Network Working Group N. Williams Request for Comments: 5587 Sun Category: Standards Track July 2009
Extended Generic Security Service Mechanism Inquiry APIs
Abstract
This document introduces new application programming interfaces (APIs) to the Generic Security Services API (GSS-API) for extended mechanism attribute inquiry. These interfaces are primarily intended to reduce instances of hardcoding of mechanism identifiers in GSS applications.
These interfaces include mechanism attributes and attribute sets, a function for inquiring the attributes of a mechanism, a function for indicating mechanisms that possess given attributes, and a function for displaying mechanism attributes.
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.
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Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................2 2. Conventions Used in This Document ...............................2 3. New GSS-API Interfaces ..........................................3 3.1. Mechanism Attributes and Attribute Sets ....................3 3.2. List of Known Mechanism Attributes .........................4 3.3. Mechanism Attribute Sets of Existing Mechs .................6 3.4. New GSS-API Function Interfaces ............................8 3.4.1. Mechanism Attribute Criticality .....................8 3.4.2. GSS_Indicate_mechs_by_attrs() .......................9 3.4.3. GSS_Inquire_attrs_for_mech() .......................10 3.4.4. GSS_Display_mech_attr() ............................10 3.4.5. New Major Status Values ............................11 3.4.6. C-Bindings .........................................11 4. Requirements for Mechanism Designers ...........................13 5. IANA Considerations ............................................13 6. Security Considerations ........................................13 7. References .....................................................13 7.1. Normative References ......................................13 7.2. Informative References ....................................14 Appendix A. Typedefs and C Bindings ..................................15
GSS-API [RFC2743] mechanisms have a number of properties that may be of interest to applications. The lack of APIs for inquiring about available mechanisms' properties has meant that many GSS-API applications must hardcode mechanism Object Identifiers (OIDs). Ongoing work may result in a variety of new GSS-API mechanisms. Applications should not have to hardcode their OIDs.
For example, the Secure Shell version 2 (SSHv2) protocol [RFC4251] supports the use of GSS-API mechanisms for authentication [RFC4462] but explicitly prohibits the use of Simple and Protected GSS-API Negotiation (SPNEGO) [RFC4178]. Future mechanisms that negotiate mechanisms would have to be forbidden as well, but there is no way to implement applications that inquire what mechanisms are available and then programmatically exclude mechanisms "like SPNEGO".
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].
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 without having to hardcode mechanism OIDs.
Section 3.1 describes the mechanism attributes concept. Sections 3.4.2, 3.4.3, and 3.4.4 describe three new interfaces that deal in mechanisms and attribute sets:
An abstraction for the features provided by mechanisms and pseudo- mechanisms is needed in order to facilitate the programmatic selection of mechanisms. Pseudo-mechanisms are mechanisms that make reference to other mechanisms in order to provide their services. For example, SPNEGO is a pseudo-mechanism, for without other mechanisms SPNEGO is useless.
Two data types are needed: one for individual mechanism attributes and one for mechanism attribute sets. To simplify the mechanism attribute 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:
<1.3.6.1.5.5.13>
Each mechanism has a set of mechanism attributes that it supports as described in its specification.
