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rfc:rfc4178

Network Working Group L. Zhu Request for Comments: 4178 P. Leach Obsoletes: 2478 K. Jaganathan Category: Standards Track Microsoft Corporation

                                                          W. Ingersoll
                                                      Sun Microsystems
                                                          October 2005
                     The Simple and Protected
  Generic Security Service Application Program Interface (GSS-API)
                       Negotiation Mechanism

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

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.  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 that forces 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 that are commonly deployed on
 the Internet.

Zhu, et al. Standards Track [Page 1] RFC 4178 The GSS-API Negotiation Mechanism October 2005

Table of Contents

 1. Introduction ....................................................2
 2. Conventions Used in This Document ...............................3
 3. Negotiation Protocol ............................................3
    3.1. Negotiation Description ....................................4
    3.2. Negotiation Procedure ......................................5
 4. Token Definitions ...............................................7
    4.1. Mechanism Types ............................................7
    4.2. Negotiation Tokens .........................................7
         4.2.1. negTokenInit ........................................8
         4.2.2. negTokenResp ........................................9
 5. Processing of mechListMIC ......................................10
 6. Extensibility ..................................................13
 7. Security Considerations ........................................13
 8. Acknowledgments ................................................14
 9. References .....................................................14
    9.1. Normative References ......................................14
    9.2. Informative References ....................................15
 Appendix A.  SPNEGO ASN.1 Module ..................................16
 Appendix B.  GSS-API Negotiation Support API ......................17
    B.1.  GSS_Set_neg_mechs Call ...................................17
    B.2.  GSS_Get_neg_mechs Call ...................................18
 Appendix C.  Changes since RFC 2478 ...............................18
 Appendix D.  mechListMIC Computation Example ......................20

1. Introduction

 The GSS-API [RFC2743] provides a generic interface that 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 that enables GSS-API
 peers to determine in-band whether their credentials support a common
 set of one or more GSS-API security mechanisms; if so, it invokes the
 normal security context establishment for a selected common security
 mechanism.  This is most useful for applications that 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

Zhu, et al. Standards Track [Page 2] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 mechanism (the first in the list) or chooses one of the available
 mechanisms from the offered list; if neither is acceptable, the
 acceptor 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 [RFC2743] 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 that 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
 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.

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].

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.

Zhu, et al. Standards Track [Page 3] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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 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 the
 initiator-selected mechanism was acceptable to the target, and 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 that 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 that
 was chosen from the list offered by the initiator.
 In case of an unsuccessful negotiation, the reject state is returned,
 and the generation of 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.

Zhu, et al. Standards Track [Page 4] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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 when trying to produce the first mechanism token before a
 mechanism can be selected.  In cases where the initiator's preferred
 mechanism is not likely to be selected by the acceptor because of the
 significant cost of its generation, implementations MAY omit the
 optimistic mechanism token.

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 on the initial mechanism token for
    the first mechanism in the list.
 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 is not the
         acceptor's most preferred mechanism (i.e., the MIC token
         exchange as described in Section 5 is required),

Zhu, et al. Standards Track [Page 5] RFC 4178 The GSS-API Negotiation Mechanism October 2005

         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().  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.
 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.  That is, 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 though 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 that internally contains GSS-API credential elements for

Zhu, et al. Standards Track [Page 6] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 all mechanisms for which the principal has credentials available,
 except for any mechanisms that are not to be negotiated, per
 implementation-, site-, or application-specific policy.  See Appendix
 B for interfaces for expressing application policy.

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
  1. - rest of definitions here
    END
 This specifies that the tagging context for the module will be
 explicit and non-automatic.
 The encoding of the 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.

