
   So at the start of handling any message the KDC request is presumed
   to be verified to the client principal.  Any pre-authentication
   mechanism that sets a new reply key not based on the principal's
   long-term secret MUST either verify the KDC response some other way
   or indicate that the response is not verified.  If a mechanism
   indicates that the response is not verified then the client
   implementation MUST return an error unless a subsequent mechanism
   verifies the response.  The KDC needs to track this state so it can
   avoid generating a response 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 response 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 to authenticate the KDC.
   Examples of this include signing the response with a well-known
   public key or providing a ticket for the client machine as a service
   in addition to the requested ticket.

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2.2  The Preauth_Required Error

   Typically a client starts an authentication session by sending  an
   initial request with no pre-authentication.  If the KDC requires
   pre-authentication, then it returns a KDC_ERR_PREAUTH_REQUIRED
   message.  This message MAY also be returned for pre-authentication
   configurations that use multi-round-trip mechanisms.  This error
   contains a sequence of padata.  Typically the padata contains the
   pre-authentication type IDs of all the available pre-authentication
   mechanisms.  IN the initial error response, most mechanisms do not
   contain data.  If a mechanism requires multiple round trips or starts
   with a challenge from the KDC to the client, then it will likely
   contain data in the initial error response.

   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 pre-authentication is desirable and where
   the KDC needs to expose ciphertext encrypted in a weak key before the
   client has proven knowledge of that key.

   In order to generate the error response, the KDC first starts by
   initializing the pre-authentication state.  Then it processes any
   padata in the client's request in the order provided by the client.
   Mechanisms that are not understood by the KDC are ignored.
   Mechanisms that are inappropriate for the client principal or request
   SHOULD also be 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 generated 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 mechanisms subsequent in the
   generated response receive this state as input.  After the padata is
   generated, the error response is sent.

2.3  Client to KDC

   This description assumes 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 optimisticly
   choose the information it would normally receive from that error

   The client starts by initializing the pre-authentication state as
   specified.  It then processes the pdata in the

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   After processing the pdata 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. When the request is complete it is sent.

2.4  KDC to Client

   When a KDC receives an AS request from a client, it needs to
   determine whether it will respond with an error or  a 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 Kerberos

   From the standpoint of evaluating the pre-authentication, the KDC
   first starts by initializing the pre-authentication state.  IT then
   processes the padata in the request.  AS mentioned in Section 2.2,
   the KDC MAY ignore padata that is inappropriate for the configuration
   and MUST ignore padata of an unknown type.

   At this point the KDC decides whether it will issue a
   pre-authentication required error or a reply.  Typically a KDC will
   issue a reply if the client's identity has been authenticated to a
   sufficient degree.  The processing of the pre-authentication required
   error is described in Section 2.2.

   The KDC generates the pdata modifying the pre-authentication state as
   necessary.  Then it generates the final response, encrypting it in
   the current pre-authentication reply key.

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3.  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 2mechanism 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.

   According to Kerberos extensibility rules (section 1.4.2 of the
   Kerberos specification [2]), an extension MUST NOT change the
   semantics of a message unless a recipient is known to understand that
   extension.  Because a client does not know that the KDC supports a
   particular pre-authentication mechanism when it sends an initial
   request, a preauth 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 clients have indicated support
   for the message.

   The only state in this model that would break the interpretation of a
   message is changing the expected reply key.  If one mechanism changed
   the reply key and a later mechanism used that reply key, then a KDC
   that interpreted the second mechanism but not the first would fail to
   interpret the request correctly.  In order to avoid this problem,
   extensions that change core semantics are typically divided into two
   parts.  The first part proposes a change to the core semantic--for
   example proposes a new reply key.  The second part acknowledges that
   the extension is understood and that the change takes effect. Section
   3.2 discusses how to design mechanisms that modify the reply key to
   be split into a proposal and acceptance without requiring additional
   round trips to use the new reply key in subsequent
   pre-authentication. Other changes in the state described in Section
   2.1 can safely be ignored by a KDC that does not understand a

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   mechanism.  Mechanisms that modify the behavior of the request
   outside the scope of this framework need to carefully consider the
   Kerberos extensibility rules to avoid similar problems.

3.1  Client Authentication

   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 of the Kerberos specification [2] and the
   single-use mechanism defined in [5]. 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
   [6] implements Client Authentication along side Replace Reply Key.

   If the reply key has been replaced, then mechanisms such as encrypted
   timestamp that rely on knowledge of the reply key to authenticate the
   client MUST NOT be used.

3.2  Strengthen Reply Key

   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 [5].  Typically these additional secrets are
   converted into a Kerberos protocol key.  Then they are combined with
   the existing reply key as discussed in Section 5.1.

   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 interpreted as if they were included

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   immediately following the proposal.  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  are included is a matter for the mechanism specification.
   Similarly, the format of the message accepting the proposal is

   Mechanisms implementing this facility and including a typed hole for
   additional padata MUST checksum that padata using a keyed checksum or
   encrypt the padata.  Typically the reply key is used to protect the
   padata.  XXX If you are only minimally increasing the strength of the
   reply key, this may give the attacker access to something too close
   to the original reply key.  However, binding the padata to the new
   reply key  seems potentially important from a security standpoint.
   There may also be objections to this from a double encryption
   standpoint because we also recommend client authentication facilities
   be tied to the reply key.

3.3  Replace Reply Key

   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
   KDC in a secure manner.    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 Strengthen 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 more common 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 3.2 if appropriate.

3.4  Verify Response

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4.  Requirements for Pre-Authentication Mechanisms

      State management for multi-round-trip mechs
      Security interactions with other mechs

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5.  Tools for Use in Pre-Authentication Mechanisms

5.1  Combine Keys

5.2  Signing Requests/Responses

5.3  Managing State for the KDC

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6.  IANA Considerations

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7.  Security Considerations

      Very little of the AS request is authenticated.  Same for padata
      in the reply or error.  Discuss implications
      Table of security requirements stated elsewhere in the document

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8.  Acknowledgements

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9.  References

9.1  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, BCP 14, March 1997.

   [2]  Neuman, C., Yu, T., Hartman, S. and K. Raeburn, "The Kerberos
        Network Authentication Service (V5)",
        draft-ietf-krb-wg-kerberos-clarifications-06.txt (work in
        progress), June 2004.

   [3]  Raeburn, K., "Encryption and Checksum Specifications for
        Kerberos 5", draft-ietf-krb-wg-crypto-03.txt (work in progress).

   [4]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

9.2  Informative References

   [5]  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.

   [6]  Tung, B., Neuman, C., Hur, M., Medvinsky, A. and S. Medvinsky,
        "Public Key Cryptography for Initial Authentication in
        Kerberos", draft-ietf-cat-kerberos-pk-init-19.txt (work in
        progress), April 2004.

Author's Address

   Sam hartman

   EMail: hartmans@mit.edu

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Appendix A.  Todo List

      Flesh out sections that are still outlines
      Discuss cookies and multiple-round-trip mechanisms.
      Talk about checksum contributions from each mechanism

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Hartman                 Expires January 17, 2005               [Page 19]