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Versions: (RFC 2478) 00 01 02 03 04 05 RFC 4178

NETWORK WORKING GROUP                                             L. Zhu
Internet-Draft                                                  P. Leach
Obsoletes: 2478 (if approved)                              K. Jaganathan
Expires: July 27, 2005                             Microsoft Corporation
                                                            W. Ingersoll
                                                        Sun Microsystems
                                                        January 23, 2005


         The Simple and Protected GSS-API Negotiation Mechanism
                      draft-ietf-kitten-2478bis-05

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 27, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document specifies a negotiation mechanism for the Generic
   Security Service Application Program Interface (GSS-API) which is
   described in RFC 2743.



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   GSS-API peers can use this negotiation mechanism to choose from a
   common set of security mechanisms.

   If per-message integrity services are available on the established
   mechanism context, then the negotiation is protected against an
   attacker forcing the selection of a mechanism not desired by the
   peers.

   This mechanism replaces RFC 2478 in order to fix defects in that
   specification and to describe how to inter-operate with
   implementations of that specification commonly deployed on the
   Internet.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  5
   3.  Negotiation Protocol . . . . . . . . . . . . . . . . . . . . .  6
     3.1   Negotiation Description  . . . . . . . . . . . . . . . . .  6
     3.2   Negotiation Procedure  . . . . . . . . . . . . . . . . . .  7
   4.  Token Definitions  . . . . . . . . . . . . . . . . . . . . . . 10
     4.1   Mechanism Types  . . . . . . . . . . . . . . . . . . . . . 10
     4.2   Negotiation Tokens . . . . . . . . . . . . . . . . . . . . 10
       4.2.1   negTokenInit . . . . . . . . . . . . . . . . . . . . . 11
       4.2.2   negTokenResp . . . . . . . . . . . . . . . . . . . . . 12
   5.  Processing of mechListMIC  . . . . . . . . . . . . . . . . . . 14
   6.  Extensibility  . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 20
   10.   References . . . . . . . . . . . . . . . . . . . . . . . . . 21
     10.1  Normative References . . . . . . . . . . . . . . . . . . . 21
     10.2  Informative References . . . . . . . . . . . . . . . . . . 21
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 21
   A.  SPNEGO ASN.1 Module  . . . . . . . . . . . . . . . . . . . . . 23
   B.  GSS-API Negotiation Support API  . . . . . . . . . . . . . . . 25
     B.1   GSS_Set_neg_mechs call . . . . . . . . . . . . . . . . . . 25
     B.2   GSS_Get_neg_mechs call . . . . . . . . . . . . . . . . . . 25
   C.  Changes since RFC2478  . . . . . . . . . . . . . . . . . . . . 27
   D.  mechListMIC Computation Example  . . . . . . . . . . . . . . . 29
       Intellectual Property and Copyright Statements . . . . . . . . 30










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1.  Introduction

   The GSS-API [RFC2743] provides a generic interface which can be
   layered atop different security mechanisms such that if communicating
   peers acquire GSS-API credentials for the same security mechanism,
   then a security context may be established between them (subject to
   policy).  However, GSS-API does not prescribe the method by which
   GSS-API peers can establish whether they have a common security
   mechanism.

   The Simple and Protected GSS-API Negotiation (SPNEGO) mechanism
   defined here is a pseudo security mechanism, which enables GSS-API
   peers to determine in-band whether their credentials support a common
   set of one or more GSS-API security mechanisms, and if so, to invoke
   the normal security context establishment for a selected common
   security mechanism.  This is most useful for applications which
   depend on GSS-API implementations and share multiple mechanisms
   between the peers.

   The SPNEGO mechanism negotiation is based on the following model: the
   initiator proposes a list of security mechanism(s), in decreasing
   preference order (favorite choice first), the acceptor (also known as
   the target) either accepts the initiator's preferred security
   mechanism (the first in the list), or chooses one that is available
   from the offered list, or rejects the proposed value(s).  The target
   then informs the initiator of its choice.

   Once a common security mechanism is chosen, mechanism-specific
   options MAY be negotiated as part of the selected mechanism's context
   establishment.  These negotiations (if any) are internal to the
   mechanism and opaque to the SPNEGO protocol.  As such they are
   outside the scope of this document.

