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NETWORK WORKING GROUP                                        N. Williams
Internet-Draft                                                       Sun
Expires: December 28, 2006                                 June 26, 2006


          Stackable Generic Security Service Pseudo-Mechanisms
            draft-ietf-kitten-stackable-pseudo-mechs-02.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document defines and formalizes the concept of stackable pseudo-
   mechanisms, and associated concept of composite mechanisms, for the
   Generic Security Service Application Programming Interface (GSS-API),
   as well as several utility functions.

   Stackable GSS-API pseudo-mechanisms allow for the composition of new
   mechanisms that combine features from multiple mechanisms.  Stackable
   mechanisms that add support for Perfect Forward Security (PFS), data
   compression, additional authentication factors, etc... are



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   facilitated by this document.


Table of Contents

   1.      Conventions used in this document  . . . . . . . . . . . .  3
   2.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
   2.1.    Glossary . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.      Mechanism Composition Issues . . . . . . . . . . . . . . .  4
   4.      Mechanism Composition  . . . . . . . . . . . . . . . . . .  5
   4.1.    Construction of Composed Mechanism OIDs  . . . . . . . . .  5
   4.2.    Mechanism Composition Rules  . . . . . . . . . . . . . . .  6
   4.3.    Interfacing with Composite Mechanisms  . . . . . . . . . .  7
   4.4.    Compatibility with the Basic GSS-APIv2u1 Interfaces  . . .  7
   4.5.    Processing of Tokens for Composite Mechanisms  . . . . . .  8
   5.      New GSS-API Interfaces . . . . . . . . . . . . . . . . . .  8
   5.1.    New GSS-API Function Interfaces  . . . . . . . . . . . . .  9
   5.1.1.  GSS_Compose_oid()  . . . . . . . . . . . . . . . . . . . .  9
   5.1.2.  GSS_Decompose_oid()  . . . . . . . . . . . . . . . . . . . 10
   5.1.3.  GSS_Release_oid()  . . . . . . . . . . . . . . . . . . . . 10
   5.1.4.  GSS_Indicate_negotiable_mechs()  . . . . . . . . . . . . . 11
   5.1.5.  GSS_Negotiate_mechs()  . . . . . . . . . . . . . . . . . . 12
   5.1.6.  C-Bindings . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.      Negotiation of Composite Mechanisms  . . . . . . . . . . . 13
   6.1.    Negotiation of Composite Mechanisms Through SPNEGO . . . . 14
   7.      Requirements for Mechanism Designers . . . . . . . . . . . 14
   8.      IANA Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.      Security considerations  . . . . . . . . . . . . . . . . . 14
   10.     Normative  . . . . . . . . . . . . . . . . . . . . . . . . 15
           Author's Address . . . . . . . . . . . . . . . . . . . . . 16
           Intellectual Property and Copyright Statements . . . . . . 17




















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1.  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].


2.  Introduction

   Recent discussions within the IETF have shown the need for a
   refactoring of the features that GSS-API mechanisms may provide and a
   way to compose new mechanisms from smaller components.

   One way to do this is to "stack" multiple mechanisms on top of each
   other such that the features of all of them are summed into a new,
   composite mechanism.

   One existing GSS-API mechanism, LIPKEY [LIPKEY], is essentially
   stacked over another, SPKM-3 [LIPKEY] (although LIPKEY does not
   conform to the stackable pseduo-mechanism framework described
   herein).

   The first truly stackable pseudo-mechanism proposed, CCM [CCM], is
   intended for signalling, during negotiation of mechanisms, the
   willingness of an initiator and/or acceptor to utilize channel
   bindings

   Since then other similar mechanism compositing needs and ideas have
   come up, along with problems such as "what combinations are possible,
   useful, reasonable and secure?"  This document addresses those
   problems.  It introduces the concepts of stackable pseudo-mechanisms,
   composite mechanisms and mechanism features or attributes, as well as
   new inquiry and related interfaces to help in the mechanism
   compositing.