+-------------------------+-----------------------------------------+ | 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_PSEUDO | Indicates that a mech is a | | | pseudo-mechanism. | | GSS_C_MA_MECH_COMPOSITE | Indicates that a mech is a composite of | | | other mechanisms. This is reserved for | | | a specification of "stackable" | | | pseudo-mechanisms. | | 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 it | | | is also known not to be the OID of any | | | GSS-API mechanism (or of 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_FRAMED | Indicates that the given mechanism's | | | initial context tokens are 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. "Initial authentication" | | | refers to the use of passwords, or keys | | | stored on tokens, for authentication. | | | Whether a mechanism supports initial | | | authentication may depend on IETF | | | consensus (see Security | | | Considerations). | | GSS_C_MA_AUTH_TARG_INIT | Indicates support for initial | | | authentication of acceptor to | | | initiator. | | GSS_C_MA_AUTH_INIT_ANON | Indicates support for | | | GSS_C_NT_ANONYMOUS as an initiator | | | principal name. |
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| GSS_C_MA_AUTH_TARG_ANON | Indicates support for | | | GSS_C_NT_ANONYMOUS as a target | | | principal name. | | 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_MIC | Indicates support for Message Integrity | | | Code (MIC) tokens. | | GSS_C_MA_WRAP | Indicates support for WRAP tokens. | | GSS_C_MA_PROT_READY | Indicates support for per-message | | | protection prior to full context | | | establishment. | | 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_PFS | Indicates support for Perfect Forward | | | Security. | | GSS_C_MA_COMPRESS | Indicates support for compression of | | | data inputs to GSS_Wrap(). | | GSS_C_MA_CTX_TRANS | Indicates support for security context | | | export/import. | +-------------------------+-----------------------------------------+
The Kerberos V mechanism [RFC1964] provides the following mechanism attributes:
o GSS_C_MA_MECH_CONCRETE
o GSS_C_MA_ITOK_FRAMED
o GSS_C_MA_AUTH_INIT
o GSS_C_MA_AUTH_TARG
o GSS_C_MA_DELEG_CRED
o GSS_C_MA_INTEG_PROT
o GSS_C_MA_CONF_PROT
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o GSS_C_MA_MIC
o GSS_C_MA_WRAP
o GSS_C_MA_PROT_READY
o GSS_C_MA_REPLAY_DET
o GSS_C_MA_OOS_DET
o GSS_C_MA_CBINDINGS
o GSS_C_MA_CTX_TRANS (some implementations, using implementation- specific exported context token formats)
The Kerberos V mechanism also has a deprecated OID that has the same mechanism attributes as above as well as GSS_C_MA_DEPRECATED.
The mechanism attributes of the Simple Public-Key GSS-API Mechanism (SPKM) [RFC2025] family of mechanisms will be provided in a separate document, as SPKM is currently being reviewed for possibly significant changes due to problems in its specifications.
The Low Infrastructure Public Key (LIPKEY) mechanism [RFC2847] offers the following attributes:
o GSS_C_MA_MECH_CONCRETE
o GSS_C_MA_ITOK_FRAMED
o GSS_C_MA_AUTH_INIT_INIT
o GSS_C_MA_AUTH_TARG (from SPKM-3)
o GSS_C_MA_AUTH_TARG_ANON (from SPKM-3)
o GSS_C_MA_INTEG_PROT
o GSS_C_MA_CONF_PROT
o GSS_C_MA_REPLAY_DET
o GSS_C_MA_OOS_DET
o GSS_C_MA_CTX_TRANS (some implementations, using implementation- specific exported context token formats)
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(LIPKEY should also provide GSS_C_MA_CBINDINGS, but SPKM-3 requires clarifications on this point.)
The SPNEGO mechanism [RFC4178] provides the following attributes:
o GSS_C_MA_MECH_NEGO
o GSS_C_MA_ITOK_FRAMED
All other mechanisms' attributes will be described elsewhere.
Several new interfaces are given by which, for example, GSS-API applications may determine what features are provided by a given mechanism and what mechanisms provide what features.
These new interfaces are all OPTIONAL.
Applications should use GSS_Indicate_mechs_by_attrs() instead of GSS_Indicate_mechs() wherever possible.
Applications can use GSS_Indicate_mechs_by_attrs() to determine what, if any, mechanisms provide a given set of features.
GSS_Indicate_mechs_by_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, etc.).
Mechanism attributes may be added at any time. Not only may attributes be added to the list of known mechanism attributes at any time, but the set of mechanism attributes supported by a mechanism can be changed at any time.
For example, new attributes might be added to reflect whether a mechanism's initiator must contact an online infrastructure and/or whether the acceptor must do so. In this example, the Kerberos V mechanism would gain a new attribute even though the mechanism itself is not modified.
