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freebsd-dev/contrib/bind9/doc/draft/draft-ietf-dnsext-ds-sha256-05.txt
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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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Internet-Draft Use of SHA-256 in DNSSEC DS RRs February 2006
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-Draft Use of SHA-256 in DNSSEC DS RRs February 2006
Intellectual Property Statement
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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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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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