   (Mechanism features are more formally referred to as "mechanism
   attributes" below.  The terms "feature" and mechanism attribute" are
   sometimes used interchangeably.)

2.1.  Glossary

   Concrete GSS-API mechanism
      A mechanism which can be used standalone.  Examples include: the
      Kerberos V mechanism [CFX], SPKM-1/2 [SPKM] and SPKM-3 [LIPKEY].

   GSS-API Pseudo-mechanism
      A mechanism which uses other mechanisms in the construction of its
      context and/or per-message tokens and security contexts.  SPNEGO



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      is an example of this.

   Stackable GSS-API pseudo-mechanism
      A mechanism which uses a single other mechanism in the
      construction of its tokens such that the OID of the composite
      result can be constructed by prepending the OID of the stackable
      pseudo-mechanism to the OID of the mechanism to be used by it.

   Mechanism-negotiation GSS-API pseudo-mechanism
      A GSS-API mechanism that negotiates the use of GSS-API mechanisms.
      SPNEGO [SPNEGO] is an example of this.


3.  Mechanism Composition Issues

   Interfacing with composite mechanisms through the existing GSS-API
   interfaces and the handling of composite mechanism tokens is
   straightforward enough and described in Section 4.

   However, the concepts of stackable and composite mechanisms do give
   rise to several minor problems:

   o  How to determine allowable combinations of mechanisms;
   o  How to encode composite mechanism OIDs;
   o  How to decompose the OID of a composite mechanism and process its
      tokens properly;
   o  Application interfacing issues such as:

      *  Whether and/or which composite mechanisms should be listed by
         GSS_Indicate_mechs();
      *  Whether and/or which composite mechanisms not listed by
         GSS_Indicate_mechs() may nonetheless be available for use by
         applications and how applications can detect their
         availability;
      *  What additional, if any, interfaces should be provided to help
         applications select appropriate mechanisms;
   o

      Mechanism negotiation issues (related to the application interface
      issues listed above), such as: vspace blankLines='1'/>
      *  Should applications advertise composite mechanisms in SPNEGO or
         other application-specific mechanism negotiation contexts?
      *  Or should applications implicitly advertise composite
         mechanisms by advertising concrete and stackable pseudo-
         mechanisms in SPNEGO or other application-specific mechanism
         negotiation contexts?

   Section 4 addresses the OID composition, decomposition and encoding



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   issues, as well as basic interfacing and token handling issues.

   Section 5 addresses interfacing issues more generally through the
   specification of additional, optional APIs.

   Section 6 addresses mechanism negotiation issues.


4.  Mechanism Composition

   Mechanism composition by stacking pseudo-mechanisms on a concrete
   mechanism is conceptually simple: join the OIDs of the several
   mechanisms in question and process GSS-API tokens and routine calls
   through the top-most pseudo-mechanism in a stack, which can then, if
   necessary, recursively call the GSS-API to process any tokens for the
   remainder of the stack.

   Some stackable pseudo-mechanisms may do nothing more than perform
   transformations on application data (e.g., compression); such pseudo-
   mechanisms will generally chain the processing of tokens and routine
   calls to the mechanisms below them in the stack.

   Other stackable pseudo-mechanisms may utilize the mechanisms below
   them only during security context setup.  For example, a stackable
   pseudo-mechanism could perform a Diffie-Hellman key exchange and
   authenticate it by binding a security context established with the
   mechanism stacked below it; such a mechanism would provide its own
   per-message tokens.

4.1.  Construction of Composed Mechanism OIDs

   Composition of mechanism OIDs is simple: prepend the OID of one
   pseudo-mechanism to the OID of another mechanism (composite or
   otherwise), but there MUST always be at least one final mechanism OID
   and it MUST be useful standalone (i.e., it MUST NOT be a pseudo-
   mechanism).  A composite mechanism OID forms, essentially, a stack.

