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Versions: 00 01 02 03 04 05 06 07 08 09 10 RFC 6542

NETWORK WORKING GROUP                                           S. Emery
Internet-Draft                                                       Sun
Intended status: Standards Track                            October 2006
Expires: April 4, 2007


        Kerberos Version 5 GSS-API Channel Binding Hash Agility
              draft-ietf-krb-wg-gss-cb-hash-agility-00.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).














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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.  Channel binding hash agility . . . . . . . . . . . . . . . . .  4
   3.  Channel binding extension hash parameters  . . . . . . . . . .  6
   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 (SHA1 based) hash
   algorithm there is a need to move to crypto-agility.  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.








































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2.  Channel binding hash agility

   When generating a channel binding identifier, Bnd, a hash is computed
   from the channel binding information.  Newer clients (initiators)
   shall continue to populate the Bnd field in order to remain
   compatible with older servers (acceptors).  In addition, newer
   clients shall populate the extension field, Exts, with TYPED-DATA as
   defined in [RFC4120].  The 0x8003 GSS checksum would 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       Type Extensions in ASN.1 DER encoding

      where Extensions ::= TYPED-DATA -- as defined in [RFC4120]


   The TYPED-DATA would have the following information:

      data-type

         This field specifies the type of channel binding extensions.
         TBD is specified when the data-value contains channel binding
         hash information.

      data-value

         This field contains specific channel binding information
         relative to data-type.  When data-type is TBD then data-value
         contains the output obtained from the get_mic() operation as
         specified in [RFC3961].  The parameters used are described in
         section 2.2.



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   When a newer server receives a token sent by a newer client the Bnd
   field will be ignored.  The newer server will then show that it
   understands the extension by sending an AP-REP with the following
   structure:

         EncAPRepPart    ::= [APPLICATION 27] SEQUENCE {
               ctime       [0] KerberosTime,
               cusec       [1] Microseconds,
               subkey      [2] EncryptionKey OPTIONAL,
               seq-number  [3] UInt32 OPTIONAL,
               extensions  [4] TYPED-DATA,
               ...
         }

         where extensions is the same data-type that the client had
          sent in the AP-REQ.



































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3.  Channel binding extension hash parameters

   The MAC value used as the channel binding identifier is calculated
   with the following parameters:

   (1)  The session key is used to derive the specific key that is used
      exclusively for the channel binding identifiers.  The key usage is
      a 32 bit integer TBD.  The specific key Kc, is therefore derived
      as:

         Kc = key_generation(session-key, TBD | 0x99)

         where key_generation is the key-derivation function

         where 0x99 is the octet that is concatenated to the key usage

   (2)  The message parameter is created the same way as described in
      section 4.1.1.2 of [RFC4121].

































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4.  Security considerations

   Servers can use a down-grade attack by ignoring the channel binding
   extensions, but client policy can prevent these attacks if the client
   has specific requirements.














































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5.  IANA Considerations

   Question for wg: Do we need to be concerned with typed hole type
   values?















































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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.












































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7.  Normative References

   [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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Full Copyright Statement

   Copyright (C) The Internet Society (2006).

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   Administrative Support Activity (IASA).





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