   If per-message integrity services are available on the established
   mechanism security context, then the negotiation is protected to
   ensure that the mechanism list has not been modified.  In cases where
   an attacker could have materially influenced the negotiation, peers
   exchange message integrity code (MIC) tokens to confirm the mechanism
   list has not been modified.  If no action of an attacker could have
   materially modified the outcome of the negotiation, the exchange of
   MIC tokens is optional (see Section 5).  Allowing MIC tokens to be
   optional in this case provides interoperability with existing
   implementations while still protecting the negotiation.  This
   interoperability comes at the cost of increased complexity.

   SPNEGO relies on the concepts developed in the GSS-API specification
   [RFC2743].  The negotiation data is encapsulated in context-level
   tokens.  Therefore, callers of the GSS-API do not need to be aware of



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   the existence of the negotiation tokens but only of the new
   pseudo-security mechanism.  A failure in the negotiation phase causes
   a major status code to be returned: GSS_S_BAD_MECH.
















































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2.  Conventions 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].














































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3.  Negotiation Protocol

   When the established mechanism context provides integrity protection,
   the mechanism negotiation can be protected.  When acquiring
   negotiated security mechanism tokens, per-message integrity services
   are always requested by the SPNEGO mechanism.

   When the established mechanism context supports per-message integrity
   services, SPNEGO guarantees that the selected mechanism is mutually
   preferred.

   This section describes the negotiation process of this protocol.

3.1  Negotiation Description

   The first negotiation token sent by the initiator contains an ordered
   list of mechanisms in decreasing preference order (favorite mechanism
   first), and optionally the initial mechanism token for the preferred
   mechanism of the initiator (i.e., the first in the list).  (Note that
   the list MUST NOT contain either this SPNEGO mechanism itself or any
   mechanism for which the client does not have appropriate
   credentials.)

   The target then processes the token from the initiator.  This will
   result in one of four possible states (as defined in Section 4.2.2)
   being returned in the reply message: accept-completed,
   accept-incomplete, reject, or request-mic.  A reject state will
   terminate the negotiation;  an accept-completed state indicates that
   not only was the initiator-selected mechanism acceptable to the
   target, but also that the security mechanism token embedded in the
   first negotiation message was sufficient to complete the
   authentication;  an accept-incomplete state indicates that further
   message exchange is needed but the MIC token exchange as described in
   Section 5 is OPTIONAL;  a request-mic state (this state can only be
   present in the first reply message from the target) indicates the MIC
   token exchange is REQUIRED if per-message integrity services are
   available.

   Unless the preference order is specified by the application, the
   policy by which the target chooses a mechanism is an
   implementation-specific local matter.  In the absence of an
   application specified preference order or other policy, the target
   SHALL choose the first mechanism in the initiator proposed list for
   which it has valid credentials.

   In case of a successful negotiation, the security mechanism in the
   first reply message represents the value suitable for the target,
   chosen from the list offered by the initiator.



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   In case of an unsuccessful negotiation, the reject state is returned
   and generating a context level negotiation token is OPTIONAL.

   Once a mechanism has been selected, context establishment tokens
   specific to the selected mechanism are carried within the negotiation
   tokens.

   Lastly, MIC tokens may be exchanged to ensure the authenticity of the
   mechanism list received by the target.

   To avoid conflicts with the use of MIC tokens by SPNEGO,
   partially-established contexts MUST NOT be used for per-message
   calls.  To guarantee this, the prot_ready_state [RFC2743] MUST be set
   to false on return from GSS_Init_sec_context() and
   GSS_Accept_sec_context() even if the underlying mechanism returned
   true.

   Note that in order to avoid an extra round trip, the first context
   establishment token of the initiator's preferred mechanism SHOULD be
   embedded in the initial negotiation message (as defined in
   Section 4.2).  (This mechanism token is referred to as the optimistic
   mechanism token in this document.) In addition, using the optimistic
   mechanism token allows the initiator to recover from non-fatal errors
   encountered trying to produce the first mechanism token before a
   mechanism can be selected.  Implementations MAY omit the optimistic
   mechanism token in cases where the likelihood of the initiator's
   preferred mechanism not being selected by the acceptor is significant
   given the cost of generating it.