   The encoding of composed mechanism OIDs is not quite the
   concatenation of the component OIDs' encodings, however.  This is
   because the first two arcs of ASN.1 OIDs are encoded differently from
   subsequent arcs (the first two arcs have a limited namespace and are
   encoded as a single octet), so were composite mechanism OIDs to be
   encoded as the concatenation of the component OIDs the result would
   not decode as the concatenation of the component OIDs.  To avoid this
   problem the first two arcs of each component of a composite mechanism
   OID, other than the leading component, will be encoded as other arcs
   would.




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   Decomposition of composite mechanism OIDs is similar, with each
   pseudo-mechanism in the stack being able to determine the OID suffix
   from knowledge of its own OID(s).

   New pseudo-mechanisms MAY be allocated OIDs from the prefix given
   below as follows by assignment of a sub-string of OID arcs to be
   appended to this prefix.  This prefix OID is:

   <TBD> [1.3.6.1.5.5.11 appears to be available, registration w/ IANA
   TBD]

   All OID allocations below this OID MUST be for stackable pseudo-
   mechanisms and MUST consist of a single arc.  This will make it
   possible to decompose the OIDs of composite mechanisms without
   necessarily knowing a priori the OIDs of the component stackable
   pseudo-mechanisms.

4.2.  Mechanism Composition Rules

   All new stackable pseudo-mechanisms MUST specify the rules for
   determining whether they can stack above a given mechanism, composite
   or otherwise.  Such rules may be based on specific mechanism
   attribute OID sets [EXTENDED-INQUIRY] and/or specific mechanism OIDs
   (composite and otherwise).

   All stackable pseudo-mechanisms MUST have the following mechanism
   composition rule relating to unknown mechanism attributes:

   o  composition with mechanisms supporting unknown mechanism
      attributes MUST NOT be permitted.

   This rule protects against compositions which cannot be considered
   today but which might nonetheless arise due to the introduction of
   new mechanisms and which might turn out to be insecure or otherwise
   undesirable.

   Mechanism composition rules for stackable pseudo-mechanisms MAY and
   SHOULD be updated as new GSS-API mechanism attributes and mechanisms
   sporting them are introduced.  The specifications of mechanisms that
   introduce new mechanism attributes or which otherwise should not be
   combined with others in ways which would be permitted under existing
   rules SHOULD also update the mechanism composition rules of affected
   pseudo-mechanisms.

   A RECOMMENDED way to describe the stacking rules for stackable
   mechanisms is as an ordered sequence of "MAY stack above X
   mechanism," "REQUIRES Y mechanism feature(s)," "MUST NOT stack above
   Z mechanism," and/or "MUST NOT stack above a mechanism with Z



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   mechanism feature(s)."

   For example a stackable mechanism that provides its own per-msg
   tokens and does not use the underlying mechnism's per-msg token
   facilities might require a rule such as "MUST NOT stack above a
   mechanism with the GSS_C_MA_COMPRESS mechanism feature."

4.3.  Interfacing with Composite Mechanisms

   The basic GSS-API [RFC2743] interfaces MUST NOT accept as input or
   provide as output the OID of any stackable pseudo-mechanism.
   Composite mechanisms MUST be treated as concrete mechanisms by the
   basic GSS-API interfaces [RFC2743].

   Thus the way in which a composite mechanism is used by applications
   with the basic GSS-API (version 2, update 1) is straightforward:
   exactly as if composite mechanisms were normal GSS-API mechanisms.

   This is facilitated by the fact that in all cases where the GSS-API
   implementation might need to know how to process or create a token it
   has the necessary contextual information, that is, the mechanism OID,
   available and can decompose composite mechanism OIDs as necessary.

