INTERNET-DRAFT RSA SIGs and KEYs in the DNS OBSOLETES RFC 2537 September 2000 Expires March 2001 RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS) --------- ---- --- --- ---- -- --- ------ ---- ------ ----- <draft-ietf-dnsext-rsa-01.txt> D. Eastlake 3rd Status of This Document This draft is intended to be become a Proposed Standard RFC. Distribution of this document is unlimited. Comments should be sent to the DNS extensions mailing list <namedroppers@ops.ietf.org> or to the author. This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. Abstract Since the adoption of a Proposed Standard for RSA signatures in the DNS [RFC 2537], advances in hashing have been made. A new DNS signature algorithm is defined to make these advances available in SIG resource records (RRs). The use of the previously specified weaker mechanism is deprecated. The algorithm number of the RSA KEY RR is changed to correspond to this new SIG algorithm. No other changes are made to DNS security. D. Eastlake 3rd [Page 1]

INTERNET-DRAFT RSA/SHA1 in the DNS Acknowledgements Material and comments from the following have been incorporated and are gratefully acknowledged: Olafur Gudmundsson Charlie Kaufman Table of Contents Status of This Document....................................1 Abstract...................................................1 Acknowledgements...........................................2 Table of Contents..........................................2 1. Introduction............................................3 2. RSA Public KEY Resource Records.........................3 3. RSA/SHA1 SIG Resource Records...........................4 4. Performance Considerations..............................5 5. IANA Considerations.....................................6 6. Security Considerations.................................6 References.................................................7 Author's Address...........................................8 Changes from last draft....................................8 Expiration and File Name...................................8 D. Eastlake 3rd [Page 2]

INTERNET-DRAFT RSA/SHA1 in the DNS 1. Introduction The Domain Name System (DNS) is the global hierarchical replicated distributed database system for Internet addressing, mail proxy, and other information [RFC 1034, 1035, etc.]. The DNS has been extended to include digital signatures and cryptographic keys as described in [RFC 2535]. Thus the DNS can now be secured and used for secure key distribution. Familiarity with the RSA and SHA-1 algorithms is assumed [Schneier, FIP180] in this document. [RFC 2537] described how to store RSA keys and RSA/MD5 based signatures in the DNS. However, since the adoption of [RFC 2537], continued cryptographic research has revealed hints of weakness in the MD5 [RFC 1321] algorithm used in [RFC 2537]. The SHA1 Secure Hash Algorithm [FIP180], which produces a larger hash, has been developed. By now there has been sufficient experience with SHA1 that it is generally acknowledged to be stronger than MD5. While this stronger hash is probably not needed today in most secure DNS zones, critical zones such a root and most TLDs are sufficiently valuable targets that it would be negligent not to provide what are generally agreed to be stronger mechanisms. Furthermore, future advances in cryptanalysis and/or computer speeds may require a stronger hash everywhere. In addition, the additional computation required by SHA1 above that required by MD5 is insignificant compared with the computational effort required by the RSA modular exponentiation. This document describes how to produce RSA/SHA1 SIG RRs in Section 3 and, so as to completely replace [RFC 2537], describes how to produce RSA KEY RRs in Section 2. Implementation of the RSA algorithm in DNS with SHA1 is MANDATORY for DNSSEC. The generation of RSA/MD5 SIG RRs as described in [RFC 2537] is NOT RECOMMENDED. The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT RECOMMENDED", and "MAY" in this document are to be interpreted as described in [RFC 2119]. 2. RSA Public KEY Resource Records RSA public keys are stored in the DNS as KEY RRs using algorithm number (TBD, suggest 5) [RFC 2535]. The structure of the algorithm specific portion of the RDATA part of such RRs is as shown below. D. Eastlake 3rd [Page 3]

INTERNET-DRAFT RSA/SHA1 in the DNS Field Size ----- ---- exponent length 1 or 3 octets (see text) exponent as specified by length field modulus remaining space For interoperability, the exponent and modulus are each limited to 4096 bits in length. The public key exponent is a variable length unsigned integer. Its length in octets is represented as one octet if it is in the range of 1 to 255 and by a zero octet followed by a two octet unsigned length if it is longer than 255 bytes. The public key modulus field is a multiprecision unsigned integer. The length of the modulus can be determined from the RDLENGTH and the preceding RDATA fields including the exponent. Leading zero octets are prohibited in the exponent and modulus. Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this algorithm number (rather than the algorithm number specified in the obsoleted [RFC 2537]). [Note: This changes the algorithm number for RSA KEY RRs to be the same as the new algorithm number for RSA/SHA1 SIGs. Or, from another point of view, adds a new KEY RR with a new algorithm number that is otherwise identical to the RSA KEY RR defined in RFC 2537. In fact, RSA KEYs do not depend on selection of hash algorithm. An alternative would be to leave RSA KEY RRs unchanged and indicated by algorithm 1.] 3. RSA/SHA1 SIG Resource Records RSA/SHA1 signatures are stored in the DNS using SIG resource records (RRs) with algorithm number (TBD, 5 suggested). The signature portion of the SIG RR RDATA area, when using the RSA/SHA1 algorithm, is calculated as shown below. The data signed is determined as specified in [RFC 2535]. See [RFC 2535] for fields in the SIG RR RDATA which precede the signature itself. hash = SHA1 ( data ) signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n) D. Eastlake 3rd [Page 4]