3.2  Negotiation Procedure

   The basic form of the procedure assumes that per-message integrity
   services are available on the established mechanism context, and it
   is summarized as follows:

   (a) The GSS-API initiator invokes GSS_Init_sec_context() as normal,
      but requests that SPNEGO be used.  SPNEGO can either be explicitly
      requested or accepted as the default mechanism.

   (b) The initiator GSS-API implementation generates a negotiation
      token containing a list of one or more security mechanisms that
      are available based on the credentials used for this context
      establishment, and optionally the initial mechanism token for the
      first mechanism in the list.







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   (c) The GSS-API initiator application sends the token to the target
      application.  The GSS-API target application passes the token by
      invoking GSS_Accept_sec_context().  The acceptor will do one of
      the following:


         (I) If none of the proposed mechanisms are acceptable, the
            negotiation SHALL be terminated.  GSS_Accept_sec_context
            indicates GSS_S_BAD_MECH.  The acceptor MAY output a
            negotiation token containing a reject state.

         (II) If either the initiator's preferred mechanism is not
            accepted by the target or this mechanism is accepted but it
            is not the acceptor's most preferred mechanism (i.e., the
            MIC token exchange as described in Section 5 is required),
            GSS_Accept_sec_context() indicates GSS_S_CONTINUE_NEEDED.
            The acceptor MUST output a negotiation token containing a
            request-mic state.

         (III) Otherwise if at least one additional negotiation token
            from the initiator is needed to establish this context,
            GSS_Accept_sec_context() indicates GSS_S_CONTINUE_NEEDED and
            outputs a negotiation token containing an accept-incomplete
            state.

         (IV) Otherwise no additional negotiation token from the
            initiator is needed to establish this context,
            GSS_Accept_sec_context() indicates GSS_S_COMPLETE and
            outputs a negotiation token containing an accept_complete
            state.

      If the initiator's preferred mechanism is accepted, and an
      optimistic mechanism token was included, this mechanism token MUST
      be passed to the selected mechanism by invoking
      GSS_Accept_sec_context() and if a response mechanism token is
      returned, it MUST be included in the response negotiation token.
      Otherwise, the target will not generate a response mechanism token
      in the first reply.

   (d) The GSS-API target application returns the negotiation token to
      the initiator application.  The GSS-API initiator application
      passes the token by invoking GSS_Init_sec_context().  The security
      context initialization is then continued according to the standard
      GSS-API conventions for the selected mechanism, where the tokens
      of the selected mechanism are encapsulated in negotiation messages
      (see Section 4) until GSS_S_COMPLETE is returned for both the
      initiator and the target by the selected security mechanism.




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   (e) MIC tokens are then either skipped or exchanged according to
      Section 5.

   Note that the *_req_flag input parameters for context establishment
   are relative to the selected mechanism, as are the *_state output
   parameters.  i.e., these parameters are not applicable to the
   negotiation process per se.

   On receipt of a negotiation token on the target side, a GSS-API
   implementation that does not support negotiation would indicate the
   GSS_S_BAD_MECH status as if a particular basic security mechanism had
   been requested and was not supported.

   When a GSS-API credential is acquired for the SPNEGO mechanism the
   implementation SHOULD produce a credential element for the SPNEGO
   mechanism which internally contains GSS-API credential elements for
   all mechanisms for which the principal has credentials available,
   except for any mechanisms which are not to be negotiated, either as
   per implementation-, site- or application-specific policy.  See
   Appendix B for interfaces for expressing application policy.































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4.  Token Definitions

   The type definitions in this section assume an ASN.1 module
   definition of the following form:


       SPNEGOASNOneSpec {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanism(5) snego (2) modules(4) spec2(2)
       } DEFINITIONS EXPLICIT TAGS ::= BEGIN

       -- rest of definitions here

       END


   This specifies that the tagging context for the module will be
   explicit and non-automatic.

   The encoding of SPNEGO protocol messages shall obey the Distinguished
   Encoding Rules (DER) of ASN.1 as described in [X690].

4.1  Mechanism Types

   In this negotiation model, each OID represents one GSS-API mechanism
   or one variant (see Section 6) of it according to [RFC2743].