   For example, for initial GSS_Init_sec_context() calls the
   implementation knows the desired mechanism OID, and if it should be
   left unspecified, it can pick a default mechanism given the initiator
   credentials provided by the application (and if none are provided
   other default mechanism and credential selections can still be made).
   For subsequent calls to GSS_Init_sec_context() the implementation
   knows which mechanism to use from the given [partially established]
   security context.  Similarly for GSS_Accept_sec_context, where on
   initial calls the mechanism OID can be determined from the given
   initial context token's framing.

   The manner in which GSS-API implementations and the various
   mechanisms and pseudo-mechanisms interface with one another is left
   as an excercise to implementors.

4.4.  Compatibility with the Basic GSS-APIv2u1 Interfaces

   In order to preserve backwards compatibility with applications that
   use only the basic GSS-API interfaces (version 2, update 1), several
   restrictions are imposed on the use of composite and stackable
   pseduo-mechanisms with the basic GSS-API interfaces:

   o  GSS_Indicate_mechs() MUST NOT indicate support for any stackable
      pseduo-mechanisms under any circumstance.
   o  GSS_Indicate_mechs() MAY indicate support for some, all or none of



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      the available composite mechanisms.
   o  Which composite mechanisms, if any, are indicated through
      GSS_Indicate_mechs() SHOULD be configurable.
   o  Composite mechanisms which are not indicated by
      GSS_Indicate_mechs() MUST NOT be considered as the default
      mechanism (GSS_C_NULL_OID) or as part of the default mechanism set
      (GSS_C_NULL_OID_SET).
   o  The OIDs of *stackable* (not composite) pseudo-mechanisms MUST NOT
      be accepted as inputs or produced in the output of any of the
      basic GSS-APIv2, update 1 API functions, except for any OID set
      construction/iteration functions.  And, if present in any OID SET
      input parameters of GSS-APIv2, update 1 functions, they MUST be
      ignored.
   o  The OIDs of *stackable* (not composite) pseudo-mechanisms MAY only
      be used as inputs or produced as outputs of functions whose
      specification explicitly allows for them or which are concerned
      with the creation/iteration of OID containters, such as OID SETs.

4.5.  Processing of Tokens for Composite Mechanisms

   The initial context token for any standard mechanism, including
   mechanisms composited from standard pseudo- and concrete mechanisms,
   MUST be encapsulated as described in section 3.1 of rfc2743
   [RFC2743], and the OID used in that framing MUST be that of the
   mechanism, but in the case of composite mechanisms this OID MUST be
   the OID of the leading component of the composite mechanism.

   Note that this has implications for pluggable multi-mechanism
   implementations of the GSS-API, namely that acceptors must route
   initial context tokens to the appropriate mechanism and they must
   allow that mechanism to determine the composite mechanism OID (such
   as by allowing that mechanism's GSS_Accept_sec_context() to output
   the actual mechanism to the application.

   In all other cases the mechanism that produced or is to produce a
   given token can be determined internally through the given security
   context.


5.  New GSS-API Interfaces

   ...

   Utility functions for mechanism OID composition and decomposition are
   given in sections 5.1.1, 5.1.2 and 5.1.3.

   Two utility functions, GSS_Indicate_negotiable_mechs() and
   GSS_Negotiate_mechs(), to aid applications in mechanism negotiation



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   are described in sections 5.1.4 and 5.1.5.  These two interfaces may
   be implemented entirely in terms of the other interfaces described
   herein.

5.1.  New GSS-API Function Interfaces

   Several new interfaces are given by which, for example, GSS-API
   applications may determine what features are provided by a given
   mechanism, what mechanisms provide what features and what
   compositions are legal.

   These new interfaces are all OPTIONAL.

   In order to preserve backwards compatibility with applications that
   do not use the new interfaces GSS_Indicate_mechs() MUST NOT indicate
   support for any stackable pseduo-mechanisms.  GSS_Indicate_mechs()
   MAY indicate support for some, all or none of the available composite
   mechanisms; which composite mechanisms, if any, are indicated through
   GSS_Indicate_mechs() SHOULD be configurable.  GSS_Acquire_cred() and
   GSS_Add_cred() MUST NOT create credentials for composite mechanisms
   not explicitly requested or, if no desired mechanism or mechanisms
   are given, for composite mechanisms not indicated by
   GSS_Indicate_mechs().

