Network Working Group Nicolas Williams
INTERNET-DRAFT Sun Microsystems
November 2004
Stackable Generic Security Service Pseudo-Mechanisms
<draft-williams-gssapi-stackable-pseudo-mechs-00.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines and formalizes the concept of stackable pseudo-
mechanisms for the Generic Security Service Application Programming
Interface (GSS-API) and introduces a framework to support stackable
pseudo-mechanisms and mechanism compositing.
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
facilitated by this document.
Table of Contents
1. Introduction pg. 3
1.1. Glossary pg. 3
2. Issues with Mechanism Composition pg. 4
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3. Mechanism Composition pg. 5
3.1. Construction of Composed Mechanism OIDs pg. 5
3.2. Mechanism Composition Rules pg. 6
3.3. Interfacing with Composite Mechanisms pg. 6
3.4. Compatibility with the Basic GSS-API Interfaces pg. 7
3.5. Processing of Tokens for Composite Mechanisms pg. 7
4. New GSS-API Interfaces pg. 8
4.1. Mechanism Attributes and Attribute Sets pg. 8
4.1.1. Determination of Attribute Sets of Composite Mechs pg. 9
4.1.2. Initial Set of Known Mechanism Attributes pg. 9
4.1.3. Mechanism Attribute Sets of Existing Mechs pg. 11
4.2. New GSS-API Function Interfaces pg. 12
4.2.1. GSS_Indicate_mechs_by_mech_attrs() pg. 12
4.2.2. GSS_Inquire_mech_attrs_for_mech() pg. 13
4.2.3. GSS_Display_mech_attr() pg. 14
4.2.4. GSS_Compose_oid() pg. 15
4.2.5. GSS_Decompose_oid() pg. 15
4.2.6. GSS_Release_oid() pg. 16
4.2.7. GSS_Indicate_negotiable_mechs() pg. 16
4.2.8. GSS_Negotiate_mechs() pg. 17
4.3. New Major Status Values pg. 18
4.4. C-Bindings pg. 18
5. Negotiation of Composite Mechanisms pg. 19
5.1. Negotiation of Composite Mechanisms Through SPNEGO pg. 20
6. Requirements for Mechanism Designers pg. 20
7. IANA Considerations pg. 20
8. Security considerations pg. 20
9. References pg. 21
9.1. Informative references pg. 21
9.2. Normative references pg. 21
10. Author's Address pg. 21
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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].
1. 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.
Mechanism features are more formally referred to as "mechanism
attributes" below. The terms "feature" and mechanism attribute" are
sometimes used interchangeably.
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] was
for signalling the willingness of an initiator and/or acceptor to
utilize channel bindings as well as to correctly implement 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?" Problems which we believe are solved
herein.
Therefore the time has come to define the concept of GSS-API
mechanism compositing through the use of stackable pseudo-mechanisms.
1.1. 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
is an example of this.
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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.
2. Issues with Mechanism Composition
Interfacing with composite mechanisms through the existing GSS-API
interfaces and the handling of composite mechanism tokens is
straightforward enough and described in section 3.
However, the concepts of stackable and composite mechanisms do give
rise to several minor problems:
- How to determine allowable combinations of mechanisms;
- How to encode composite mechanism OIDs;
- How to decompose the OID of a composite mechanism and process its
tokens properly;
- 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 interfaces should be provided to help
applications select appropriate mechanisms;
- Mechanism negotiation issues (related to the application interface
issues listed above), such as:
- Should applications advertise composite mechanisms in SPNEGO or
other application-specific mechanism negotiation contexts?
- Or should applications advertise concrete and stackable pseudo-
mechanisms in SPNEGO or other application-specific mechanism
negotiation contexts?
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Section 3 addresses the OID composition, decomposition and encoding
issues, as well as basic interfacing and token handling issues.
Section 4 addresses the application interfacing issues.
Section 5 addresses the mechanism negotiation issues.
3. 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, do the same with 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.
3.1. 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.
Decomposition of 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), from the composite mechanism OID in its
OID or OID SET arguments and from the the composite OID from the
initial context token for a given context.
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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.
3.2. 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 sets and/or specific OIDs (composite and otherwise) of
mechanisms stacked below them.
