INTERNET-DRAFT DSA Information in the DNS OBSOLETES: RFC 2536 Donald E. Eastlake 3rd Motorola Laboratories Expires: January 2006 July 2005 DSA Keying and Signature Information in the DNS --- ------ --- --------- ----------- -- --- --- <draft-ietf-dnsext-rfc2536bis-dsa-06.txt> Donald E. Eastlake 3rd Status of This Document 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. Distribution of this document is unlimited. Comments should be sent to the DNS extensions working group mailing list <namedroppers@ops.ietf.org>. 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 a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract The standard method of encoding US Government Digital Signature Algorithm keying and signature information for use in the Domain Name System is specified. Copyright Notice Copyright (C) The Internet Society 2005. All Rights Reserved. D. Eastlake 3rd [Page 1]
INTERNET-DRAFT DSA Information in the DNS Table of Contents Status of This Document....................................1 Abstract...................................................1 Copyright Notice...........................................1 Table of Contents..........................................2 1. Introduction............................................3 2. DSA Keying Information..................................3 3. DSA Signature Information...............................4 4. Performance Considerations..............................4 5. Security Considerations.................................5 6. IANA Considerations.....................................5 Copyright and Disclaimer...................................5 Normative References.......................................7 Informative References.....................................7 Authors Address............................................8 Expiration and File Name...................................8 D. Eastlake 3rd [Page 2]
INTERNET-DRAFT DSA Information 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]. The DNS has been extended to include digital signatures and cryptographic keys as described in [RFC 4033, 4034, 4035] and additional work is underway which would require the storage of keying and signature information in the DNS. This document describes how to encode 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 Keying Information When DSA public keys are stored in the DNS, the structure of the relevant part of the RDATA part of the RR being used is the fields listed below in the order given. The period of key validity is not included in this data but is indicated separately, for example by an RR such as RRSIG which signs and authenticates the RR containing the keying information. 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 if the T octet is greater than 8 is reserved and the remainder of the data 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. Thus 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 Information in the DNS Y = G**X mod P 3. DSA Signature Information The portion of the RDATA area used for US Digital Signature Algorithm signature information is shown below with fields in the order they are listed and the contents of each multi-octet field in "big-endian" network order. Field Size ----- ---- T 1 octet R 20 octets S 20 octets First, the data signed must be determined. 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 information on the SHA-1 hash function see [FIPS 180-2] 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 standardized. 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 some applications. D. Eastlake 3rd [Page 4]
INTERNET-DRAFT DSA Information in the DNS 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 RR sets containing keying and/or signature inforamtion consistent with adequate security. 5. Security Considerations 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 [random] for guidance. DSA is designed so that if biased 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 (i.e., > 8 ) requires an IETF standards actions. It is intended that values unallocated herein be used to cover future extensions of the DSS standard. Copyright and Disclaimer Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. D. Eastlake 3rd [Page 5]
INTERNET-DRAFT DSA Information in the DNS This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. D. Eastlake 3rd [Page 6]
INTERNET-DRAFT DSA Information in the DNS Normative References [FIPS 186-2] - U.S. Federal Information Processing Standard: Digital Signature Standard, 27 January 2000. [RFC 4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, March 2005. Informative References [RFC 1034] - "Domain names - concepts and facilities", P. Mockapetris, 11/01/1987. [RFC 1035] - "Domain names - implementation and specification", P. Mockapetris, 11/01/1987. [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. [RFC 4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC 4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, March 2005. [RFC 4086] - Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. [Schneier] - "Applied Cryptography Second Edition: protocols, algorithms, and source code in C" (second edition), Bruce Schneier, 1996, John Wiley and Sons, ISBN 0-471-11709-9. D. Eastlake 3rd [Page 7]
INTERNET-DRAFT DSA Information in the DNS Authors Address Donald E. Eastlake 3rd Motorola Labortories 155 Beaver Street Milford, MA 01757 USA Telephone: +1-508-786-7554(w) EMail: Donald.Eastlake@motorola.com Expiration and File Name This draft expires in January 2006. Its file name is draft-ietf-dnsext-rfc2536bis-dsa-06.txt. D. Eastlake 3rd [Page 8]