Applications making use of attributes not defined herein would then have no way of knowing whether a GSS-API implementation and its mechanisms know about new mechanism attributes. To address this problem, GSS_Indicate_mechs_by_attrs() and GSS_Inquire_attrs_for_mech() support a notion of critical mechanism attributes. Applications can search for mechanisms that understand
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mechanism attributes that are critical to the application, and the application may ask what mechanism attributes are understood by a given mechanism.
o desired_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_* OIDs that the mechanisms indicated in the mechs output parameter 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.
o critical_mech_attrs SET OF OBJECT IDENTIFIER -- set of GSS_C_MA_* OIDs that the mechanisms indicated in the mechs output parameter MUST understand (i.e., mechs must know whether critical attributes are or are not supported).
Outputs:
o major_status INTEGER
o minor_status INTEGER
o mechs SET OF OBJECT IDENTIFIER -- set of mechanisms that support the given desired_mech_attrs but not the except_mech_attrs, and all of which understand the given critical_mech_attrs (the caller must release this output with GSS_Release_oid_set()).
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_FAILURE indicates that the request failed for some other reason.
GSS_Indicate_mechs_by_attrs() returns the set of OIDs corresponding to mechanisms that offer at least the desired_mech_attrs but none of the except_mech_attrs, and that understand all of the attributes listed in critical_mech_attrs.
When all three sets of OID input parameters are the empty set, this function acts as a version of GSS_indicate_mechs() that outputs the set of all supported mechanisms.
o mech_attrs SET OF OBJECT IDENTIFIER -- set of mech_attrs OIDs (GSS_C_MA_*) supported by the mechanism (the caller must release this output with GSS_Release_oid_set()).
o known_mech_attrs SET OF OBJECT IDENTIFIER -- set of mech_attrs OIDs known to the mechanism implementation (the caller must release this output with GSS_Release_oid_set()).
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 the 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_attrs_for_mech() indicates the set of mechanism attributes supported by a given mechanism.
A single, new, major status code is added for GSS_Display_mech_attr():
o 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 of the GSS-API [RFC2744], 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).
Note that there is a bug in the C bindings of the GSS-APIv2u1 [RFC2744] in that the C 'const' attribute is applied to types that are pointer typedefs. This is a bug because it declares that the pointer argument is 'const' rather than that the object pointed by it is const. To avoid this error, we hereby define new typedefs, which include const properly:
Note that only gss_const_OID and gss_const_OID_set are used below. We include the other const typedefs for convenience since the C bindings of the GSS-API do use const with pointer typedefs when it should often instead use the above typedefs instead.
The namespace of programming-language symbols with names beginning with GSS_C_MA_* is reserved for allocation by IETF Consensus. IANA allocated a base OID, as an arc of 1.3.6.1.5.5, for the set of GSS_C_MA_* described herein, and registered all of the GSS_C_MA_* values described in Section 3.2.
This document specifies extensions to a security-related API. It imposes new requirements on future GSS-API mechanisms, and the specifications of future protocols that use the GSS-API should make reference to this document where applicable. The ability to inquire about specific properties of mechanisms should improve security.
The semantics of each mechanism attribute may include a security component.
Application developers must understand that mechanism attributes may be added at any time -- both to the set of known mechanism attributes as well as to existing mechanisms' sets of supported mechanism attributes. Therefore, application developers using the APIs described herein must understand what mechanism attributes their applications depend critically on, and must use the mechanism attribute criticality features of these APIs.
[RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996.
[RFC2025] Adams, C., "The Simple Public-Key GSS-API Mechanism (SPKM)", RFC 2025, October 1996.
[RFC2847] Eisler, M., "LIPKEY - A Low Infrastructure Public Key Mechanism Using SPKM", RFC 2847, June 2000.
[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The Simple and Protected Generic Security Service Application Program Interface (GSS-API) Negotiation Mechanism", RFC 4178, October 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, January 2006.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch, "Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol", RFC 4462, May 2006.
This appendix contains the full set of code fragments defined in this document.
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