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+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
+
+Status of this Memo
+
+ By submitting this Internet-Draft, each author represents that any
+ applicable patent or other IPR claims of which he or she is aware
+ have been or will be disclosed, and any of which he or she becomes
+ aware will be disclosed, in accordance with Section 6 of BCP 79.
+
+ Internet-Drafts are working documents of the Internet Engineering
+ Task Force (IETF), its areas, and its working groups. Note that
+ other groups may also distribute working documents as Internet-
+ Drafts.
+
+ Internet-Drafts are draft documents valid for a maximum of six months
+ and may be updated, replaced, or obsoleted by other documents at any
+ time. It is inappropriate to use Internet-Drafts as reference
+ material or to cite them other than as "work in progress."
+
+ The list of current Internet-Drafts can be accessed at
+ http://www.ietf.org/ietf/1id-abstracts.txt.
+
+ The list of Internet-Draft Shadow Directories can be accessed at
+ http://www.ietf.org/shadow.html.
+
+ This Internet-Draft will expire on August 25, 2006.
+
+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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+
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+Hardaker Expires August 25, 2006 [Page 1]
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+Internet-Draft Use of SHA-256 in DNSSEC DS RRs February 2006
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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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+Hardaker Expires August 25, 2006 [Page 2]
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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]:
+
+
+
+Hardaker Expires August 25, 2006 [Page 3]
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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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+Hardaker Expires August 25, 2006 [Page 4]
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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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+Hardaker Expires August 25, 2006 [Page 5]
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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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+Hardaker Expires August 25, 2006 [Page 6]
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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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+Hardaker Expires August 25, 2006 [Page 7]
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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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+Hardaker Expires August 25, 2006 [Page 8]
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+Intellectual Property Statement
+
+ The IETF takes no position regarding the validity or scope of any
+ Intellectual Property Rights or other rights that might be claimed to
+ pertain to the implementation or use of the technology described in
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+Disclaimer of Validity
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+ This document and the information contained herein are provided on an
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+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.
+
+
+Acknowledgment
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+Hardaker Expires August 25, 2006 [Page 9]
+
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