INTERNET-DRAFT DSA KEYs and SIGs in the DNS OBSOLETES: RFC 2536 Donald Eastlake 3rd Motorola Expires: May 2002 November 2001 DSA KEYs and SIGs in the Domain Name System (DNS) --- ---- --- ---- -- --- ------ ---- ------ ----- <draft-ietf-dnsext-rfc2536bis-dsa-01.txt> Donald E. Eastlake 3rd Status of This Document This draft is intended to be become a Draft Standard RFC. Distribution of this document is unlimited. Comments should be sent to the DNS extensions working group 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 A standard method for storing US Government Digital Signature Algorithm keys and signatures in the Domain Name System is described which utilizes DNS KEY and SIG resource records. D. Eastlake 3rd [Page 1]
INTERNET-DRAFT DSA in the DNS Table of Contents Status of This Document....................................1 Abstract...................................................1 Table of Contents..........................................2 1. Introduction............................................3 2. DSA KEY Resource Records................................3 3. DSA SIG Resource Records................................4 4. Performance Considerations..............................4 5. Security Considerations.................................5 6. IANA Considerations.....................................5 References.................................................6 Author's Address...........................................6 Expiration and File Name...................................7 D. Eastlake 3rd [Page 2]
INTERNET-DRAFT DSA 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 [RFC 2535]. Thus the DNS can now be secured and can be used for key distribution. This document describes how to store US Government Digital Signature Algorithm (DSA) keys and signatures in the DNS. Familiarity with the US Digital Signature Algorithm is assumed [FIPS 186-2, Schneier]. 2. DSA KEY Resource Records DSA public keys are stored in the DNS as KEY RRs using algorithm number 3 [RFC 2535]. The structure of the algorithm specific portion of the RDATA part of this RR is as shown below. These fields, from Q through Y are the "public key" part of the DSA KEY RR. The period of key validity is not in the KEY RR but is indicated by the SIG RR(s) which signs and authenticates the KEY RR(s) at that domain name. Field Size ----- ---- T 1 octet Q 20 octets P 64 + T*8 octets G 64 + T*8 octets Y 64 + T*8 octets As described in [FIPS 186-2] and [Schneier], T is a key size parameter chosen such that 0 <= T <= 8. (The meaning for algorithm 3 if the T octet is greater than 8 is reserved and the remainder of the RDATA portion may have a different format in that case.) Q is a prime number selected at key generation time such that 2**159 < Q < 2**160 so Q is always 20 octets long and, as with all other fields, is stored in "big-endian" network order. P, G, and Y are calculated as directed by the [FIPS 186-2] key generation algorithm [Schneier]. P is in the range 2**(511+64T) < P < 2**(512+64T) and thus is 64 + 8*T octets long. G and Y are quantities modulo P and so can be up to the same length as P and are allocated fixed size fields with the same number of octets as P. During the key generation process, a random number X must be generated such that 1 <= X <= Q-1. X is the private key and is used in the final step of public key generation where Y is computed as D. Eastlake 3rd [Page 3]
INTERNET-DRAFT DSA in the DNS Y = G**X mod P 3. DSA SIG Resource Records The signature portion of the SIG RR RDATA area, when using the US Digital Signature Algorithm, is shown below with fields in the order they occur. See [RFC 2535] for fields in the SIG RR RDATA which precede the signature itself. Field Size ----- ---- T 1 octet R 20 octets S 20 octets The data signed is determined as specified in [RFC 2535]. Then the following steps are taken, as specified in [FIPS 186-2], where Q, P, G, and Y are as specified in the public key [Schneier]: hash = SHA-1 ( data ) Generate a random K such that 0 < K < Q. R = ( G**K mod P ) mod Q S = ( K**(-1) * (hash + X*R) ) mod Q For infromation on the SHA-1 has funcation see [FIPS 180-1] and [RFC 3174]. Since Q is 160 bits long, R and S can not be larger than 20 octets, which is the space allocated. T is copied from the public key. It is not logically necessary in the SIG but is present so that values of T > 8 can more conveniently be used as an escape for extended versions of DSA or other algorithms as later specified. 4. Performance Considerations General signature generation speeds are roughly the same for RSA [RFC 3110] and DSA. 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 than RSA when the RSA public exponent is chosen to be small as is recommended for KEY RRs used in domain name system (DNS) data D. Eastlake 3rd [Page 4]
INTERNET-DRAFT DSA in the DNS 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 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 [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. The key size limitation of a maximum of 1024 bits ( T = 8 ) in the current DSA standard may limit the security of DSA. For particular applications, implementors are encouraged to consider the range of available algorithms and key sizes. DSA assumes the ability to frequently generate high quality random numbers. See [RFC 1750] for guidance. DSA is designed so that if manipulated rather than random numbers are used, high bandwidth covert channels are possible. See [Schneier] and more recent research. The leakage of an entire DSA private key in only two DSA signatures has been demonstrated. DSA provides security only if trusted implementations, including trusted random number generation, are used. 6. IANA Considerations Allocation of meaning to values of the T parameter that are not defined herein requires an IETF standards actions. It is intended that values unallocated herein be used to cover future extensions of the DSS standard. D. Eastlake 3rd [Page 5]
INTERNET-DRAFT DSA in the DNS References [FIPS 180-1] - U.S. Federal Information Processing Standard: Secure Hash Standard, April 1995. [FIPS 186-2] - U.S. Federal Information Processing Standard: Digital Signature Standard, January 2000. [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 1750] - D. Eastlake, S. Crocker, J. Schiller, "Randomness Recommendations for Security", December 1994. [RFC 2535] - Domain Name System Security Extensions, D. Eastlake, March 1999. [RFC 2671] - Extension Mechanisms for DNS (EDNS0), P. Vixie, August 1999. [RFC 3110] - RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS), D. Eastlake 3rd. May 2001. [RFC 3174] - US Secure Hash Algorithm 1 (SHA1), D. Eastlake, P. Jones, September 2001. [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 Motorola 155 Beaver Street Milford, MA 01757 USA Telephone: +1-508-261-5434(w) +1-508-634-2066(h) FAX: +1-508-261-4447(w) EMail: Donald.Eastlake@motorola.com D. Eastlake 3rd [Page 6]
INTERNET-DRAFT DSA in the DNS Expiration and File Name This draft expires in May 2002. Its file name is draft-ietf-dnsext-rfc2536bis-dsa-01.txt. D. Eastlake 3rd [Page 7]