All stackable pseudo-mechanisms MUST have the following mechanism
composition rule relating to unknown mechanism attributes:
- 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.
3.3. 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
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implementation might need to know how to process or create a token it
has the necessary contextual information, 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 more
interesting; it is also left as an excercise to implementors.
3.4. Compatibility with the Basic GSS-API 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 the available composite mechanisms.
o Which composite mechanisms, if any, are indicated through
GSS_Indicate_mechs() SHOULD be configurable.
o GSS_Acquire_cred() and GSS_Add_cred() MUST NOT create credentials
for composite mechanisms not explicitly requested or, if
GSS_C_NULL_OID is given as a desired for composite mechanisms not
indicated by GSS_Indicate_mechs().
o GSS_Acquire_cred() and GSS_Add_cred() MUST NOT (and truly cannot)
create credentials for stackable pseudo-mechanisms.
3.5. Processing of Tokens for Composite Mechanisms
The initial context token for any mechanism, composite or otherwise,
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 multi-mechanism implementations
of the GSS-API, namely that acceptors MUST route initial context
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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 a given token can be
determined by the given security context.
4. New GSS-API Interfaces
GSS-API applications face, today, the problem of how to select from
multiple GSS-API mechanisms that may be available. This problem is
likely to be exacerbated by the introduction of stackable.
To address this problem we introduce a new concept: that of mechanism
attributes. By allowing applications to query the set of attributes
associated with individual mechanisms and to find out which
mechanisms support a given set of attributes we allow applications to
select mechanisms based on their attributes yet without having to
hardcode mechanism OIDs.
Section 4.1 describes the mechanism mechanism attributes concept.
Sections 4.2.1, 4.2.2 and 4.2.3 describe three new interfaces that
deal in mechanisms and attribute sets:
- GSS_Indicate_mechs_by_attrs()
- GSS_Inquire_attrs_for_mech()
- GSS_Display_mech_attr()
Additional utility functions for mechanism OID composition and
decomposition are given in sections 4.2.4, 4.2.5 and 4.2.6.
Finally, two utility functions, GSS_Indicate_negotiable_mechs() and
GSS_Negotiate_mechs(), to aid applications in mechanism negotiation
are described in sections 4.2.7 and 4.2.8. These two interfaces may
be implemented entirely in terms of the other interfaces described
herein.
4.1. Mechanism Attributes and Attribute Sets
An abstraction for the features provided by pseudo-mechanisms is
needed in order to facilitate the programmatic composition of
mechanisms.
Two data types are needed: one for individual mechanism attributes
and one for mechanism attribute sets. To simplify the mechanism
attributes interfaces we reuse the 'OID' and 'OID set' data types and
model individual mechanism attribute types as OIDs.
To this end we define an open namespace of mechanism attributes and
assign them arcs off of this OID:
<TBD>
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[1.3.6.1.5.5.12 appears to be available, registration w/ IANA TBD]
4.1.1. Determination of Attribute Sets of Composite Mechs
Each mechanism, composite or otherwise, has a set of mechanism
attributes that it supports as specified.
The mechanism attribute set of a composite mechanism is to be
determined by the top-most stackable pseudo-mechanism of the
composite according to its own attribute set and that of the
mechanism stacked directly below it.
It may well be that some composite mechanisms' attribute sets consist
of the union of those of their every component, however this need not
be the case and should not be assumed.
Every stackable pseudo-mechanism's specification MUST specify the
rules for determining the mechanism attribute set of mechanisms
composed by it.
