


NETWORK WORKING GROUP                                           S. Emery
Internet-Draft                                                    Oracle
Updates: 4121 (if approved)                              January 6, 2012
Intended status: Standards Track
Expires: July 9, 2012


        Kerberos Version 5 GSS-API Channel Binding Hash Agility
              draft-ietf-krb-wg-gss-cb-hash-agility-10.txt

Abstract

   Currently, channel bindings are implemented using a MD5 hash in the
   Kerberos Version 5 Generic Security Services Application Programming
   Interface (GSS-API) mechanism [RFC4121].  This document updates
   RFC4121 to allow channel bindings using algorithms negotiated based
   on Kerberos crypto framework as defined in RFC3961.  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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 9, 2012.

Copyright Notice

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   document authors.  All rights reserved.

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Channel Binding Hash Agility . . . . . . . . . . . . . . . . .  5
     3.1.  Structure of the Exts Field  . . . . . . . . . . . . . . .  5
     3.2.  The Channel Binding Extension  . . . . . . . . . . . . . .  6
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11































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1.  Introduction

   With the recently discovered weaknesses in the MD5 hash algorithm,
   see [RFC6151], there is a need to use stronger hash algorithms.
   Kerberos Version 5 Generic Security Services 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.








































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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 for the most-significant-octet-first encoding.










































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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.

3.1.  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 in
                            section 4.1.1.2 [RFC4121].
      20..23    Flags      Four-octet context-establishment flags in
                            little-endian order as described in section
                            4.1.1.1 [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
                            in section 4.1.1.1 [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, 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.




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3.2.  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 a MD5 hash of the channel
   bindings MUST set the Bnd field to sixteen octets of hex value FF.































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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 to enforce the use of channel
   bindings or not.  [RFC5056] and [RFC5554] give guidance for
   application developers on channel bindings usage.











































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5.  IANA Considerations

   The IANA is hereby requested to create a new top-level registry
   titled "Kerberos V GSS-API Mechanism Parameters," separate from the
   existing Kerberos parameters registry.  Within this registry, IANA is
   requested to create a sub-registry of "Kerberos V GSS-API mechanism
   extension types" with four-field entries (type number, type name,
   description, and normative reference) and, initially, a single
   registration: 0x00000000, "Channel Binding MIC," "Extension for the
   verifier of the channel bindings," <this RFC>.

   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 Use
































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6.  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 feed-back of the draft.














































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7.  References

7.1.  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.



















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Author's Address

   Shawn Emery
   Oracle
   500 Eldorado Blvd
   Building 1
   Broomfield, CO  80021
   USA

   Email: shawn.emery@oracle.com









































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