       MechType ::= OBJECT IDENTIFIER
           -- OID represents each security mechanism as suggested by
           -- [RFC2743]

       MechTypeList ::= SEQUENCE OF MechType


4.2  Negotiation Tokens

   The syntax of the initial negotiation tokens follows the
   initialContextToken syntax defined in Section 3.1 of [RFC2743].  The
   SPNEGO pseudo mechanism is identified by the Object Identifier
   iso.org.dod.internet.security.mechanism.snego (1.3.6.1.5.5.2).
   Subsequent tokens MUST NOT be encapsulated in this GSS-API generic
   token framing.

   This section specifies the syntax of the inner token for the initial
   message and the syntax of subsequent context establishment tokens.





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       NegotiationToken ::= CHOICE {
           negTokenInit    [0] NegTokenInit,
           negTokenResp    [1] NegTokenResp
       }



4.2.1  negTokenInit

       NegTokenInit ::= SEQUENCE {
           mechTypes       [0] MechTypeList,
           reqFlags        [1] ContextFlags  OPTIONAL,
             -- inherited from RFC 2478 for backward compatibility,
             -- RECOMMENDED to be left out
           mechToken       [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }
       ContextFlags ::= BIT STRING {
           delegFlag       (0),
           mutualFlag      (1),
           replayFlag      (2),
           sequenceFlag    (3),
           anonFlag        (4),
           confFlag        (5),
           integFlag       (6)
       } (SIZE (32))

   This is the syntax for the inner token of the initial negotiation
   message.

   mechTypes

         This field contains one or more security mechanisms available
         for the initiator in decreasing preference order (favorite
         choice first).

   reqFlags

         This field, if present, contains the service options that are
         requested to establish the context (the req_flags parameter of
         GSS_Init_sec_context()).  This field is inherited from RFC 2478
         and it is not integrity protected.  For implementations of this
         specification the initiator SHOULD omit this reqFlags field,
         and the acceptor MUST ignore this reqFlags field.






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   mechToken

         This field, if present, contains the optimistic mechanism
         token.

   mechlistMIC

         This field, if present, contains a MIC token for the mechanism
         list in the initial negotiation message.  This MIC token is
         computed according to Section 5.


4.2.2  negTokenResp

       NegTokenResp ::= SEQUENCE {
           negState       [0] ENUMERATED {
               accept-completed    (0),
               accept-incomplete   (1),
               reject              (2),
               request-mic         (3)
           }                                 OPTIONAL,
             -- REQUIRED in the first reply from the target
           supportedMech   [1] MechType      OPTIONAL,
             -- present only in the first reply from the target
           responseToken   [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }

   This is the syntax for all subsequent negotiation messages.

   negState

         This field, if present, contains the state of the negotiation.
         This can be:

         accept-completed

            No further negotiation message from the peer is expected,
            and the security context is established for the sender.

         accept-incomplete

            At least one more negotiation message from the peer is
            needed to establish the security context.






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         reject

            The sender terminates the negotiation.

         request-mic

            The sender indicates that the exchange of MIC tokens, as
            described in Section 5, will be REQUIRED if per-message
            integrity services are available on the mechanism context to
            be established.  This value SHALL only be present in the
            first reply from the target.

         This field is REQUIRED in the first reply from the target, and
         it is OPTIONAL thereafter.  When negState is absent the actual
         state should be inferred from the state of the negotiated
         mechanism context.

   supportedMech

         This field SHALL only be present in the first reply from the
         target.  It MUST be one of the mechanism(s) offered by the
         initiator.

   ResponseToken

         This field, if present, contains tokens specific to the
         mechanism selected.

   mechlistMIC

         This field, if present, contains a MIC token for the mechanism
         list in the initial negotiation message.  This MIC token is
         computed according to Section 5.


















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5.  Processing of mechListMIC

   If the mechanism selected by the negotiation does not support
   integrity protection, then no mechlistMIC token is used.

   Otherwise, if the accepted mechanism is the most preferred mechanism
   of both the initiator and the acceptor, then the MIC token exchange,
   as described later in this section, is OPTIONAL.  A mechanism is the
   acceptor's most preferred mechanism if there is no other mechanism
   which, had it been present in the mechanism list, the acceptor would
   have preferred over the accepted mechanism.