   Applications SHOULD use GSS_Indicate_mechs_by_mech_attrs() instead of
   GSS_Indicate_mechs() wherever possible.

   Applications can use GSS_Indicate_mechs_by_mech_attrs() to determine
   what, if any, mechanisms provide a given set of features.

   GSS_Indicate_mechs_by_mech_attrs() can also be used to indicate (as
   in GSS_Indicate_mechs()) the set of available mechanisms of each type
   (concrete, mechanism negotiation pseudo-mechanism, stackable pseudo-
   mechanism and composite mechanisms).

   Applications may use GSS_Inquire_mech_attrs_for_mech() to test
   whether a given composite mechanism is available and the set of
   features that it offers.

   GSS_Negotiate_mechs() may be used to negotiate the use of mechanisms
   such that composite mechanisms need not be advertised but instead be
   implied by offering stackable pseudo-mechanisms.

5.1.1.  GSS_Compose_oid()

   Inputs:
   o  mech1 OBJECT IDENTIFIER, -- mechanism OID
   o  mech2 OBJECT IDENTIFIER -- mechanism OID



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   Outputs:
   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  composite OBJECT IDENTIFIER -- OID composition of mech1 with mech2
      ({mech1 mech2})

   Return major_status codes:
   o  GSS_S_COMPLETE indicates success.
   o  GSS_S_BAD_MECH indicates that mech1 is not supported.
   o  GSS_S_FAILURE indicates that the request failed for some other
      reason.  The minor status will be specific to mech1 and may
      provide further information.

5.1.2.  GSS_Decompose_oid()

   Inputs:
   o  input_mech OBJECT IDENTIFIER, -- mechanism OID.
   o  mechs SET OF OBJECT IDENTIFIER -- mechanism OIDs (if
      GSS_C_NULL_OID_SET defaults to the set of stackable pseudo-
      mechanism OIDs indicated by GSS_Indicate_mechs_by_mech_attrs()).

   Outputs:
   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  lead_mech OBJECT IDENTIFIER, -- leading stackable pseudo-
      mechanism OID.
   o  trail_mech OBJECT IDENTIFIER -- input_mech with lead_mech removed
      from the front.

   Return major_status codes:
   o  GSS_S_COMPLETE indicates success.
   o  GSS_S_BAD_MECH indicates that the input_mech could not be
      decomposed as no stackable pseudo-mechanism is available whose OID
      is a prefix of the input_mech.
   o  GSS_S_FAILURE indicates that the request failed for some other
      reason.

5.1.3.  GSS_Release_oid()

   The following text is adapted from the obsoleted rfc2078 [RFC2078].

   Inputs:
   o  oid OBJECT IDENTIFIER

   Outputs:
   o  major_status INTEGER,
   o  minor_status INTEGER




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   Return major_status codes:
   o  GSS_S_COMPLETE indicates successful completion
   o  GSS_S_FAILURE indicates that the operation failed

   Allows the caller to release the storage associated with an OBJECT
   IDENTIFIER buffer allocated by another GSS-API call, specifically
   GSS_Compose_oid() and GSS_Decompose_oid().  This call's specific
   behavior depends on the language and programming environment within
   which a GSS-API implementation operates, and is therefore detailed
   within applicable bindings specifications; in particular, this call
   may be superfluous within bindings where memory management is
   automatic.

5.1.4.  GSS_Indicate_negotiable_mechs()

   Inputs:
   o  input_cred_handle CREDENTIAL HANDLE, -- credential handle to be
      used with GSS_Init_sec_context(); may be GSS_C_NO_CREDENTIAL.
   o  peer_type_known BOOLEAN, -- indicates whether the peer is known to
      support or not supprot the stackable pseudo-mechanism framework.
   o  peer_has_mech_stacking BOOLEAN -- indicates whether the peer
      supports the stackable pseudo-mechanism framework; ignore if
      peer_type_known is FALSE.

