rfc2078.txt

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1.1.3:  Security Contexts

   Security contexts are established between peers, using credentials
   established locally in conjunction with each peer or received by
   peers via delegation. Multiple contexts may exist simultaneously
   between a pair of peers, using the same or different sets of
   credentials. Coexistence of multiple contexts using different
   credentials allows graceful rollover when credentials expire.
   Distinction among multiple contexts based on the same credentials
   serves applications by distinguishing different message streams in a
   security sense.

   The GSS-API is independent of underlying protocols and addressing
   structure, and depends on its callers to transport GSS-API-provided
   data elements. As a result of these factors, it is a caller



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   responsibility to parse communicated messages, separating GSS-API-
   related data elements from caller-provided data.  The GSS-API is
   independent of connection vs. connectionless orientation of the
   underlying communications service.

   No correlation between security context and communications protocol
   association is dictated. (The optional channel binding facility,
   discussed in Section 1.1.6 of this document, represents an
   intentional exception to this rule, supporting additional protection
   features within GSS-API supporting mechanisms.) This separation
   allows the GSS-API to be used in a wide range of communications
   environments, and also simplifies the calling sequences of the
   individual calls. In many cases (depending on underlying security
   protocol, associated mechanism, and availability of cached
   information), the state information required for context setup can be
   sent concurrently with initial signed user data, without interposing
   additional message exchanges.

1.1.4:  Mechanism Types

   In order to successfully establish a security context with a target
   peer, it is necessary to identify an appropriate underlying mechanism
   type (mech_type) which both initiator and target peers support. The
   definition of a mechanism embodies not only the use of a particular
   cryptographic technology (or a hybrid or choice among alternative
   cryptographic technologies), but also definition of the syntax and
   semantics of data element exchanges which that mechanism will employ
   in order to support security services.

   It is recommended that callers initiating contexts specify the
   "default" mech_type value, allowing system-specific functions within
   or invoked by the GSS-API implementation to select the appropriate
   mech_type, but callers may direct that a particular mech_type be
   employed when necessary.

   The means for identifying a shared mech_type to establish a security
   context with a peer will vary in different environments and
   circumstances; examples include (but are not limited to):

      use of a fixed mech_type, defined by configuration, within an
      environment

      syntactic convention on a target-specific basis, through
      examination of a target's name

      lookup of a target's name in a naming service or other database in
      order to identify mech_types supported by that target




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      explicit negotiation between GSS-API callers in advance of
      security context setup

   When transferred between GSS-API peers, mech_type specifiers (per
   Section 3, represented as Object Identifiers (OIDs)) serve to qualify
   the interpretation of associated tokens. (The structure and encoding
   of Object Identifiers is defined in ISO/IEC 8824, "Specification of
   Abstract Syntax Notation One (ASN.1)" and in ISO/IEC 8825,
   "Specification of Basic Encoding Rules for Abstract Syntax Notation
   One (ASN.1)".) Use of hierarchically structured OIDs serves to
   preclude ambiguous interpretation of mech_type specifiers. The OID
   representing the DASS MechType, for example, is 1.3.12.2.1011.7.5,
   and that of the Kerberos V5 mechanism, once advanced to the level of
   Proposed Standard, will be 1.2.840.113554.1.2.2.

1.1.5:  Naming

   The GSS-API avoids prescribing naming structures, treating the names
   which are transferred across the interface in order to initiate and
   accept security contexts as opaque objects.  This approach supports
   the GSS-API's goal of implementability atop a range of underlying
   security mechanisms, recognizing the fact that different mechanisms
   process and authenticate names which are presented in different
   forms. Generalized services offering translation functions among
   arbitrary sets of naming environments are outside the scope of the
   GSS-API; availability and use of local conversion functions to
   translate among the naming formats supported within a given end
   system is anticipated.

   Different classes of name representations are used in conjunction
   with different GSS-API parameters:

      - Internal form (denoted in this document by INTERNAL NAME),
      opaque to callers and defined by individual GSS-API
      implementations.  GSS-API implementations supporting multiple
      namespace types must maintain internal tags to disambiguate the
      interpretation of particular names.  A Mechanism Name (MN) is a
      special case of INTERNAL NAME, guaranteed to contain elements
      corresponding to one and only one mechanism; calls which are
      guaranteed to emit MNs or which require MNs as input are so
      identified within this specification.

      - Contiguous string ("flat") form (denoted in this document by
      OCTET STRING); accompanied by OID tags identifying the namespace
      to which they correspond.  Depending on tag value, flat names may
      or may not be printable strings for direct acceptance from and
      presentation to users. Tagging of flat names allows GSS-API
      callers and underlying GSS-API mechanisms to disambiguate name



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      types and to determine whether an associated name's type is one
      which they are capable of processing, avoiding aliasing problems
      which could result from misinterpreting a name of one type as a
      name of another type.

