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Internet Engineering Task Force (IETF)                          S. EmeryRequest for Comments: 6542                                        OracleUpdates:4121                                                 March 2012Category: Standards TrackISSN: 2070-1721Kerberos Version 5Generic Security Service Application Program Interface (GSS-API)Channel Binding Hash AgilityAbstract   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 updatesRFC4121 to allow channel bindings using algorithms negotiated based on   Kerberos crypto framework as defined inRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 mustEmery                        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 Contents1. Introduction ....................................................22. Conventions Used in This Document ...............................23. Channel Binding Hash Agility ....................................23.1. Structure of the Exts Field ................................33.2. The Channel Binding Extension ..............................44. Security Considerations .........................................45. IANA Considerations .............................................46. Acknowledgments .................................................57. References ......................................................57.1. Normative References .......................................57.2. Informative References .....................................51.  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.Emery                        Standards Track                    [Page 2]

RFC 6542              Channel Binding Hash Agility            March 20123.1.  Structure of the Exts Field   The 0x8003 GSS checksum has the same structure described in [RFC4121]   except that the Exts field is now defined; the entire structure of   the 0x8003 checksum, including the now defined Exts field, follows:      Octet     Name       Description      -----------------------------------------------------------------      0..3      Lgth       Number of octets in Bnd field, represented                            in little-endian order;  currently contains                            hex value 10 00 00 00 (16).      4..19     Bnd        Channel binding information, as described inSection 4.1.1.2 of [RFC4121].      20..23    Flags      Four-octet context-establishment flags in                            little-endian order as described inSection4.1.1.1 of [RFC4121].      24..25    DlgOpt     The delegation option identifier (=1) in                            little-endian order [optional].  This field                            and the next two fields are present if and                            only if GSS_C_DELEG_FLAG is set as described                            inSection 4.1.1.1 of [RFC4121].      26..27    Dlgth      The length of the Deleg field in                            little-endian order [optional].      28..(n-1) Deleg      KRB_CRED message (n = Dlgth + 28) [optional].      n..last   Exts       Extensions.      where Exts is the concatenation of zero, one, or more individual      extensions, each of which consists of the following, in order:         type --   big-endian-order unsigned integer, 32 bits, which                   contains the type of extension         length -- big-endian-order unsigned integer, 32 bits, which                   contains the length, in octets, of the extension data                   encoded as an array of octets immediately following                   this field         data --   octet string of extension information      If multiple extensions are present, then there MUST be at most one      instance of a given extension type.Emery                        Standards Track                    [Page 3]

RFC 6542              Channel Binding Hash Agility            March 20123.2.  The Channel Binding Extension   When channel binding is used, the Exts MUST include the following   extension:      data-type 0x00000000      data-value         The output obtained by applying the Kerberos V get_mic         operation [RFC3961] with key usage number 43 to the channel         binding data as described in[RFC4121], Section 4.1.1.2 (using         get_mic instead of MD5).  The key used is the sub-session key         from the authenticator, if it is present; otherwise, the key         used is the session key from the ticket.  The get_mic algorithm         is chosen as the "required checksum mechanism" for the         encryption type of the key used.   Initiators that are unwilling to use an MD5 hash of the channel   bindings MUST set the Bnd field to sixteen octets of hex value FF.4.  Security Considerations   With this mechanism, initiators get no indication as to whether the   acceptors check or ignore channel bindings.   It is up to the application whether or not to enforce the use of   channel bindings.  [RFC5056] and [RFC5554] give guidance for   application developers on channel binding usage.5.  IANA Considerations   IANA has created a new top-level registry titled "Kerberos V GSS-API   Mechanism Parameters," separate from the existing Kerberos parameters   registry.  Within this registry, IANA has created a sub-registry of   "Kerberos V GSS-API Mechanism Extension Types" with four-field   entries (Type Number, Type Name, Description, and Reference) and,   initially, a single registration: 0x00000000, "Channel Binding MIC,"   "Extension for the verifier of the channel bindings," [RFC6542].   Using the guidelines for allocation as described in [RFC5226], type   number assignments are as follows:       0x00000000 - 0x000003FF IETF Review       0x00000400 - 0xFFFFF3FF Specification Required       0xFFFFF400 - 0xFFFFFFFF Private UseEmery                        Standards Track                    [Page 4]

RFC 6542              Channel Binding Hash Agility            March 20126.  Acknowledgments   The author would like to thank Larry Zhu, Nicolas Williams, Sam   Hartman, Jeffrey Hutzelman, and Simon Josefsson for their help in   reviewing and providing valuable feedback on this document.7.  References7.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for              Kerberos 5",RFC 3961, February 2005.   [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.   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 5226,              May 2008.7.2.  Informative References   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure              Channels",RFC 5056, November 2007.   [RFC5554]  Williams, N., "Clarifications and Extensions to the              Generic Security Service Application Program Interface              (GSS-API) for the Use of Channel Bindings",RFC 5554, May              2009.   [RFC6151]  Turner, S. and L. Chen, "Updated Security Considerations              for the MD5 Message-Digest and the HMAC-MD5 Algorithms",RFC 6151, March 2011.Emery                        Standards Track                    [Page 5]

RFC 6542              Channel Binding Hash Agility            March 2012Author's Address   Shawn Emery   Oracle   500 Eldorado Blvd, Building 1   Broomfield, CO  80021   USA   EMail: shawn.emery@oracle.comEmery                        Standards Track                    [Page 6]