   Outputs:
   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  offer_mechs SET OF OBJECT IDENTIFIER, -- mechanisms to offer.

   Return major_status codes:
   o  GSS_S_COMPLETE indicates success.
   o  GSS_S_NO_CREDENTIAL indicates that the caller's credentials are
      expired or, if input_cred_handle is GSS_C_NO_CREDENTIAL, that no
      credentials could be acquired for GSS_C_NO_NAME.
   o  GSS_S_FAILURE indicates that the request failed for some other
      reason.

   This function produces a set of mechanism OIDs, optimized for space,
   that its caller should advertise to peers during mechanism
   negotiation.

   The output offer_mechs parameter will include all of the mechanisms
   for which the input_cred_handle has elements (as indicated by
   GSS_Inquire_cred()), but composite mechanisms will be included either
   implicitly or implicitly as per the following rules:
   o  if peer_type_known is TRUE and peer_has_mech_stacking is FALSE
      then no composite mechanisms not indicated by GSS_Indicate_mechs()
      will be advertised, explictly or implicitly;



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   o  if peer_type_known is FALSE then all composite mechanisms
      indicated by GSS_Indicate_mechs() for which input_cred_handle has
      elements will be indicated in offer_mechs explicitly and all
      others may be indicated in offer_mechs implicitly, by including
      their component stackable pseduo-mechanism OIDs (see below);
   o  if peer_type_known is TRUE and peer_has_mech_stacking is TRUE
      composite mechanisms will generally not be advertised explicitly,
      but will be advertised implicitly, by including their component
      stackable pseduo-mechanism OIDs (see below); no composite
      mechanisms will be advertised explicitly
   o  if the input_cred_handle does not have elements for all of the
      possible composite mechanisms that could be constructed from the
      its elements' decomposed mechanisms, then all composite mechanisms
      for which the input_cred_handle does have elements will be
      advertised explicitly in offer_mechs.

5.1.5.  GSS_Negotiate_mechs()

   Inputs:
   o  input_credential_handle CREDENTIAL HANDLE, -- mechanisms offered
      by the caller.
   o  peer_mechs SET OF OBJECT IDENTIFIER -- mechanisms offered by the
      caller's peer.

   Outputs:
   o  major_status INTEGER,
   o  minor_status INTEGER,
   o  mechs SET OF OBJECT IDENTIFIER -- mechanisms common to the
      caller's credentials and the caller's peer.

   Return major_status codes:
   o  GSS_S_COMPLETE indicates success; the output mechs parameter MAY
      be the empty set (GSS_C_NO_OID_SET).
   o  GSS_S_NO_CREDENTIAL indicates that the caller's credentials are
      expired or, if input_cred_handle is GSS_C_NO_CREDENTIAL, that no
      credentials could be acquired for GSS_C_NO_NAME.
   o  GSS_S_FAILURE indicates that the request failed for some other
      reason.

   This function matches the mechanisms for which the caller has
   credentials with the mechanisms offered by the caller's peer and
   returns the set of mechanisms in common to both, accounting for any
   composite mechanisms offered by the peer implicitly.

