


NETWORK WORKING GROUP                                           S. Emery
Internet-Draft                                          Sun Microsystems
Updates: 4121 (if approved)                                July 14, 2008
Intended status: Standards Track
Expires: January 15, 2009


        Kerberos Version 5 GSS-API Channel Binding Hash Agility
              draft-ietf-krb-wg-gss-cb-hash-agility-04.txt

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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 the channel binding restriction to be lifted 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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Conventions Used in This Document  . . . . . . . . . . . . . .  4
   3.  Channel binding hash agility . . . . . . . . . . . . . . . . .  5
   4.  Security considerations  . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12




























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1.  Introduction

   With the recently discovered weaknesses in the MD5 hash algorithm
   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 negotiating algorithms
   securely.  This will allow deploying new algorithms incrementally
   without break 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, with
   TYPED-DATA as defined in [RFC4120].  All fields before Exts, do not
   change from what is described in [RFC4121], they are listed for
   convenience.  The 0x8003 GSS checksum MUST have the following
   structure:

      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
      data -- octet string of extension information

      If multiple extensions are present then there MUST be at most one
      instance of a given extension type.

   When channel binding is used the Exts MUST include the following
   extension:








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      data-type 0x00000000

      data-value

         The output obtained by applying the Kerberos V get_mic()
         operation [RFC3961], using the sub-session key from the
         authenticator and key usage number 43, to the channel binding
         data as described in [RFC4121], section 4.1.1.2 (using get_mic
         instead of MD5).

   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

   Initiators do not know if the acceptor had ignored channel bindings
   or whether it validated the MD5 hash of the channel bindings
   [RFC4121].

   Ultimately, it is up to the application whether to use channel
   binding or not.  This is dependent upon the security policy of these
   applications.










































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5.  IANA Considerations

   The IANA is hereby requested to create a new 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 Consensus

      0x00000400 - 0xFFFFF3FF Specification Required

      0xFFFFF400 - 0xFFFFFFFF Private Use



































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6.  Acknowledgements

   Larry Zhu helped in the review of this document overall and provided
   the suggestions of typed-data.

   Nicolas Williams and Sam Hartman suggested that the Bnd and Exts
   fields be populated simultaneously.

   Nicolas Williams and Jeffrey Hutzelman had also suggested a number
   changes to this document.









































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7.  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.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 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.































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Author's Address

   Shawn Emery
   Sun Microsystems
   500 Eldorado Blvd
   M/S UBRM05-171
   Broomfield, CO  80021
   US

   Email: shawn.emery@sun.com









































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