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DNS Extensions working group J. Jansen
Internet-Draft NLnet Labs
Expires: August 19, 2008 February 16, 2008
Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records
for DNSSEC
draft-ietf-dnsext-dnssec-rsasha256-03
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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on August 19, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
This document describes how to produce RSA/SHA-256 and RSA/SHA-512
DNSKEY and RRSIG resource records for use in the Domain Name System
Security Extensions (DNSSEC, RFC 4033, RFC 4034, and RFC 4035).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. DNSKEY Resource Records . . . . . . . . . . . . . . . . . . . . 3
2.1. RSA/SHA-256 DNSKEY Resource Records . . . . . . . . . . . . 3
2.2. RSA/SHA-512 DNSKEY Resource Records . . . . . . . . . . . . 3
3. RRSIG Resource Records . . . . . . . . . . . . . . . . . . . . 4
3.1. RSA/SHA-256 RRSIG Resource Records . . . . . . . . . . . . 4
3.2. RSA/SHA-512 RRSIG Resource Records . . . . . . . . . . . . 4
4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 5
4.1. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Signature Sizes . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation Considerations . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource
Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.2. Signature Type Downgrade Attacks . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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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
cryptographic keys and digital signatures for the verification of the
integrity of its data. RFC 4033 [1], RFC 4034 [2], and RFC 4035 [3]
describe these DNS Security Extensions, called DNSSEC.
RFC 4034 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 algorithms RSA/SHA-256 and RSA/
SHA-512, and specifies how to store DNSKEY data and how to produce
RRSIG resource records with these hash algorithms.
Familiarity with DNSSEC, RSA [7] and the SHA-2 [5] family of
algorithms is assumed in this document.
To refer to both SHA-256 and SHA-512, this document will use the name
SHA-2. This is done to improve readability. When a part of text is
specific for either SHA-256 or SHA-512, their specific names are
used. The same goes for RSA/SHA-256 and RSA/SHA-512, which will be
grouped using the name RSA/SHA-2.
2. DNSKEY Resource Records
The format of the DNSKEY RR can be found in RFC 4034 [2] and RFC 3110
[6].
2.1. 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 {TBA1}.
For use with NSEC3, the algorithm number of RSA/SHA-256 will be
{TBA2}.
The key size for RSA/SHA-256 keys MUST NOT be less than 512 bits, and
MUST NOT be more than 4096 bits.
2.2. RSA/SHA-512 DNSKEY Resource Records
RSA public keys for use with RSA/SHA-512 are stored in DNSKEY
resource records (RRs) with the algorithm number {TBA3}.
For use with NSEC3, the algorithm number of RSA/SHA-512 will be
{TBA4}.
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The key size for RSA/SHA-512 keys MUST NOT be less than 1024 bits,
and MUST NOT be more than 4096 bits.
3. RRSIG Resource Records
The value of the signature field in the RRSIG RR follow the RSASSA-
PKCS1-v1_5 signature scheme, and is calculated as follows. The
values for the RDATA fields that precede the signature data are
specified in RFC 4034 [2].
hash = SHA-XXX(data)
Where XXX is either 256 or 512, depending on the algorithm used.
signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)
Where SHA-XXX is the message digest algorithm as specified in FIPS
PUB 180-2 [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 RFC 4034 [2].
The "prefix" is intended to make the use of standard cryptographic
libraries easier. These specifications are taken directly from the
specification of EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2
[4]. The prefixes for the different algorithms are specified below.
3.1. RSA/SHA-256 RRSIG Resource Records
RSA/SHA-256 signatures are stored in the DNS using RRSIG resource
records (RRs) with algorithm number {TBA1} for use with NSEC, or
{TBA2} for use with NSEC3.