5.1.6.  C-Bindings


      OM_uint32 gss_compose_oid(



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         OM_uint32         *minor_status,
         const gss_OID      mech1,
         const gss_OID      mech2,
         gss_OID           *composite);

      OM_uint32 gss_decompose_oid(
         OM_uint32         *minor_status,
         const gss_OID      input_mech,
         const gss_OID_set  mechs,
         gss_OID           *lead_mech,
         gss_OID           *trail_mech);

      OM_uint32 gss_release_oid(
         OM_uint32         *minor_status,
         gss_OID           *oid);

      OM_uint32 gss_indicate_negotiable_mechs(
         OM_uint32                 *minor_status,
         const gss_cred_id_t        input_cred_handle,
         OM_uint32                  peer_type_known,
         OM_uint32                  peer_has_mech_stacking,
         gss_OID_set               *offer_mechs);

      OM_uint32 gss_negotiate_mechs(
         OM_uint32                 *minor_status,
         const gss_cred_id_t        input_cred_handle,
         const gss_OID_set          peer_mechs,
         const gss_OID_set         *mechs);


   Figure 1


6.  Negotiation of Composite Mechanisms

   Where GSS-API implementations do not support the stackable mechanism
   framework interfaces applications may only negotiate explicitly from
   a set of concrete and composite mechanism OIDs as indicated by
   GSS_Indicate_mechs() and for which suitable credentials are
   available.  GSS_Indicate_mechs(), as described in Section 4.4, MUST
   NOT indicate support for individual stackable pseudo-mechanisms, so
   there will not be any composite mechanisms implied but not explicitly
   offered in the mechanism negotiation.

   Applications that support the stackable mechanism framework SHOULD
   use GSS_Indicate_negotiable_mechs() to construct the set of mechanism
   OIDs to offer to their peers.  GSS_Indicate_negotiable_mechs()
   optimizes for bandwidth consumption by using decomposed OIDs instead



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   of composed OIDs, where possible.  See Section 5.1.4.

   Peers that support the stackable mechanism framework interfaces
   SHOULD use GSS_Negotiate_mechs() to select a mechanism as that
   routine accounts for composite mechanisms implicit in the mechanism
   offers.

6.1.  Negotiation of Composite Mechanisms Through SPNEGO

   SPNEGO applications MUST advertise either the set of mechanism OIDs
   for which they have suitable credentials or the set of mechanism OIDs
   produced by calling GSS_Indicate_negotiable_mechs() with the
   available credentials and the peer_type_known parameter as FALSE.


7.  Requirements for Mechanism Designers

   Stackable pseudo-mechanisms specifications MUST:
   o  list the set of GSS-API mechanism attributes associated with them
   o  list their initial mechanism composition rules
   o  specify a mechanism for updating their mechanism composition rules

   All other mechanism specifications MUST:
   o  list the set of GSS-API mechanism attributes associated with them


8.  IANA Considerations

   Allocation of arcs in the namespace of OIDs relative to the base
   stackable pseduo-mechanism OID specified in Section 4.1 is reserved
   to the IANA.


9.  Security considerations

   Some composite mechanisms may well not be secure.  The mechanism
   composition rules of pseudo-mechanisms (including the default
   composition rule given in Section 4 for unknown mechanism attributes)
   should be used to prevent the use of unsafe composite mechanisms.

   Designers of pseudo-mechanisms should study the possible combinations
   of their mechanisms with others and design mechanism composition
   rules accordingly.

   Similarly, pseudo-mechanism designers MUST specify, and implementors
   MUST implement, composite mechanism attribute set determination rules
   appropriate to the subject pseduo-mechanism, as described in section
   4.2.  Failure to do so may lead to inappropriate composite mechanisms



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   being deemed permissible by programmatic application of flawed
   mechanism composition rules or to by their application with incorrect
   mechanism attribute sets.

10.  Normative

   [EXTENDED-INQUIRY]
              Williams, N., "Extended Generic Security Service Mechanism
              Inquiry APIs",
              draft-ietf-kitten-extended-mech-inquiry-00.txt (work in
              progress).

   [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.

   [RFC2744]  Wray, J., "Generic Security Service API Version 2 :
              C-bindings", RFC 2744, January 2000.































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Author's Address

   Nicolas Williams
   Sun Microsystems
   5300 Riata Trace Ct
   Austin, TX  78727
   US

   Email: Nicolas.Williams@sun.com










































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