   In all other cases, MIC tokens MUST be exchanged after the mechanism
   context is fully established.

   a) The mechlistMIC token (or simply the MIC token) is computed over
      the mechanism list in the initial negotiation message by invoking
      GSS_GetMIC() as follows: the input context_handle is the
      established mechanism context, the input qop_req is 0, and the
      input message is the DER encoding of the value of type
      MechTypeList which is contained in the "mechTypes" field of the
      NegTokenInit.  The input message is NOT the DER encoding of the
      type "[0] MechTypeList".

   b) If the selected mechanism exchanges an even number of mechanism
      tokens (i.e., the acceptor sends the last mechanism token), the
      acceptor does the following when generating the negotiation
      message containing the last mechanism token: if the MIC token
      exchange is optional, GSS_Accept_sec_context() either indicates
      GSS_S_COMPLETE and does not include a mechlistMIC token, or
      indicates GSS_S_CONTINUE_NEEDED and includes a mechlistMIC token
      and an accept-incomplete state; if the MIC token exchange is
      required, GSS_Accept_sec_context() indicates
      GSS_S_CONTINUE_NEEDED, and includes a mechlistMIC token.
      Acceptors that wish to be compatible with legacy Windows SPNEGO
      implementations as described in Appendix C should not generate a
      mechlistMIC token when the MIC token exchange is not required.
      The initiator then processes the last mechanism token, and does
      one of the following:

      (I) If a mechlistMIC token was included, and is correctly
         verified, GSS_Init_sec_context() indicates GSS_S_COMPLETE.  The
         output negotiation message contains a mechlistMIC token, and an
         accept_complete state.  The acceptor MUST then verify this
         mechlistMIC token.






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      (II) If a mechlistMIC token was included but is incorrect, the
         negotiation SHALL be terminated.  GSS_Init_sec_context()
         indicates GSS_S_DEFECTIVE_TOKEN.

      (III) If no mechlistMIC token was included, and the MIC token
         exchange is not required, GSS_Init_sec_context() indicates
         GSS_S_COMPLETE with no output token.

      (IV) If no mechlistMIC token was included, but the MIC token
         exchange is required, the negotiation SHALL be terminated.
         GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.

   c) In the case that the chosen mechanism exchanges an odd number of
      mechanism tokens (i.e., the initiator sends the last mechanism
      token), the initiator does the following when generating the
      negotiation message containing the last mechanism token: if the
      negState was request-mic in the first reply from the target, a
      mechlistMIC token MUST be included, otherwise the mechlistMIC
      token is OPTIONAL.  (Note that the MIC token exchange is required
      if a mechanism other than the initiator's first choice is chosen.)
      In the case that the optimistic mechanism token is the only
      mechanism token for the initiator's preferred mechanism, the
      mechlistMIC token is OPTIONAL.  Whether or not the mechlistMIC
      token is included, GSS_Init_sec_context() indicates
      GSS_S_CONTINUE_NEEDED.  Initiators that wish to be compatible with
      legacy Windows SPNEGO implementations as described in Appendix C
      should not generate a mechlistMIC token when the MIC token
      exchange is not required.  The acceptor then processes the last
      mechanism token and does one of the following:

      (I) If a mechlistMIC token was included and is correctly verified,
         GSS_Accept_sec_context() indicates GSS_S_COMPLETE.  The output
         negotiation message contains a mechlistMIC token and an
         accept_complete state.  The initiator MUST then verify this
         mechlistMIC token.

      (II) If a mechlistMIC token was included but is incorrect, the
         negotiation SHALL be terminated.  GSS_Accept_sec_context()
         indicates GSS_S_DEFECTIVE_TOKEN.

      (III) If no mechlistMIC token was included and the mechlistMIC
         token exchange is not required, GSS_Accept_sec_context()
         indicates GSS_S_COMPLETE.  The output negotiation message
         contains an accept_complete state.







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      (IV) In the case that the optimistic mechanism token is also the
         last mechanism token (when the initiator's preferred mechanism
         is accepted by the target) and the target sends a request-mic
         state but the initiator did not send a mechlistMIC token, the
         target then MUST include a mechlistMIC token in that first
         reply.  GSS_Accept_sec_context() indicates
         GSS_S_CONTINUE_NEEDED.  The initiator MUST verify the received
         mechlistMIC token and generate a mechlistMIC token to send back
         to the target.  The target SHALL in turn verify the returned
         mechlistMIC token and complete the negotiation.