The prefix is the ASN.1 BER SHA-256 algorithm designator prefix as
specified in PKCS #1 v2.1 [4]:
hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20
3.2. RSA/SHA-512 RRSIG Resource Records
RSA/SHA-512 signatures are stored in the DNS using RRSIG resource
records (RRs) with algorithm number {TBA3} for use with NSEC, or
{TBA4} for use with NSEC3.
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The prefix is the ASN.1 BER SHA-512 algorithm designator prefix as
specified in PKCS #1 v2.1 [4]:
hex 30 51 30 0d 06 09 60 86 48 01 65 03 04 02 03 05 00 04 40
4. Deployment Considerations
4.1. Key Sizes
Apart from prohibiting RSA/SHA-512 signatures smaller than 1024
bytes, this document will not specify what size of keys to use. That
is more an operational issue and depends largely on the environment
and intended use. Some good starting points might be DNSSEC
Operational Practises [9], section 3.5, and NIST SP 800-57 Part 1
[10] and Part 3 [11].
4.2. Signature Sizes
In this family of signing algorithms, the size of signatures is
related to the size of the key, and not the hashing algorithm used in
the signing process. Therefore, RRSIG resource records produced with
RSA/SHA256 or RSA/SHA512 shall have the same size as those produced
with RSA/SHA1, if the keys have the same length.
5. Implementation Considerations
DNSSEC aware implementations SHOULD be able to support RRSIG resource
records with the RSA/SHA-2 algorithms.
If both RSA/SHA-2 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-2 signature does
not verify the data, and the RSA/SHA-1 signature does, the validator
SHOULD mark the data with the security status from the RSA/SHA-2
signature.
6. IANA Considerations
IANA has not yet assigned an algorithm number for RSA/SHA-256 and
RSA/SHA-512.
The algorithm list from RFC 4034 Appendix A.1 [2] is extended with
the following entries:
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Zone
Value Algorithm [Mnemonic] Signing References Status
----- ----------- ----------- ------- ----------- --------
{TBA1} RSA/SHA-256 RSASHA256 y {this memo} OPTIONAL
{TBA2} RSA/SHA-256-NSEC3 RSASHA256NSEC3 y {this memo} OPTIONAL
{TBA3} RSA/SHA-512 RSASHA512 y {this memo} OPTIONAL
{TBA4} RSA/SHA-512-NSEC3 RSASHA512NSEC3 y {this memo} OPTIONAL
7. Security Considerations
7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource Records
Users of DNSSEC are encouraged to deploy SHA-2 as soon as software
implementations allow for it. SHA-2 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-2 is the better choice for use in
DNSSEC records.
SHA-2 is considered sufficiently strong for the immediate future, but
predictions about future development in cryptography and
cryptanalysis are beyond the scope of this document.
The signature scheme RSASSA-PKCS1-v1_5 is chosen to match the one
used for RSA/SHA-1 signatures. This should ease implementation of
the new hashing algorithms in DNSSEC software.
7.2. 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/SHA-2 RRSIG, and force the validator
to use the RSA/SHA-1 signature if both are present in the zone.
Together with the implementation considerations from Section 5 of
this document, this provides resilience against algorithm downgrade
attacks, if the validator supports RSA/SHA-2.
8. Acknowledgments
This document is a minor extension to RFC 4034 [2]. Also, we try to
follow the documents RFC 3110 [6] and RFC 4509 [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
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Akkerhuis, Roy Arends, Rob Austein, Miek Gieben, Alfred Hoenes,
Michael St. Johns, Scott Rose and Wouter Wijngaards.
9. References
9.1. 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.
9.2. 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)", RFC 4509, May 2006.
[9] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
RFC 4641, September 2006.
[10] Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
"Recommendations for Key Management Part 1: General", NIST
SP 800-57 Part 1, March 2007.
[11] Barker, E., Barker, W., Burr, W., Jones, A., Polk, W., Smid,
M., and S. Rose, "Recommendations for Key Management Part 3:
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Application-Specific Key Guidance", NIST SP 800-57 Part 3,
March 2007.
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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