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-rsasha256-00draft-ietf-dnsext-dnssec-rsasha256-01 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 July 5, 2006. Copyright Notice 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). 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 . . . . . . . . . . . . . . . . . . . . . . . . . . 5 220.127.116.11 Normative References . . . . . . . . . . . . . . . . . . . 5 18.104.22.168 Informative References . . . . . . . . . . . . . . . . . . 56 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .6 Intellectual Property and Copyright Statements . . . . . . . . . .7 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 , RFC4034 , and RFC4035  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  and SHA-256  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  and RFC3110 . 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 . 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 , | 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 . The prefix is the ASN.1 BER SHA-256 algorithm designator prefix as specified in PKCS 2.1 : 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. 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  is extended with the following entry: Zone Value Algorithm [Mnemonic] Signing References Status ----- ----------- ----------- -------- ---------- --------- [tba] RSA/SHA-256 [RSASHA256] y [TBA] MANDATORY 6. Security Considerations Recently, weaknesses have been discovered in the6.1 SHA-1 hashing algorithm. It is therefore stronglyversus SHA-256 Considerations for RRSIG resource records Users of DNSSEC are encouraged to deploy SHA-256 where SHA-1 is used now,as soon as the DNSsoftware supportsimplementations 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. 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  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 . Also, we try to follow the documents RFC3110  and draft-ietf-dnsext-ds-sha256.txt  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 22.214.171.124 Normative References  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005.  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005.  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005.  Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.  National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-2, August 2002.  Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)", RFC 3110, May 2001. 126.96.36.199 Informative References  Schneier, B., "Applied Cryptography Second Edition: protocols, algorithms, and source code in C", Wiley and Sons , ISBN 0-471- 11709-9, 1996.  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/ 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 this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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