4.1.2. Initial Set of Known Mechanism Attributes
Mech Attr name OID Arc Arc name
-------------- ------- --------
GSS_C_MA_MECH_CONCRETE (1) concrete-mech
GSS_C_MA_MECH_STACKABLE (2) pseudo-mech
GSS_C_MA_MECH_COMPOSITE (3) composite-mech
GSS_C_MA_MECH_NEGO (4) mech-negotiation-mech
GSS_C_MA_MECH_GLUE (5) mech-glue
GSS_C_MA_NOT_MECH (6) not-mech
GSS_C_MA_DEPRECATED (7) mech-deprecated
GSS_C_MA_NOT_DFLT_MECH (8) mech-not-default
GSS_C_MA_ITOK_NOT_FRAMED (9) initial-token-not-framed
GSS_C_MA_AUTH_INIT (10) auth-init-princ
GSS_C_MA_AUTH_TARG (11) auth-targ-princ
GSS_C_MA_AUTH_INIT_INIT (12) auth-init-princ-initial
GSS_C_MA_AUTH_TARG_INIT (13) auth-targ-princ-initial
GSS_C_MA_AUTH_INIT_ANON (14) auth-init-princ-anon
GSS_C_MA_AUTH_TARG_ANON (15) auth-targ-princ-anon
GSS_C_MA_DELEG_CRED (16) deleg-cred
GSS_C_MA_INTEG_PROT (17) integ-prot
GSS_C_MA_CONF_PROT (18) conf-prot
GSS_C_MA_PROT_READY (19) prot-ready
GSS_C_MA_REPLAY_DET (20) replay-detection
GSS_C_MA_OOS_DET (21) oos-detection
GSS_C_MA_CBINDINGS (22) channel-bindings
GSS_C_MA_CBINDINGS_BIDI (23) channel-bindings-bidirectional
GSS_C_MA_CBINDINGS_NEGO (24) channel-bindings-negotiate
GSS_C_MA_PFS (25) pfs
GSS_C_MA_COMPRESS (26) compress
<reserved> (27..)
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Mech Attr name Purpose
-------------- -------
GSS_C_MA_MECH_CONCRETE Indicates that a mech is neither a pseudo-
mechanism nor a composite mechanism.
GSS_C_MA_MECH_STACKABLE Indicates that a mech is a pseudo-mechanism.
GSS_C_MA_MECH_COMPOSITE Indicates that a mech is a composite
mechanism.
GSS_C_MA_MECH_NEGO Indicates that a mech negotiates other
mechs (e.g., SPNEGO has this attribute).
GSS_C_MA_MECH_GLUE Indicates that the OID is not for a
mechanism but for the GSS-API itself.
GSS_C_MA_NOT_MECH Indicates that the OID is known, yet also
known not to be the OID of any GSS-API
mechanism (or the GSS-API itself).
GSS_C_MA_DEPRECATED Indicates that a mech (or its OID) is
deprecated and MUST NOT be used as a default
mechanism.
GSS_C_MA_NOT_DFLT_MECH Indicates that a mech (or its OID) MUST NOT
be used as a default mechanism.
GSS_C_MA_ITOK_NOT_FRAMED Indicates that the given mechanism's initial
context tokens are not properly framed as
per-section 3.1 of rfc2743.
GSS_C_MA_AUTH_INIT Indicates support for authentication of
initiator to acceptor.
GSS_C_MA_AUTH_TARG Indicates support for authentication of
acceptor to initiator.
GSS_C_MA_AUTH_INIT_INIT Indicates support for initial authentication
of initiator to acceptor.
GSS_C_MA_AUTH_TARG_INIT Indicates support for initial authentication
of acceptor to initiator.
GSS_C_MA_AUTH_INIT_ANON Indicates support for initiator anonymity.
GSS_C_MA_AUTH_TARG_ANON Indicates support for acceptor anonymity.
GSS_C_MA_DELEG_CRED Indicates support for credential delegation.
GSS_C_MA_INTEG_PROT Indicates support for per-message integrity
protection.
GSS_C_MA_CONF_PROT Indicates support for per-message
confidentiality protection.
GSS_C_MA_PROT_READY Indicates support for per-message protection
prior to full context establishment.
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GSS_C_MA_REPLAY_DET Indicates support for replay detection.
GSS_C_MA_OOS_DET Indicates support for out-of-sequence
detection.
GSS_C_MA_CBINDINGS Indicates support for channel bindings.
GSS_C_MA_CBINDINGS_BIDI Indicates support for bidirectional channel
bindings.
GSS_C_MA_CBINDINGS_NEGO Indicates that the mech acts as a signal for
application support for and willingness to
use channel bindings.
GSS_C_MA_PFS Indicates support for Perfect Forward
Security.
GSS_C_MA_COMPRESS Indicates support for compression of data
inputs to GSS_Wrap().