      - The GSS-API Exported Name Object, a special case of flat name
      designated by a reserved OID value, carries a canonicalized form
      of a name suitable for binary comparisons.

   In addition to providing means for names to be tagged with types,
   this specification defines primitives to support a level of naming
   environment independence for certain calling applications. To provide
   basic services oriented towards the requirements of callers which
   need not themselves interpret the internal syntax and semantics of
   names, GSS-API calls for name comparison (GSS_Compare_name()),
   human-readable display (GSS_Display_name()), input conversion
   (GSS_Import_name()), internal name deallocation (GSS_Release_name()),
   and internal name duplication (GSS_Duplicate_name()) functions are
   defined. (It is anticipated that these proposed GSS-API calls will be
   implemented in many end systems based on system-specific name
   manipulation primitives already extant within those end systems;
   inclusion within the GSS-API is intended to offer GSS-API callers a
   portable means to perform specific operations, supportive of
   authorization and audit requirements, on authenticated names.)

   GSS_Import_name() implementations can, where appropriate, support
   more than one printable syntax corresponding to a given namespace
   (e.g., alternative printable representations for X.500 Distinguished
   Names), allowing flexibility for their callers to select among
   alternative representations. GSS_Display_name() implementations
   output a printable syntax selected as appropriate to their
   operational environments; this selection is a local matter. Callers
   desiring portability across alternative printable syntaxes should
   refrain from implementing comparisons based on printable name forms
   and should instead use the GSS_Compare_name()  call to determine
   whether or not one internal-format name matches another.

   The GSS_Canonicalize_name() and GSS_Export_name() calls enable
   callers to acquire and process Exported Name Objects, canonicalized
   and translated in accordance with the procedures of a particular
   GSS-API mechanism.  Exported Name Objects can, in turn, be input to
   GSS_Import_name(), yielding equivalent MNs. These facilities are
   designed specifically to enable efficient storage and comparison of
   names (e.g., for use in access control lists).







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   The following diagram illustrates the intended dataflow among name-
   related GSS-API processing routines.

                        GSS-API library defaults
                               |
                               |
                               V                         text, for
   text -------------->  internal_name (IN) -----------> display only
         import_name()          /          display_name()
                               /
                              /
                             /
    accept_sec_context()    /
          |                /
          |               /
          |              /  canonicalize_name()
          |             /
          |            /
          |           /
          |          /
          |         /
          |        |
          V        V     <---------------------
    single mechanism        import_name()         exported name: flat
    internal_name (MN)                            binary "blob" usable
                         ---------------------->  for access control
                            export_name()

1.1.6:  Channel Bindings

   The GSS-API accommodates the concept of caller-provided channel
   binding ("chan_binding") information.  Channel bindings are used to
   strengthen the quality with which peer entity authentication is
   provided during context establishment, by limiting the scope within
   which an intercepted context establishment token can be reused by an
   attacker. Specifically, they enable GSS-API callers to bind the
   establishment of a security context to relevant characteristics
   (e.g., addresses, transformed representations of encryption keys) of
   the underlying communications channel, of protection mechanisms
   applied to that communications channel, and to application-specific
   data.

   The caller initiating a security context must determine the
   appropriate channel binding values to provide as input to the
   GSS_Init_sec_context() call, and consistent values must be provided
   to GSS_Accept_sec_context() by the context's target, in order for
   both peers' GSS-API mechanisms to validate that received tokens
   possess correct channel-related characteristics. Use or non-use of



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   the GSS-API channel binding facility is a caller option.  GSS-API
   mechanisms can operate in an environment where NULL channel bindings
   are presented; mechanism implementors are encouraged, but not
   required, to make use of caller-provided channel binding data within
   their mechanisms. Callers should not assume that underlying
   mechanisms provide confidentiality protection for channel binding
   information.

   When non-NULL channel bindings are provided by callers, certain
   mechanisms can offer enhanced security value by interpreting the
   bindings' content (rather than simply representing those bindings, or
   integrity check values computed on them, within tokens) and will
   therefore depend on presentation of specific data in a defined
   format. To this end, agreements among mechanism implementors are
   defining conventional interpretations for the contents of channel
   binding arguments, including address specifiers (with content
   dependent on communications protocol environment) for context
   initiators and acceptors. (These conventions are being incorporated
   in GSS-API mechanism specifications and into the GSS-API C language
   bindings specification.) In order for GSS-API callers to be portable
   across multiple mechanisms and achieve the full security
   functionality which each mechanism can provide, it is strongly
   recommended that GSS-API callers provide channel bindings consistent