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+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
+
+
+
+
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+
+
+Hardaker Standards Track [Page 1]
+\f
+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.
+
+
+
+
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+Hardaker Standards Track [Page 2]
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+RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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+
+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]
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+RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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+
+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.
+
+
+
+
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+Hardaker Standards Track [Page 4]
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+RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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+ 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]
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+RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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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)", Work in
+ Progress.
+
+Author's Address
+
+ Wes Hardaker
+ Sparta
+ P.O. Box 382
+ Davis, CA 95617
+ USA
+
+ EMail: hardaker@tislabs.com
+
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+Hardaker Standards Track [Page 6]
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+RFC 4509 Use of SHA-256 in DNSSEC DS RRs May 2006
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+
+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.
+
+Intellectual Property
+
+ The IETF takes no position regarding the validity or scope of any
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+ pertain to the implementation or use of the technology described in
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+
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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]
+\f
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