


Network Working Group                                        W. Hardaker
Internet-Draft                                                    Sparta
Expires: August 25, 2006                               February 21, 2006


 Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)
                   draft-ietf-dnsext-ds-sha256-05.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies how to use the SHA-256 digest type in DNS
   Delegation Signer (DS) Resource Records (RRs).  DS records, when
   stored in a parent zone, point to key signing DNSKEY key(s) in a
   child zone.








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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Implementing the SHA-256 algorithm for DS record support  . . . 3
     2.1.  DS record field values  . . . . . . . . . . . . . . . . . . 3
     2.2.  DS Record with SHA-256 Wire Format  . . . . . . . . . . . . 3
     2.3.  Example DS Record Using SHA-256 . . . . . . . . . . . . . . 4
   3.  Implementation Requirements . . . . . . . . . . . . . . . . . . 4
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . . . 4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
     6.1.  Potential Digest Type Downgrade Attacks . . . . . . . . . . 5
     6.2.  SHA-1 vs SHA-256 Considerations for DS Records  . . . . . . 6
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8
   Intellectual Property and Copyright Statements  . . . . . . . . . . 9
































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1.  Introduction

   The DNSSEC [RFC4033] [RFC4034] [RFC4035] DS RR is published in parent
   zones to distribute a cryptographic digest of a child's Key Signing
   Key (KSK) DNSKEY RR.  The DS RRset is signed by at least one of the
   parent zone's private zone data signing keys for each algorithm in
   use by the parent.  Each signature is published in an RRSIG resource
   record, owned by the same domain as the DS RRset and with a type
   covered of DS.

   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].


2.  Implementing the SHA-256 algorithm for DS record support

   This document specifies that the digest type code [XXX: To be
   assigned by IANA; likely 2] is to be assigned to SHA-256 [SHA256]
   [SHA256CODE] for use within DS records.  The results of the digest
   algorithm MUST NOT be truncated and the entire 32 byte digest result
   is to be published in the DS record.

2.1.  DS record field values

   Using the SHA-256 digest algorithm within a DS record will make use
   of the following DS-record fields:

   Digest type: [XXX: To be assigned by IANA; likely 2]

   Digest: A SHA-256 bit digest value calculated by using the following
      formula ("|" denotes concatenation).  The resulting value is not
      truncated and the entire 32 byte result is to used in the
      resulting DS record and related calculations.

        digest = SHA_256(DNSKEY owner name | DNSKEY RDATA)

      where DNSKEY RDATA is defined by [RFC4034] as:

        DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key

   The Key Tag field and Algorithm fields remain unchanged by this
   document and are specified in the [RFC4034] specification.

2.2.  DS Record with SHA-256 Wire Format

   The resulting on-the-wire format for the resulting DS record will be
   [XXX: IANA assignment should replace the 2 below]:



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                           1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Key Tag             |  Algorithm    | DigestType=2  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                                                               /
      /            Digest  (length for SHA-256 is 32 bytes)           /
      /                                                               /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|

2.3.  Example DS Record Using SHA-256

   The following is an example DNSKEY and matching DS record.  This
   DNSKEY record comes from the example DNSKEY/DS records found in
   section 5.4 of [RFC4034].

   The DNSKEY record:

   dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz
                                                fwJr1AYtsmx3TGkJaNXVbfi/
                                                2pHm822aJ5iI9BMzNXxeYCmZ
                                                DRD99WYwYqUSdjMmmAphXdvx
                                                egXd/M5+X7OrzKBaMbCVdFLU
                                                Uh6DhweJBjEVv5f2wwjM9Xzc
                                                nOf+EPbtG9DMBmADjFDc2w/r
                                                ljwvFw==
                                                ) ;  key id = 60485

   The resulting DS record covering the above DNSKEY record using a SHA-
   256 digest: [RFC Editor: please replace XXX with the assigned digest
   type (likely 2):]

   dskey.example.com. 86400 IN DS 60485 5 XXX ( D4B7D520E7BB5F0F67674A0C
                                                CEB1E3E0614B93C4F9E99B83
                                                83F6A1E4469DA50A )


3.  Implementation Requirements

   Implementations MUST support the use of the SHA-256 algorithm in DS
   RRs.  Validator implementations SHOULD ignore DS RRs containing SHA-1
   digests if DS RRs with SHA-256 digests are present in the DS RRset.


4.  Deployment Considerations

   If a validator does not support the SHA-256 digest type and no other
   DS RR exists in a zone's DS RRset with a supported digest type, then



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   the validator has no supported authentication path leading from the
   parent to the child.  The resolver should treat this case as it would
   the case of an authenticated NSEC RRset proving that no DS RRset
   exists, as described in [RFC4035], section 5.2.

   Because zone administrators can not control the deployment speed of
   support for SHA-256 in validators that may be referencing any of
   their zones, zone operators should consider deploying both SHA-1 and
   SHA-256 based DS records.  This should be done for every DNSKEY for
   which DS records are being generated.  Whether to make use of both
   digest types and for how long is a policy decision that extends
   beyond the scope of this document.


5.  IANA Considerations

   Only one IANA action is required by this document:

   The Digest Type to be used for supporting SHA-256 within DS records
   needs to be assigned by IANA.  This document requests that the Digest
   Type value of 2 be assigned to the SHA-256 digest algorithm.

   At the time of this writing, the current digest types assigned for
   use in DS records are as follows:

      VALUE     Digest Type          Status
        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: hardaker@tislabs.com










































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   Internet Society.




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