      (V) If no mechlistMIC token was included and the acceptor sent a
         request-mic state in the first reply message (the exchange of
         MIC tokens is required), the negotiation SHALL be terminated.
         GSS_Accept_sec_context() indicates GSS_S_DEFECTIVE_TOKEN.




































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6.  Extensibility

   Two mechanisms are provided for extensibility.  First, the ASN.1
   structures in this specification MAY be expanded by IETF standards
   action.  Implementations receiving unknown fields MUST ignore these
   fields.

   Secondly, OIDs corresponding to a desired mechanism attribute (i.e.,
   mechanism variants) may be included in the set of preferred
   mechanisms by an initiator.  The acceptor can choose to honor this
   request by preferring mechanisms that have the included attributes.
   Future work within the Kitten working group is expected to
   standardize common attributes that SPNEGO mechanisms may wish to
   support.  At this time it is sufficient to say that initiators MAY
   include OIDs that do not correspond to mechanisms.  Such OIDs MAY
   influence the acceptor's choice of mechanism.  As discussed in
   Section 5, if there are mechanisms that if present in the initiator's
   list of mechanisms might be preferred by the acceptor to the
   initiator's preferred mechanism, the acceptor MUST demand the MIC
   token exchange.  As the consequence, acceptors MUST demand the MIC
   token exchange if they support negotiation of attributes not
   available in the initiator's preferred mechanism regardless of
   whether the initiator actually requested these attributes.




























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7.  Security Considerations

   In order to produce the MIC token for the mechanism list, the
   mechanism must provide integrity protection.  When the selected
   mechanism does not support integrity protection, the negotiation is
   vulnerable: an active attacker can force it to use a security
   mechanism that is not mutually preferred but is acceptable to the
   target.

   This protocol provides the following guarantees when per-message
   integrity services are available on the established mechanism context
   and the mechanism list was altered by an adversary such that a
   mechanism which is not mutually preferred could be selected:

   a) If the last mechanism token is sent by the initiator, both peers
      shall fail;
   b) If the last mechanism token is sent by the acceptor, the acceptor
      shall not complete and the initiator at worst shall complete with
      its preferred mechanism being selected.

   The negotiation may not be terminated if an alteration was made but
   it had no material impact.

   The protection of the negotiation depends on the strength of the
   integrity protection.  In particular, the strength of SPNEGO is no
   stronger than the integrity protection of the weakest mechanism
   acceptable to GSS-API peers.

   Note that where there exist multiple mechanisms with similar context
   tokens, but different semantics, such that some or all of the
   mechanisms' context tokens can be easily altered so that one
   mechanism's context tokens may pass for another of the similar
   mechanism's context tokens, then there may exist downgrade or similar
   attacks.  For example, if a given family of mechanisms uses the same
   context token syntax for two or more variants and depends on the OID
   in the initial token's pseudo-ASN.1/DER wrapper, but does not provide
   integrity protection for that OID, then there may exist an attack
   against those mechanisms.  SPNEGO does not generally defeat such
   attacks.

   In all cases, the communicating peers are exposed to the denial of
   service threat.









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8.  IANA Considerations

   This document has no actions for IANA.
















































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9.  Acknowledgments

   The authors wish to thank Sam Hartman, Nicolas Williams, Ken Raeburn,
   Martin Rex, Jeff Altman, Tom Yu, Cristian Ilac, Simon Spero and Bill
   Sommerfeld for their comments and suggestions during development of
   this document.

   Luke Howard provided a prototype of this protocol in Heimdal and
   resolved several issues in the initial draft.

   Eric Baize and Denis Pinkas wrote the original SPNEGO specification
   [RFC2478] of which some of the text has been retained in this
   document.






































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10.  References

10.1  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2743]  Linn, J., "Generic Security Service Application Program
              Interface Version 2, Update 1", RFC 2743, January 2000.

   [X690]     ASN.1 encoding rules: Specification of Basic Encoding
              Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER), ITU-T Recommendation
              X.690 (1997) | ISO/IEC International Standard 8825-1:1998.

10.2  Informative References

   [RFC2478]  Baize, E. and D. Pinkas, "The Simple and Protected GSS-API
              Negotiation Mechanism", RFC 2478, December 1998.


