INTERNET-DRAFT RSA/MD5 KEYs and SIGs in the DNS January 1998 Expires July 1998 RSA/MD5 KEYs and SIGs in the Domain Name System (DNS) ------- ---- --- ---- -- --- ------ ---- ------ ----- Donald E. Eastlake 3rd Status of This Document This draft, file name draft-ietf-dnssec-rsa-00.txt, is intended to be become a Proposed Standard RFC. Distribution of this document is unlimited. Comments should be sent to the DNS security mailing list <dns-security@tis.com> or to the author. This document is an Internet-Draft. 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. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net (East USA), ftp.isi.edu (West USA), nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe), munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa). Abstract A standard method for storing RSA keys and and RSA/MD5 based signatures in the Domain Name System is described which utilizes DNS KEY and SIG resource records. Donald E. Eastlake 3rd [Page 1]

INTERNET-DRAFT RSA/MD5 in the DNS Table of Contents Status of This Document....................................1 Abstract...................................................1 Table of Contents..........................................2 1. Introduction............................................3 2. RSA Public KEY Resource Records.........................3 3. RSA/MD5 SIG Resource Records............................4 4. Performance Considerations..............................5 5. Security Considerations.................................5 References.................................................6 Author's Address...........................................6 Expiration and File Name...................................6 Donald E. Eastlake 3rd [Page 2]

INTERNET-DRAFT RSA/MD5 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. The DNS has been extended to include digital signatures and cryptographic keys as described in [draft-ietf- dnssec-secext2-*]. Thus the DNS can now be secured and used for secure key distribution. This document describes how to store RSA keys and and RSA/MD5 based signatures in the DNS. Familiarity with the RSA algorithm is assumed [Schneier]. Implementation of the RSA algorithm in DNS is recommended. 2. RSA Public KEY Resource Records RSA public keys are stored in the DNS as KEY RRs using algorithm number 1 [draft-ietf-dnssec-secext2-*]. The structure of the algorithm specific portion of the RDATA part of such RRs is as shown below. 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 currently 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. Donald E. Eastlake 3rd [Page 3]

INTERNET-DRAFT RSA/MD5 in the DNS 3. RSA/MD5 SIG Resource Records The signature portion of the SIG RR RDATA area, when using the RSA/MD5 algorithm, is calculated as shown below. The data signed is determined as specified in [draft-ietf-dnssec-secext2-*]. See [draft-ietf-dnssec-secext2-*] for fields in the SIG RR RDATA which precede the signature itself. hash = MD5 ( data ) signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n) where MD5 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 MD5 algorithm designator prefix specified in PKCS1, that is, hex 3020300c06082a864886f70d020505000410 [NETSEC]. This prefix is included to make it easier to use RSAREF (or similar packages such as EuroRef). 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 [PKCS1].) 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. Leading zero bytes are permitted in the RSA/MD5 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. Donald E. Eastlake 3rd [Page 4]

INTERNET-DRAFT RSA/MD5 in the DNS 4. Performance Considerations General signature generation speeds are roughly the same for RSA and DSA [RFC xDSA]. 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 overhead. While larger transfers will perform correctly and work is underway to make larger transfers more efficient, it 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. Security Considerations Many of the general security consideration in [draft-ietf-dnssec- secext2-*] 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 interoperability, the RSA key size is limited to 4096 bits. For particularly critical applications, implementors are encouraged to consider the range of available algorithms and key sizes. Donald E. Eastlake 3rd [Page 5]

INTERNET-DRAFT RSA/MD5 in the DNS References [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. [PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc., 3 June 1991, Version 1.4. [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. [draft-ietf-dnssec-secext2-*] - Domain Name System Security Extensions, D. Eastlake, C. Kaufman, January 1997. [RFC xDSA] - draft-ietf-dnssec-dss-*.txt [Schneier] - Bruce Schneier, "Applied Cryptography Second Edition: protocols, algorithms, and source code in C", 1996, John Wiley and Sons, ISBN 0-471-11709-9. Author's Address Donald E. Eastlake 3rd CyberCash, Inc. 318 Acton Street Carlisle, MA 01741 USA Telephone: +1 978 287 4877 +1 703 620-4200 (main office, Reston, Virginia) FAX: +1 978 371 7148 EMail: dee@cybercash.com Expiration and File Name This draft expires in July 1998. Its file name is draft-ietf-dnssec-rsa-00.txt. Donald E. Eastlake 3rd [Page 6]