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rfc:rfc4509

Network Working Group W. Hardaker Request for Comments: 4509 Sparta Category: Standards Track May 2006

Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

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 DNSKEYs in a child zone.

Table of Contents

 1. Introduction ....................................................2
 2. Implementing the SHA-256 Algorithm for DS Record Support ........2
    2.1. DS Record Field Values .....................................2
    2.2. DS Record with SHA-256 Wire Format .........................3
    2.3. Example DS Record Using SHA-256 ............................3
 3. Implementation Requirements .....................................3
 4. Deployment Considerations .......................................4
 5. IANA Considerations .............................................4
 6. Security Considerations .........................................4
    6.1. Potential Digest Type Downgrade Attacks ....................4
    6.2. SHA-1 vs SHA-256 Considerations for DS Records .............5
 7. Acknowledgements ................................................5
 8. References ......................................................6
    8.1. Normative References .......................................6
    8.2. Informative References .....................................6

Hardaker Standards Track [Page 1] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

1. Introduction

 The DNSSEC [RFC4033] [RFC4034] [RFC4035] DS RR is published in parent
 zones to distribute a cryptographic digest of one key in a child's
 DNSKEY RRset.  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, with a type covered of DS.
 In this document, the key words "MUST", "MUST NOT", "REQUIRED",
 "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
 and "OPTIONAL" 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 2 has been 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: 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 be 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.

Hardaker Standards Track [Page 2] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

2.2. DS Record with SHA-256 Wire Format

 The resulting on-the-wire format for the resulting DS record will be
 as follows:
                        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:
 dskey.example.com. 86400 IN DS 60485 5 2   ( 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.

Hardaker Standards Track [Page 3] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

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
 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 cannot 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
 has been assigned by IANA.
 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 uses a different digest type.
 o  A validator accepts a weaker digest even if a stronger one is
    present but invalid.

Hardaker Standards Track [Page 4] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

 For example, if the following conditions are all true:
 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
 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. Acknowledgements

 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.

Hardaker Standards Track [Page 5] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

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)", Work in
              Progress.

Author's Address

 Wes Hardaker
 Sparta
 P.O. Box 382
 Davis, CA  95617
 USA
 EMail: hardaker@tislabs.com

Hardaker Standards Track [Page 6] RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006

Full Copyright Statement

 Copyright (C) The Internet Society (2006).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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Acknowledgement

 Funding for the RFC Editor function is provided by the IETF
 Administrative Support Activity (IASA).

Hardaker Standards Track [Page 7]

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