draft-ietf-dnsext-ds-sha256-05.txt

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        0       Reserved                -
        1       SHA-1                MANDATORY
        2       SHA-256              MANDATORY
      3-255    Unassigned               -


6.  Security Considerations

6.1.  Potential Digest Type Downgrade Attacks

   A downgrade attack from a stronger digest type to a weaker one is
   possible if all of the following are true:

   o  A zone includes multiple DS records for a given child's DNSKEY,
      each of which use a different digest type.

   o  A validator accepts a weaker digest even if a stronger one is
      present but invalid.

   For example, if the following conditions are all true:





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   o  Both SHA-1 and SHA-256 based digests are published in DS records
      within a parent zone for a given child zone's DNSKEY.

   o  The DS record with the SHA-1 digest matches the digest computed
      using the child zone's DNSKEY.

   o  The DS record with the SHA-256 digest fails to match the digest
      computed using the child zone's DNSKEY.

   Then if the validator accepts the above situation as secure then this
   can be used as a downgrade attack since the stronger SHA-256 digest
   is ignored.

6.2.  SHA-1 vs SHA-256 Considerations for DS Records

   Users of DNSSEC are encouraged to deploy SHA-256 as soon as software
   implementations allow for it.  SHA-256 is widely believed to be more
   resilient to attack than SHA-1, and confidence in SHA-1's strength is
   being eroded by recently-announced attacks.  Regardless of whether or
   not the attacks on SHA-1 will affect DNSSEC, it is believed (at the
   time of this writing) that SHA-256 is the better choice for use in DS
   records.

   At the time of this publication, the SHA-256 digest algorithm is
   considered sufficiently strong for the immediate future.  It is also
   considered sufficient for use in DNSSEC DS RRs for the immediate
   future.  However, future published attacks may weaken the usability
   of this algorithm within the DS RRs.  It is beyond the scope of this
   document to speculate extensively on the cryptographic strength of
   the SHA-256 digest algorithm.

   Likewise, it is also beyond the scope of this document to specify
   whether or for how long SHA-1 based DS records should be
   simultaneously published alongside SHA-256 based DS records.


7.  Acknowledgments

   This document is a minor extension to the existing DNSSEC documents
   and those authors are gratefully appreciated for the hard work that
   went into the base documents.

   The following people contributed to portions of this document in some
   fashion: Mark Andrews, Roy Arends, Olafur Gudmundsson, Paul Hoffman,
   Olaf M. Kolkman, Edward Lewis, Scott Rose, Stuart E. Schechter, Sam
   Weiler.





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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [SHA256]   National Institute of Standards and Technology, "Secure
              Hash Algorithm. NIST FIPS 180-2", August 2002.

8.2.  Informative References

   [SHA256CODE]
              Eastlake, D., "US Secure Hash Algorithms (SHA)",
              June 2005.
























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Author's Address

   Wes Hardaker
   Sparta
   P.O. Box 382
   Davis, CA  95617
   US

   Email: [e-mail]










































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