4.1.3. Mechanism Attribute Sets of Existing Mechs
The Kerberos V mechanism [RFC1964] [CFX] provides the following
mechanism attributes:
GSS_C_MA_MECH_CONCRETE
GSS_C_MA_AUTH_INIT
GSS_C_MA_AUTH_TARG
GSS_C_MA_DELEG_CRED
GSS_C_MA_INTEG_PROT
GSS_C_MA_CONF_PROT
GSS_C_MA_PROT_READY (varies by initiator implementation)
GSS_C_MA_REPLAY_DET
GSS_C_MA_OOS_DET
GSS_C_MA_CBINDINGS
The Kerberos V mechanism also has a deprecated OID which has the same
mechanism attributes as above, and GSS_C_MA_DEPRECATED.
[The mechanism attributes of the SPKM family of mechanisms will be
provided in a separate document as SPKM is current being reviewed
for possibly significant changes due to problems in its
specifications.]
The LIPKEY mechanism offers the following attributes:
GSS_C_MA_MECH_CONCRETE (should be stackable, but does not compose)
GSS_C_MA_AUTH_INIT_INIT
GSS_C_MA_AUTH_TARG (from SPKM-3)
GSS_C_MA_AUTH_TARG_ANON (from SPKM-3)
GSS_C_MA_INTEG_PROT
GSS_C_MA_CONF_PROT
GSS_C_MA_REPLAY_DET
GSS_C_MA_OOS_DET
(LIPKEY should also provide GSS_C_MA_CBINDINGS, but SPKM-3
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requires clarifications on this point.)
The SPNEGO mechanism [SPNEGO] provides the following attribute:
GSS_C_MA_MECH_NEGO
All other mechanisms' attrivutes will be described elsewhere.
4.2. 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.
4.2.1. GSS_Indicate_mechs_by_mech_attrs()
Inputs:
o desired_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_*
-- OIDs that the mechanisms indicated in the mechs output parameter
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-- MUST offer.
o except_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_*
-- OIDs that the mechanisms indicated in the mechs output parameter
-- MUST NOT offer.
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o mechs SET OF OBJECT IDENTIFIER -- set of mechanisms that support
-- the desired_mech_attrs but not the except_mech_attrs.
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_BAD_MECH_ATTR indicates that at least one mechanism attribute
OID in desired_mech_attrs or except_mech_attrs is unknown to the
implementation.
o GSS_S_FAILURE indicates that the request failed for some other
reason.
GSS_Indicate_mechs_by_mech_attrs() returns the set of mechanism OIDs
that offer at least the desired_mech_attrs but none of the
except_mech_attrs.
When desired_mech_attrs and except_mech_attrs are the empty set this
function acts as a version of GSS_indicate_mechs() that outputs the
set of all supported mechanisms of all types. By setting the
desired_mechs input parameter to a set of a single GSS_C_MA_MECH*
feature applications can obtain the list of all supported mechanisms
of a given type (concrete, stackable, etc...).
4.2.2. GSS_Inquire_mech_attrs_for_mech()
Inputs:
o mech OBJECT IDENTIFIER -- mechanism OID
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o mech_attrs SET OF OBJECT IDENTIFIER -- set of mech_attrs OIDs
-- (GSS_C_MA_*)
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Return major_status codes:
o GSS_S_COMPLETE indicates success; the output mech_attrs parameter
MAY be the empty set (GSS_C_NO_OID_SET).
o GSS_S_BAD_MECH indicates that the mechanism named by the mech
parameter does not exist or that mech is GSS_C_NO_OID and no default
mechanism could be determined.
o GSS_S_FAILURE indicates that the request failed for some other
reason.
GSS_Inquire_mech_attrs_for_mech() indicates the set of mechanism
attributes supported by a given mechanism.
Because the mechanism attribute sets of composite mechanisms need not
be the union of their components', when called to obtain the feature
set of a composite mechanism GSS_Inquire_mech_attrs_for_mech()
obtains it by querying the mechanism at the top of the stack. See
section 4.1.
4.2.3. GSS_Display_mech_attr()
Inputs:
o mech_attr OBJECT IDENTIFIER -- mechanism attribute OID
Outputs:
o major_status INTEGER,
o minor_status INTEGER,
o name OCTET STRING, -- name of mechanism attribute (e.g.,
-- GSS_C_MA_*)
o short_desc OCTET STRING, -- a short description of the mechanism
-- attribute
o long_desc OCTET STRING -- a longer description of the mechanism
-- attribute
Return major_status codes:
o GSS_S_COMPLETE indicates success.
o GSS_S_BAD_MECH_ATTR indicates that the mechanism attribute
referenced by the mech_attr parameter is unknown to the
implementation.
o GSS_S_FAILURE indicates that the request failed for some other
reason.