INTERNET-DRAFT RSA/SHA1 in the DNS where SHA1 is the message digest algorithm documented in [RFC 1321], "|" is concatenation, "e" is the private key exponent of the signer, and "n" is the modulus of the signer's public key. 01, FF, and 00 are fixed octets of the corresponding hexadecimal value. "prefix" is the ASN.1 BER SHA1 algorithm designator prefix required in PKCS1 [RFC 2437], that is, hex 30 21 30 1F 06 05 2B 0E 03 02 1A 05 00 04 14 This prefix is included to make it easier to use standard cryptographic libraries. The FF octet MUST be repeated the maximum number of times such that the value of the quantity being exponentiated is one octet shorter than the value of n. (The above specifications are identical to the corresponding part of Public Key Cryptographic Standard #1 [RFC 2437]. The value for prefix was provided by Charlie Kaufman.) The size of "n", including most and least significant bits (which will be 1) MUST be not less than 512 bits and not more than 4096 bits. "n" and "e" SHOULD be chosen such that the public exponent is small. These are protocol limits. For a discussion of key size see [RFC 2541]. Leading zero bytes are permitted in the RSA/SHA1 algorithm signature. A public exponent of 3 minimizes the effort needed to verify a signature. Use of 3 as the public exponent is weak for confidentiality uses since, if the same data can be collected encrypted under three different keys with an exponent of 3 then, using the Chinese Remainder Theorem [NETSEC], the original plain text can be easily recovered. This weakness is not significant for DNS security because we seek only authentication, not confidentiality. 4. Performance Considerations General signature generation speeds are roughly the same for RSA and DSA [RFC 2536]. With sufficient pre-computation, signature generation with DSA is faster than RSA. Key generation is also faster for DSA. However, signature verification is an order of magnitude slower with DSA when the RSA public exponent is chosen to be small as is recommended for KEY RRs used in domain name system (DNS) data authentication. Current DNS implementations are optimized for small transfers, typically less than 512 bytes including DNS overhead. Larger transfers will perform correctly and extensions have been standardized [RFC 2671] to make larger transfers more efficient, it D. Eastlake 3rd [Page 5]

INTERNET-DRAFT RSA/SHA1 in the DNS is still advisable at this time to make reasonable efforts to minimize the size of KEY RR sets stored within the DNS consistent with adequate security. Keep in mind that in a secure zone, at least one authenticating SIG RR will also be returned. 5. IANA Considerations The DNSSEC algorithm number (TBD, 5 suggested) is allocated for RSA/SHA1 SIG RRs and RSA KEY RRs. 6. Security Considerations Many of the general security consideration in [RFC 2535] apply. Keys retrieved from the DNS should not be trusted unless (1) they have been securely obtained from a secure resolver or independently verified by the user and (2) this secure resolver and secure obtainment or independent verification conform to security policies acceptable to the user. As with all cryptographic algorithms, evaluating the necessary strength of the key is essential and dependent on local policy. For particularly critical applications, implementers are encouraged to consider the range of available algorithms and key sizes. See also [RFC 2541], "DNS Security Operational Considerations". D. Eastlake 3rd [Page 6]

INTERNET-DRAFT RSA/SHA1 in the DNS References [FIP180] -U.S. Department of Commerce, "Secure Hash Standard", FIPS PUB 180-1, 17 Apr 1995. [NETSEC] - Network Security: PRIVATE Communications in a PUBLIC World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall Series in Computer Networking and Distributed Communications, 1995. [RFC 1034] - P. Mockapetris, "Domain names - concepts and facilities", 11/01/1987. [RFC 1035] - P. Mockapetris, "Domain names - implementation and specification", 11/01/1987. [RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April 1992. [RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", March 1997. [RFC 2437] - B. Kaliski, J. Staddon, "PKCS #1: RSA Cryptography Specifications Version 2.0", October 1998. [RFC 2535] - D. Eastlake, "Domain Name System Security Extensions", March 1999. [RFC 2536] - D. Eastlake, "DSA KEYs and SIGs in the Domain Name System (DNS)", March 1999. [RFC 2537] - D. Eastlake, "RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)", March 1999. [RDC 2541] - D. Eastlake, "DNS Security Operational Considerations", March 1999. [RFC 2671] - P. Vixie, "Extension Mechanisms for DNS (EDNS0)", August 1999. [Schneier] - Bruce Schneier, "Applied Cryptography Second Edition: protocols, algorithms, and source code in C", 1996, John Wiley and Sons, ISBN 0-471-11709-9. D. Eastlake 3rd [Page 7]

INTERNET-DRAFT RSA/SHA1 in the DNS Author's Address Donald E. Eastlake 3rd Motorola 140 Forest Avenue Hudson, MA 01749 USA Telephone: +1-978-562-2827 (h) +1-508-261-5434 (w) FAX: +1-508-261-4777 (w) EMail: Donald.Eastlake@motorola.com Changes from last draft Add acknowledgements and hex value for signature "prefix". Expiration and File Name This draft expires in March 2001. Its file name is draft-ietf-dnsext-rsa-01.txt. D. Eastlake 3rd [Page 8]