DNS Extensions working group J. Jansen Internet-Draft NLnet Labs Intended status: Standards Track February 27, 2009 Expires: August 31, 2009 Use of SHA-2 algorithms with RSA in DNSKEY and RRSIG Resource Records for DNSSEC draft-ietf-dnsext-dnssec-rsasha256-11 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 31, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. 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 Jansen Expires August 31, 2009 [Page 1]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 Security Extensions (DNSSEC, RFC 4033, RFC 4034, and RFC 4035). 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 . . . . . . . . . . . . 4 3. RRSIG Resource Records . . . . . . . . . . . . . . . . . . . . 4 3.1. RSA/SHA-256 RRSIG Resource Records . . . . . . . . . . . . 4 3.2. RSA/SHA-512 RRSIG Resource Records . . . . . . . . . . . . 5 4. Deployment Considerations . . . . . . . . . . . . . . . . . . . 5 4.1. Key Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.2. Signature Sizes . . . . . . . . . . . . . . . . . . . . . . 5 5. Implementation Considerations . . . . . . . . . . . . . . . . . 5 5.1. Support for SHA-2 signatures . . . . . . . . . . . . . . . 5 5.2. Support for NSEC3 Denial of Existence . . . . . . . . . . . 5 5.2.1. NSEC3 in Authoritative servers . . . . . . . . . . . . 6 5.2.2. NSEC3 in Validators . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 7.1. SHA-1 versus SHA-2 Considerations for RRSIG Resource Records . . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.2. Signature Type Downgrade Attacks . . . . . . . . . . . . . 7 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Jansen Expires August 31, 2009 [Page 2]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 1. Introduction The Domain Name System (DNS) is the global hierarchical distributed database for Internet Naming. The DNS has been extended to use cryptographic keys and digital signatures for the verification of the authenticity and integrity of its data. RFC 4033 [RFC4033], RFC 4034 [RFC4034], and RFC 4035 [RFC4035] 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 and the SHA-2 [FIPS.180-3.2008] 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. The term "SHA-2" is not officially defined, but is usually used to refer to the collection of the algorithms SHA-224, SHA-256, SHA-384 and SHA-512. Since SHA-224 and SHA-384 are not used in DNSSEC, SHA-2 will only refer to SHA-256 and SHA-512 in this document. 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. DNSKEY Resource Records The format of the DNSKEY RR can be found in RFC 4034 [RFC4034]. RFC 3110 [RFC3110] describes the use of RSA/SHA-1 for DNSSEC signatures. 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 interoperability, as in RFC 3110 [RFC3110], the key size of RSA/ SHA-256 keys MUST NOT be less than 512 bits, and MUST NOT be more than 4096 bits. Jansen Expires August 31, 2009 [Page 3]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 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 {TBA2}. The key size of 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 follows 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 [RFC4034]. hash = SHA-XXX(data) Here XXX is either 256 or 512, depending on the algorithm used, as specified in FIPS PUB 180-3 [FIPS.180-3.2008], and "data" is the wire format data of the resource record set that is signed, as specified in RFC 4034 [RFC4034]. signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n) Here "|" 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 exact number of times so that the total length of the concatenated term in parentheses equals the length of the modulus of the signer's public key ("n"). The "prefix" is intended to make the use of standard cryptographic libraries easier. These specifications are taken directly from the specifications of RSASSA-PKCS1-v1_5 in PKCS #1 v2.1 section 8.2 [RFC3447], and EMSA-PKCS1-v1_5 encoding in PKCS #1 v2.1 section 9.2 [RFC3447]. 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}. The prefix is the ASN.1 DER SHA-256 algorithm designator prefix as specified in PKCS #1 v2.1 [RFC3447]: hex 30 31 30 0d 06 09 60 86 48 01 65 03 04 02 01 05 00 04 20 Jansen Expires August 31, 2009 [Page 4]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 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 {TBA2}. The prefix is the ASN.1 DER SHA-512 algorithm designator prefix as specified in PKCS #1 v2.1 [RFC3447]: 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 the restrictions in section 2, this document will not specify what size of keys to use. That is an operational issue and depends largely on the environment and intended use. A good starting point for more information would be NIST SP 800-57 [NIST800-57]. 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 will have the same size as those produced with RSA/SHA1, if the keys have the same length. 5. Implementation Considerations 5.1. Support for SHA-2 signatures DNSSEC aware implementations SHOULD be able to support RRSIG and DNSKEY resource records created with the RSA/SHA-2 algorithms as defined in this document. 5.2. Support for NSEC3 Denial of Existence RFC5155 [RFC5155] defines new algorithm identifiers for existing signing algorithms, to indicate that zones signed with these algorithm identifiers use NSEC3 instead of NSEC records to provide denial of existence. That mechanism was chosen to protect implementations predating RFC5155 from encountering resource records they could not know about. This document does not define such algorithm aliases, and support for NSEC3 denial of existence is implicitly signaled with support for one of the algorithms defined in this document. Jansen Expires August 31, 2009 [Page 5]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 5.2.1. NSEC3 in Authoritative servers An authoritative server that does not implement NSEC3 MAY still serve zones that use RSA/SHA2 with NSEC denial of existence. 5.2.2. NSEC3 in Validators A DNSSEC validator that implements RSA/SHA2 MUST be able to handle both NSEC and NSEC3 [RFC5155] negative answers. If this is not the case, the validator MUST treat a zone signed with RSA/SHA256 or RSA/ SHA512 as signed with an unknown algorithm, and thus as insecure. 6. IANA Considerations This document updates the IANA registry "DNS SECURITY ALGORITHM NUMBERS -- per [RFC4035] " (http://www.iana.org/assignments/dns-sec-alg-numbers). The following entries are added to the registry: Zone Value Algorithm Mnemonic Signing References {TBA1} RSA/SHA-256 RSASHA256 y {this memo} {TBA2} RSA/SHA-512 RSASHA512 y {this memo} 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. Jansen Expires August 31, 2009 [Page 6]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 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 [RFC4035] 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. This should provide resilience against algorithm downgrade attacks, if the validator supports RSA/SHA-2. 8. Acknowledgments This document is a minor extension to RFC 4034 [RFC4034]. Also, we try to follow the documents RFC 3110 [RFC3110] and RFC 4509 [RFC4509] 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, Mark Andrews, Roy Arends, Rob Austein, Francis Dupont, Miek Gieben, Alfred Hoenes, Paul Hoffman, Peter Koch, Michael St. Johns, Scott Rose and Wouter Wijngaards. 9. References 9.1. Normative References [FIPS.180-3.2008] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-3, October 2008. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)", RFC 3110, May 2001. [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. Jansen Expires August 31, 2009 [Page 7]
Internet-Draft DNSSEC RSA/SHA-2 February 2009 9.2. Informative References [NIST800-57] Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid, "Recommendations for Key Management", NIST SP 800-57, March 2007. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC4509] Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer (DS) Resource Records (RRs)", RFC 4509, May 2006. [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, March 2008. Author's Address Jelte Jansen NLnet Labs Kruislaan 419 Amsterdam 1098VA NL Email: jelte@NLnetLabs.nl URI: http://www.nlnetlabs.nl/ Jansen Expires August 31, 2009 [Page 8]