Zhu, et al. Standards Track [Page 7] RFC 4178 The GSS-API Negotiation Mechanism October 2005

    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 is not integrity protected.  For implementations of this
    specification, the initiator SHOULD omit this reqFlags field and
    the acceptor MUST ignore this reqFlags field.
    The size constraint on the ContextFlags ASN.1 type only applies to
    the abstract type.  The ASN.1 DER requires that all trailing zero
    bits be truncated from the encoding of a bit string type whose
    abstract definition includes named bits.  Implementations should

Zhu, et al. Standards Track [Page 8] RFC 4178 The GSS-API Negotiation Mechanism October 2005

    not expect to receive exactly 32 bits in an encoding of
    ContextFlags.
 mechToken
    This field, if present, contains the optimistic mechanism token.
 mechlistMIC
    This field, if present, contains an 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 additional negotiation message from the peer is
       needed to establish the security context.

Zhu, et al. Standards Track [Page 9] RFC 4178 The GSS-API Negotiation Mechanism October 2005

    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 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 an MIC token for the mechanism
    list in the initial negotiation message.  This MIC token is
    computed according to Section 5.

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
 that the acceptor would have preferred over the accepted mechanism
 had it been present in the mechanism list.
 In all other cases, MIC tokens MUST be exchanged after the mechanism
 context is fully established.

Zhu, et al. Standards Track [Page 10] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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.
     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

Zhu, et al. Standards Track [Page 11] RFC 4178 The GSS-API Negotiation Mechanism October 2005

    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 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.
     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
 instead of 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.

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 that 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
 had no material impact.

Zhu, et al. Standards Track [Page 13] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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 a 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.

8. 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 the development
 of this document.
 Luke Howard provided a prototype of this protocol in Heimdal and
 resolved several issues in the initial version of this document.
 Eric Baize and Denis Pinkas wrote the original SPNEGO specification
 [RFC2478] of which some of the text has been retained in this
 document.

9. References

9.1. 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.

Zhu, et al. Standards Track [Page 14] RFC 4178 The GSS-API Negotiation Mechanism October 2005

9.2. Informative References

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

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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),

Zhu, et al. Standards Track [Page 16] RFC 4178 The GSS-API Negotiation Mechanism October 2005

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

Appendix B. GSS-API Negotiation Support API

 In order to provide to a GSS-API caller (the initiator or the target
 or both) with the ability to choose among the set of supported
 mechanisms, a reduced set of mechanisms for negotiation and 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.
 This allows callers to specify the set of security mechanisms that
 may be negotiated with the credential identified by cred_handle.
 This call is intended to support 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

Zhu, et al. Standards Track [Page 17] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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:
 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.
 This allows callers to determine the set of security mechanisms
 available for negotiation with the credential identified by
 cred_handle.  This call is intended to support 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.

Appendix C. Changes since RFC 2478

 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.

Zhu, et al. Standards Track [Page 18] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 The following changes have been made to be compatible with these
 legacy implementations.
  • NegTokenTarg is changed to negTokenResp and is the message format

for all subsequent negotiation tokens.

  • NegTokenInit is the message for the initial negotiation message,

and only that message.

  • 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 RFC 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 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.
 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, to
 maintain interoperability with the Microsoft implementations, and to
 maintain interoperability with correct implementations of RFC 2478.

Zhu, et al. Standards Track [Page 19] RFC 4178 The GSS-API Negotiation Mechanism October 2005

 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.

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
  1. - contents octets of the SEQUENCE begin with
  2. - DER encoding of "[0] MechTypeList":

A0 – identifier octet for constructed [0]

       nn -- length
  1. - contents of the constructed [0] are DER encoding
  2. - of MechTypeList (which is a SEQUENCE):

30 – identifier octet for constructed SEQUENCE

           nn -- length
  1. - contents octets of the SEQUENCE begin with
  2. - 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 ...

Zhu, et al. Standards Track [Page 20] RFC 4178 The GSS-API Negotiation Mechanism October 2005

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

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
 Wyllys Ingersoll
 Sun Microsystems
 1775 Wiehle Avenue, 2nd Floor
 Reston, VA  20190
 US
 EMail: wyllys.ingersoll@sun.com

Zhu, et al. Standards Track [Page 21] RFC 4178 The GSS-API Negotiation Mechanism October 2005

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 contained in BCP 78, and except as set forth therein, the authors
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Zhu, et al. Standards Track [Page 22]

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