Network Working Group                                        N. Williams
Request for Comments: 5554                                           Sun
Updates: 2743                                                   May 2009
Category: Standards Track


                   Clarifications and Extensions to
 the Generic Security Service Application Program Interface (GSS-API)
                    for the Use of Channel Bindings

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.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 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 Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. New Requirements for GSS-API Mechanisms .........................2
   4. Generic Structure for GSS-API Channel Bindings ..................2
   5. Security Considerations .........................................3
   6. References ......................................................4
      6.1. Normative References .......................................4
      6.2. Informative References .....................................4

1.  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 of RFC 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 in RFC 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 part

   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,      -- See RFC2744
              initiator-address       OCTET STRING, -- See RFC2744
              acceptor-address-type   INTEGER,      -- See RFC2744
              acceptor-address        OCTET STRING, -- See RFC2744
              application-data        OCTET STRING  -- See RFC5056
      }

                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.

6.  References

6.1.  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.com