Authors' Addresses

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

   Email: lzhu@microsoft.com


   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: paulle@microsoft.com


   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: karthikj@microsoft.com




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   Wyllys Ingersoll
   Sun Microsystems
   1775 Wiehle Avenue, 2nd Floor
   Reston, VA  20190
   US

   Email: wyllys.ingersoll@sun.com












































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Appendix A.  SPNEGO ASN.1 Module

       SPNEGOASNOneSpec {
          iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanism(5) snego (2) modules(4) spec2(2)
       } DEFINITIONS EXPLICIT TAGS ::= BEGIN

       MechType ::= OBJECT IDENTIFIER
           -- OID represents each security mechanism as suggested by
           -- [RFC2743]

       MechTypeList ::= SEQUENCE OF MechType

       NegotiationToken ::= CHOICE {
           negTokenInit    [0] NegTokenInit,
           negTokenResp    [1] NegTokenResp
       }

       NegTokenInit ::= SEQUENCE {
           mechTypes       [0] MechTypeList,
           reqFlags        [1] ContextFlags  OPTIONAL,
             -- inherited from RFC 2478 for backward compatibility,
             -- RECOMMENDED to be left out
           mechToken       [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }
       NegTokenResp ::= SEQUENCE {
           negState       [0] ENUMERATED {
               accept-completed    (0),
               accept-incomplete   (1),
               reject              (2),
               request-mic         (3)
           }                                 OPTIONAL,
             -- REQUIRED in the first reply from the target
           supportedMech   [1] MechType      OPTIONAL,
             -- present only in the first reply from the target
           responseToken   [2] OCTET STRING  OPTIONAL,
           mechListMIC     [3] OCTET STRING  OPTIONAL,
           ...
       }

       ContextFlags ::= BIT STRING {
           delegFlag       (0),
           mutualFlag      (1),
           replayFlag      (2),
           sequenceFlag    (3),
           anonFlag        (4),



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           confFlag        (5),
           integFlag       (6)
       } (SIZE (32))

       END














































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Appendix B.  GSS-API Negotiation Support API

   In order to provide to a GSS-API caller (either the initiator or the
   target or both) the ability to choose among the set of supported
   mechanisms a reduced set of mechanisms for negotiation, two
   additional APIs are defined:

   o  GSS_Get_neg_mechs() indicates the set of security mechanisms
      available on the local system to the caller for negotiation, for
      which appropriate credentials are available.
   o  GSS_Set_neg_mechs() specifies the set of security mechanisms to be
      used on the local system by the caller for negotiation, for the
      given credentials.

B.1  GSS_Set_neg_mechs call

   Inputs:

   o  cred_handle CREDENTIAL HANDLE, -- NULL specifies default
      -- credentials
   o  mech_set SET OF OBJECT IDENTIFIER

   Outputs:

   o  major_status INTEGER,
   o  minor_status INTEGER

   Return major_status codes:

   o  GSS_S_COMPLETE indicates that the set of security mechanisms
      available for negotiation has been set to mech_set.
   o  GSS_S_FAILURE indicates that the requested operation could not be
      performed for reasons unspecified at the GSS-API level.

   Allows callers to specify the set of security mechanisms that may be
   negotiated with the credential identified by cred_handle.  This call
   is intended for support of specialized callers who need to restrict
   the set of negotiable security mechanisms from the set of all
   security mechanisms available to the caller (based on available
   credentials).  Note that if more than one mechanism is specified in
   mech_set, the order in which those mechanisms are specified implies a
   relative preference.

B.2  GSS_Get_neg_mechs call

   Input:





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   o  cred_handle CREDENTIAL HANDLE -- NULL specifies default
      -- credentials

   Outputs:

   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  mech_set SET OF OBJECT IDENTIFIER

   Return major_status codes:

   o  GSS_S_COMPLETE indicates that the set of security mechanisms
      available for negotiation has been returned in mech_set.
   o  GSS_S_FAILURE indicates that the requested operation could not be
      performed for reasons unspecified at the GSS-API level.

   Allows callers to determine the set of security mechanisms available
   for negotiation with the credential identified by cred_handle.  This
   call is intended for support of specialized callers who need to
   reduce the set of negotiable security mechanisms from the set of
   supported security mechanisms available to the caller (based on
   available credentials).

   Note: The GSS_Indicate_mechs() function indicates the full set of
   mechanism types available on the local system.  Since this call has
   no input parameter, the returned set is not necessarily available for
   all credentials.
