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This function can be used to obtain human-readable descriptions of
GSS-API mechanism attributes.
4.2.4. GSS_Compose_oid()
Inputs:
o mech1 OBJECT IDENTIFIER, -- mechanism OID
o mech2 OBJECT IDENTIFIER -- mechanism OID
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.
4.2.5. 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.
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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.
4.2.6. 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
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.
4.2.7. 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
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-- 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:
- 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;
- 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);
- 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
- 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.
4.2.8. GSS_Negotiate_mechs()
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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.
4.3. New Major Status Values
A single new major status code is added for GSS_Display_mech_attr():
GSS_S_BAD_MECH_ATTR
roughly corresponding to GSS_S_BAD_MECH, but applicable to mechanism
attribute OIDs, rather than to mechanism OIDs.
For the C-bindings GSS_S_BAD_MECH_ATTR shall have a routine error
number of 19 (this is shifted to the left by
GSS_C_ROUTINE_ERROR_OFFSET).
4.4. C-Bindings
#define GSS_S_BAD_MECH_ATTR (19ul << GSS_C_ROUTINE_ERROR_OFFSET)
OM_uint32 gss_inquire_mechs_for_mech_attrs(
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OM_uint32 *minor_status,
const gss_OID_set desired_mech_attrs,
gss_OID_set *mechs);
OM_uint32 gss_inquire_mech_attrs_for_mech(
OM_uint32 *minor_status,
const gss_OID mech,
gss_OID_set *mech_attrs);
OM_uint32 gss_display_mech_attr(
OM_uint32 *minor_status,
const gss_OID mech_attr,
gss_buffer_t name,
gss_buffer_t short_desc,
gss_buffer_t long_desc);
OM_uint32 gss_compose_oid(
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);
5. 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 3.4, MUST
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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
of composed OIDs, where possible. See section 4.2.7.
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.
5.1. 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.
6. Requirements for Mechanism Designers
Stackable pseudo-mechanisms specifications MUST:
- list the set of GSS-API mechanism attributes associated with them
- list their initial mechanism composition rules
- specify a mechanism for updating their mechanism composition rules
All other mechanism specifications MUST:
- list the set of GSS-API mechanism attributes associated with them
7. IANA Considerations
The namsepace of programming language symbols with names beginning
with GSS_C_MA_* is reserved for allocation by the IANA.
Allocation of arcs in the namespace of OIDs relative to the base
mechanism attribute OID specified in section 4 is reserved to the
IANA.
Allocation of arcs in the namespace of OIDs relative to the base
stackable pseduo-mechanism OID specified in section 3 is reserved to
the IANA.
8. Security considerations
Some composite mechanisms may well not be secure. The mechanism
composition rules of pseudo-mechanisms (including the default
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composition rule given in section 3 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
4.1.1. Failure to do so may lead to inappropriate composite
mechanisms being deemed permissible by programmatic application of
flawed mechanism composition rules or to by their application with
incorrect mechanism attribute sets.
9. References
9.1. Informative references
[?]
...
9.2. Normative references
[RFC2026]
S. Bradner, RFC2026: "The Internet Standard Process - Revision
3," October 1996, Obsoletes - RFC 1602, Status: Best Current
Practice.
[RFC2119]
S. Bradner, RFC2119 (BCP14): "Key words for use in RFCs to
Indicate Requirement Levels," March 1997, Status: Best Current
Practice.
[RFC2743]
J. Linn, RFC2743: "Generic Security Service Application Program
Interface Version 2, Update 1," January 2000, Status: Proposed
Standard.
[RFC2744]
J. Wray, RFC2744: "Generic Security Service API Version 2 :
C-bindings," January 2000, Status: Proposed Standard.
[MUST add references to LIPKEY, SPKM, the Kerberos V mechanism,
SPNEGO, CCM, etc...]
...
10. Author's Address
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
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Email: nicolas.williams@sun.com
Full Copyright Statement
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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