DNS Extensions working group J. Jansen
Internet-Draft NLnet Labs
Expires: July 5, 2006 January 2006
Use of RSA/SHA-256 DNSKEY and RRSIG Resource Records in DNSSEC
draft-ietf-dnsext-dnssec-rsasha256-01
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Copyright (C) The Internet Society (2006).
Abstract
This document describes how to produce RSA/SHA-256 DNSKEY and RRSIG
resource records for use in the Domain Name System Security
Extensions (DNSSEC, RFC4033, RFC4034, and RFC4035).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . . 3
3. RSA/SHA-256 RRSIG Resource Records . . . . . . . . . . . . . . 3
4. Implementation Considerations . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6.1 SHA-1 versus SHA-256 Considerations for RRSIG resource
records . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.2 Signature Type Downgrade Attacks . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1 Normative References . . . . . . . . . . . . . . . . . . . 5
8.2 Informative References . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . 7
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1. Introduction
The Domain Name System (DNS) is the global hierarchical distributed
database for Internet Addressing. The DNS has been extended to use
digital signatures and cryptographic keys for the verification of
data. RFC4033 [1], RFC4034 [2], and RFC4035 [3] describe these DNS
Security Extensions.
RFC4034 describes how to store DNSKEY and RRSIG resource records, and
specifies a list of cryptographic algorithms to use. This document
extends that list with the algorithm RSA/SHA-256, and specifies how
to store RSA/SHA-256 DNSKEY data and how to produce RSA/SHA-256 RRSIG
resource records.
Familiarity with the RSA [7] and SHA-256 [5] algorithms is assumed in
this document.
2. RSA/SHA-256 DNSKEY Resource Records
RSA public keys for use with RSA/SHA-256 are stored in DNSKEY
resource records (RRs) with the algorithm number [TBA].
The format of the DNSKEY RR can be found in RFC4034 [2] and RFC3110
[6].
3. RSA/SHA-256 RRSIG Resource Records
RSA/SHA-256 signatures are stored in the DNS using RRSIG resource
records (RRs) with algorithm number [TBA].
The value of the signature field in the RRSIG RR is calculated as
follows. The values for the fields that precede the signature data
are specified in RFC4034 [2].
hash = SHA-256(data)
signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)
Where SHA-256 is the message digest algorithm as specified in FIPS
180 [5], | is concatenation, 00, 01, FF and 00 are fixed octets of
corresponding hexadecimal value, "e" is the private exponent of the
signing RSA key, and "n" is the public modulus of the signing key.
The FF octet MUST be repeated the maximum number of times so that the
total length of the signature equals the length of the modulus of the
signer's public key ("n"). "data" is the data of the resource record
set that is signed, as specified in RFC4034 [2].
The prefix is the ASN.1 BER SHA-256 algorithm designator prefix as
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specified in PKCS 2.1 [4]:
hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
This prefix should make the use of standard cryptographic libraries
easier. These specifications are taken directly from PKCS #1 v2.1
section 9.2 [4].
4. Implementation Considerations
DNSSEC aware implementations MUST be able to support RRSIG resource
records with the RSA/SHA-256 algorithm.
If both RSA/SHA-256 and RSA/SHA-1 RRSIG resource records are
available for a certain rrset, with a secure path to their keys, the
validator SHOULD ignore the SHA-1 signature. If the RSA/SHA-256
signature does not verify the data, and the RSA/SHA-1 does, the
validator SHOULD mark the data with the security status from the RSA/
SHA-256 signature.
5. IANA Considerations
IANA has not yet assigned an algorithm number for RSA/SHA-256.
The algorithm list from RFC4034 Appendix A.1 [2] is extended with the
following entry:
Zone
Value Algorithm [Mnemonic] Signing References Status
----- ----------- ----------- -------- ---------- ---------
[tba] RSA/SHA-256 [RSASHA256] y [TBA] MANDATORY
6. Security Considerations
6.1 SHA-1 versus SHA-256 Considerations for RRSIG resource 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.
SHA-256 is considered sufficiently strong for the immediate future,
but predictions about future development in cryptography and
cryptanalysis are beyond the scope of this document.
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6.2 Signature Type Downgrade Attacks
Since each RRset MUST be signed with each algorithm present in the
DNSKEY RRset at the zone apex (see [3] Section 2.2), a malicious
party cannot filter out the RSA/SHA256 RRSIG, and force the validator
to use the RSA/SHA1 signature if both are present in the zone.
Together with the implementation considerations from Section 4 of
this document, this provides resilience against algorithm downgrade
attacks, if the validator supports RSA/SHA256.
7. Acknowledgments
This document is a minor extension to RFC4034 [2]. Also, we try to
follow the documents RFC3110 [6] and draft-ietf-dnsext-ds-sha256.txt
[8] for consistency. The authors of and contributors to these
documents are gratefully acknowledged for their hard work.
The following people provided additional feedback and text: Jaap
Akkerhuis, Rob Austein, Miek Gieben and Wouter Wijngaards.
8. References
8.1 Normative References
[1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[4] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards
(PKCS) #1: RSA Cryptography Specifications Version 2.1",
RFC 3447, February 2003.
[5] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-2, August 2002.
[6] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
System (DNS)", RFC 3110, May 2001.
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8.2 Informative References
[7] Schneier, B., "Applied Cryptography Second Edition: protocols,
algorithms, and source code in C", Wiley and Sons , ISBN 0-471-
11709-9, 1996.
[8] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS)
Resource Records (RRs)", Work in Progress Feb 2006.
Author's Address
Jelte Jansen
NLnet Labs
Kruislaan 419
Amsterdam 1098VA
NL
Email: jelte@NLnetLabs.nl
URI: http://www.nlnetlabs.nl/
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