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Network Working Group                                        N. WilliamsRequest for Comments: 5554                                           SunUpdates:2743                                                   May 2009Category: Standards TrackClarifications and Extensions tothe Generic Security Service Application Program Interface (GSS-API)for the Use of Channel BindingsStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (c) 2009 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents in effect on the date of   publication of this document (http://trustee.ietf.org/license-info).   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.Abstract   This document clarifies and generalizes the Generic Security Service   Application Programming Interface (GSS-API) "channel bindings"   facility, and imposes requirements on future GSS-API mechanisms and   programming language bindings of the GSS-API.Table of Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. New Requirements for GSS-API Mechanisms .........................24. Generic Structure for GSS-API Channel Bindings ..................25. Security Considerations .........................................36. References ......................................................46.1. Normative References .......................................46.2. Informative References .....................................4Williams                    Standards Track                     [Page 1]

RFC 5554                GSS-API Channel Bindings                May 20091.  Introduction   The base GSS-API version 2, update 1 specification [RFC2743] provides   a facility for channel binding (see also [RFC5056]), but its   treatment is incomplete.  The GSS-API C-bindings specification   [RFC2744] expands somewhat on this facility in what should be a   generic way, but is instead a C-specific way, thus leaving the   treatment of this facility incomplete.   This document clarifies the GSS-API's channel binding facility and   generalizes the parts of it that are specified in the C-bindings   document but that should have been generic from the start.2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].3.  New Requirements for GSS-API Mechanisms   Given the publication ofRFC 5056, we now assert that all new GSS-API   mechanisms that support channel binding MUST conform to [RFC5056].4.  Generic Structure for GSS-API Channel Bindings   The base GSS-API version 2, update 1 specification [RFC2743] provides   a facility for channel binding.  It models channel bindings as an   OCTET STRING and leaves it to the GSS-API version 2, update 1   C-bindings specification to specify the structure of the contents of   the channel bindings OCTET STRINGs.  The C-bindings specification   [RFC2744] then defines, in terms of C, what should have been a   generic structure for channel bindings.  The Kerberos V GSS mechanism   [RFC4121] also defines a method for encoding GSS channel bindings in   a way that is independent of the C-bindings -- otherwise, the   mechanism's channel binding facility would not be useable with other   language bindings.   In other words, the structure of GSS channel bindings given in   [RFC2744] is actually generic in spite of being specified in terms of   C concepts and syntax.   We generalize it as shown below, using the same pseudo-ASN.1 as is   used inRFC 2743.  Although the figure below is, indeed, a valid   ASN.1 [CCITT.X680] type, we do not provide a full ASN.1 module as   none is needed because no standard encoding of this structure is   needed -- the definition below is part of an abstract API, not partWilliams                    Standards Track                     [Page 2]

RFC 5554                GSS-API Channel Bindings                May 2009   of a protocol defining bits on the wire.  GSS-API mechanisms do need   to encode the contents of this structure, but that encoding will be   mechanism specific (see below).      GSS-CHANNEL-BINDINGS ::= SEQUENCE {              initiator-address-type  INTEGER,      -- SeeRFC2744              initiator-address       OCTET STRING, -- SeeRFC2744              acceptor-address-type   INTEGER,      -- SeeRFC2744              acceptor-address        OCTET STRING, -- SeeRFC2744              application-data        OCTET STRING  -- SeeRFC5056      }                Abstract GSS-API Channel Bindings Structure   The values for the address fields are described in [RFC2744].   New language-specific bindings of the GSS-API SHOULD specify a   language-specific formulation of this structure.   Where a language binding of the GSS-API models channel bindings as   OCTET STRINGs (or the language's equivalent), then the implementation   MUST assume that the given bindings correspond only to the   application-data field of GSS-CHANNEL-BINDINGS as shown above, rather   than some encoding of GSS-CHANNEL-BINDINGS.   As mentioned above, [RFC4121] describes an encoding of the above GSS-   CHANNEL-BINDINGS structure and then hashes that encoding.  Other GSS-   API mechanisms are free to use that encoding.5.  Security Considerations   For general security considerations relating to channel bindings, see   [RFC5056].   Language bindings that use OCTET STRING (or equivalent) for channel   bindings will not support the use of network addresses as channel   bindings.  This should not cause any security problems, as the use of   network addresses as channel bindings is not generally secure.   However, it is important that "end-point channel bindings" not be   modeled as network addresses; otherwise, such channel bindings may   not be useable with all language bindings of the GSS-API.Williams                    Standards Track                     [Page 3]

RFC 5554                GSS-API Channel Bindings                May 20096.  References6.1.  Normative References   [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate                 Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2743]     Linn, J., "Generic Security Service Application Program                 Interface Version 2, Update 1",RFC 2743, January 2000.   [RFC2744]     Wray, J., "Generic Security Service API Version 2 :                 C-bindings",RFC 2744, January 2000.   [RFC4121]     Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos                 Version 5 Generic Security Service Application Program                 Interface (GSS-API) Mechanism: Version 2",RFC 4121,                 July 2005.   [RFC5056]     Williams, N., "On the Use of Channel Bindings to Secure                 Channels",RFC 5056, November 2007.6.2.  Informative References   [CCITT.X680]  International Telephone and Telegraph Consultative                 Committee, "Abstract Syntax Notation One (ASN.1):                 Specification of basic notation", CCITT Recommendation                 X.680, July 2002.Author's Address   Nicolas Williams   Sun Microsystems   5300 Riata Trace Ct   Austin, TX  78727   US   EMail: Nicolas.Williams@sun.comWilliams                    Standards Track                     [Page 4]