Kerberos Version 5 Generic Security Service Application Program Interface (GSS-API) Channel Binding Hash Agility
RFC 6542

Internet Engineering Task Force (IETF)                          S. Emery
Request for Comments: 6542                                        Oracle
Updates: 4121                                                 March 2012
Category: Standards Track
ISSN: 2070-1721

                           Kerberos Version 5
    Generic Security Service Application Program Interface (GSS-API)
                      Channel Binding Hash Agility

Abstract

   Currently, channel bindings are implemented using an MD5 hash in the
   Kerberos Version 5 Generic Security Service Application Programming
   Interface (GSS-API) mechanism (RFC 4121).  This document updates RFC
   4121 to allow channel bindings using algorithms negotiated based on
   Kerberos crypto framework as defined in RFC 3961.  In addition,
   because this update makes use of the last extensible field in the
   Kerberos client-server exchange message, extensions are defined to
   allow future protocol extensions.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6542.

Copyright Notice

   Copyright (c) 2012 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

Emery                        Standards Track                    [Page 1]
RFC 6542              Channel Binding Hash Agility            March 2012

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Conventions Used in This Document ...............................2
   3. Channel Binding Hash Agility ....................................2
      3.1. Structure of the Exts Field ................................3
      3.2. The Channel Binding Extension ..............................4
   4. Security Considerations .........................................4
   5. IANA Considerations .............................................4
   6. Acknowledgments .................................................5
   7. References ......................................................5
      7.1. Normative References .......................................5
      7.2. Informative References .....................................5

1.  Introduction

   With the recently discovered weaknesses in the MD5 hash algorithm
   (see [RFC6151]), there is a need to use stronger hash algorithms.
   The Kerberos Version 5 Generic Security Service Application
   Programming Interface (GSS-API) mechanism [RFC4121] uses MD5 to
   calculate channel binding verifiers.  This document specifies an
   update to the mechanism that allows it to create channel binding
   information based on negotiated algorithms.  This will allow
   deploying new algorithms incrementally without breaking
   interoperability with older implementations when new attacks arise in
   the future.

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].

   The term "little-endian order" is used for brevity to refer to the
   least-significant-octet-first encoding, while the term "big-endian
   order" is used for the most-significant-octet-first encoding.

3.  Channel Binding Hash Agility

   When generating a channel binding verifier, Bnd, a hash is computed
   from the channel binding fields.  Initiators MUST populate the Bnd
   field in order to maintain interoperability with existing acceptors.
   In addition, initiators MUST populate the extension field (Exts)
   defined below.

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