


NETWORK WORKING GROUP                                           S. Emery
Internet-Draft                                          Sun Microsystems
Updates: 4121 (if approved)                             November 9, 2007
Intended status: Standards Track
Expires: May 12, 2008


        Kerberos Version 5 GSS-API Channel Binding Hash Agility
              draft-ietf-krb-wg-gss-cb-hash-agility-03.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2007).













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Abstract

   Currently, the Kerberos Version 5 Generic Security Services
   Application Programming Interface (GSS-API) mechanism [RFC4121] does
   not have the ability to utilize better hash algorithms used to
   generate channel binding identities.  The current mechanism for doing
   this is hard coded to use MD5 only.  The purpose of this document is
   to outline changes required to update the protocol so that more
   secure algorithms can be used to create channel binding identities.
   The extensibility of this solution also provides an eventual
   replacement of identities based solely on hash algorithms.


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 move stronger hash alogrithms.  Kerberos Version 5
   Generic Security Services Application Programming Interface (GSS-API)
   mechanism [RFC4121] uses MD5 to calculate channel binding identities
   that are required to be unique.  This document specifies an update to
   the mechanism that allows it to create channel binding identities
   based on negotiating algorithms securely.  This will prevent lengthy
   standardizations in the future when new attacks arise and will allow
   an incremental update to the protocol so that this will interoperate
   with older implementations.







































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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 identifier, Bnd, a hash is computed
   from the channel binding information.  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].  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:
        type -- big endian order unsigned integer, 32-bits
        length -- big endian order unsigned integer, 32-bits
        data -- octet string of length octets
      in that order

   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], using the sub-session key from the
         authenticator and key usage number TBD, to the channel binding
         data as described in [RFC4121], section 4.1.1.2 (using get_mic
         instead of MD5).



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   Initiators that are unwilling to use a MD5 hash of the channel
   bindings should set the Bnd field to all ones (1).

















































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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 hash function-agile channel binding," <this RFC>.

   Registration of additional extensions SHALL be by IESG Protocol
   Action.









































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6.  Acknowledgements

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

   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.



































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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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