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Appendix C.  Changes since RFC2478

      SPNEGO implementations in Microsoft Windows 2000/Windows
      XP/Windows Server 2003 have the following behavior: no mechlistMIC
      is produced and mechlistMIC is not processed if one is provided;
      if the initiator sends the last mechanism token, the acceptor will
      send back a negotiation token with an accept_complete state and no
      mechlistMIC token.  In addition, an incorrect OID
      (1.2.840.48018.1.2.2) can be used to identify the GSS-API Kerberos
      Version 5 mechanism.

      The following changes have been made to be compatible with these
      legacy implementations.

      *  NegTokenTarg is changed to negTokenResp and it is the message
         format for all subsequent negotiation tokens.
      *  NegTokenInit is the message for the initial negotiation message
         and that message only.
      *  mechTypes in negTokenInit is not optional.
      *  If the selected mechanism is also the most preferred mechanism
         for both peers, it is safe to omit the MIC tokens.

      If at least one of the two peers implements the updated pseudo
      mechanism in this document, the negotiation is protected.

      The following changes are to address problems in RFC 2478.

      *  reqFlags is not protected therefore it should not impact the
         negotiation.
      *  DER encoding is required.
      *  GSS_GetMIC() input is clarified.
      *  Per-message integrity services are requested for the negotiated
         mechanism.
      *  Two MIC tokens are exchanged, one in each direction.

   An implementation that conforms to this specification will not
   inter-operate with a strict 2748 implementation.  Even if the new
   implementation always sends a mechlistMIC token, it will still fail
   to inter-operate.  If it is a server, it will fail because it
   requests a mechlistMIC token using an option that older
   implementations simply do not support.  Clients will tend to fail as
   well.

   As an alternative to the approach chosen in this specification, we
   could have documented a correct behavior that is fully backward
   compatible with RFC 2478 and included an appendix on how to
   inter-operate with existing incorrect implementations of RFC 2478.




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   As a practical matter, the SPNEGO implementers within the IETF have
   valued interoperability with the Microsoft implementations.  We were
   unable to choose to maintain reasonable security guarantees, maintain
   interoperability with the Microsoft implementations and maintain
   interoperability with correct implementations of RFC 2478.  The
   working group was not aware of any RFC 2478 implementations deployed
   on the Internet.  Even if there are such implementations, it is
   unlikely that they will inter-operate because of a critical flaw in
   the description of the encoding of the mechanism list in RFC 2478.

   With the approach taken in this specification, security is ensured
   between new implementations all the time while maintaining
   interoperability with the implementations deployed within the IETF
   community.  The working group believes that this justifies breaking
   compatibility with a correct implementation of RFC 2478.




































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Appendix D.  mechListMIC Computation Example

   The following is an example to illustrate how the mechListMIC field
   would be computed.

   The initial part of the DER encoding of NegTokenInit is constructed
   as follows (the "nn" are length encodings, possibly longer than one
   octet):

      30 -- identifier octet for constructed SEQUENCE (NegTokenInit)
      nn -- length

         -- contents octets of the SEQUENCE begin with
         -- DER encoding of "[0] MechTypeList":
         A0 -- identifier octet for constructed [0]
         nn -- length

             -- contents of the constructed [0] are DER encoding
             -- of MechTypeList (which is a SEQUENCE):
             30 -- identifier octet for constructed SEQUENCE
             nn -- length

                -- contents octets of the SEQUENCE begin with
                -- DER encoding of OBJECT IDENTIFIER:
                06 -- identifier octet for primitive OBJECT IDENTIFIER
                09 -- length
                2A 86 48 86 F7 12 01 02 02 -- Kerberos V5
                                           -- {1 2 840 113554 1 2 2}

   If a mechlistMIC needs to be generated (according to the rules in
   Section 5), it is computed by using the DER encoding of the type
   MechTypeList data from the initiator's NegTokenInit token as input to
   the GSS_GetMIC() function.  In this case, the MIC would be computed
   over the following octets:

      DER encoding of MechTypeList:
      30 nn 06 09 2A 86 48 86 F7 12 01 02 02 ...

   Note that the identifier octet and length octet(s) for constructed
   [0] (A0 nn) are not included in the MIC computation.











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