INTERNET-DRAFT Diffie-Hellman Keys in the DNS OBSOLETES: RFC 2539 Donald Eastlake 3rd Motorola Expires: January 2002 July 2001 Storage of Diffie-Hellman Keys in the Domain Name System (DNS) ------- -- -------------- ---- -- --- ------ ---- ------ ----- <draft-ietf-dnsext-rfc2539bis-dhk-00.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. Donald Eastlake 3rd [Page 1]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS Abstract A standard method for storing Diffie-Hellman keys in the Domain Name System is described which utilizes DNS KEY resource records. Acknowledgements Part of the format for Diffie-Hellman keys and the description thereof was taken from a work in progress by Ashar Aziz, Tom Markson, and Hemma Prafullchandra. In addition, the following persons provided useful comments that were incorporated into the predecessor of this document: Ran Atkinson, Thomas Narten. Donald Eastlake 3rd [Page 2]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS Table of Contents Status of This Document....................................1 Abstract...................................................2 Acknowledgements...........................................2 Table of Contents..........................................3 1. Introduction............................................4 1.1 About This Document....................................4 1.2 About Diffie-Hellman...................................4 2. Diffie-Hellman KEY Resource Records.....................5 3. Performance Considerations..............................6 4. IANA Considerations.....................................6 5. Security Considerations.................................6 References.................................................7 Author's Address...........................................7 Expiration and File Name...................................7 Appendix A: Well known prime/generator pairs...............8 A.1. Well-Known Group 1: A 768 bit prime..................8 A.2. Well-Known Group 2: A 1024 bit prime.................8 A.3. Well-Known Group 3: A 1536 bit prime.................9 Donald Eastlake 3rd [Page 3]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS 1. Introduction The Domain Name System (DNS) is the current global hierarchical replicated distributed database system for Internet addressing, mail proxy, and similar information. The DNS has been extended to include digital signatures and cryptographic keys as described in [RFC 2535]. Thus the DNS can now be used for secure key distribution. 1.1 About This Document This document describes how to store Diffie-Hellman keys in the DNS. Familiarity with the Diffie-Hellman key exchange algorithm is assumed [Schneier]. 1.2 About Diffie-Hellman Diffie-Hellman requires two parties to interact to derive keying information which can then be used for authentication. Since DNS SIG RRs are primarily used as stored authenticators of zone information for many different resolvers, no Diffie-Hellman algorithm SIG RR is defined. For example, assume that two parties have local secrets "i" and "j". Assume they each respectively calculate X and Y as follows: X = g**i ( mod p ) Y = g**j ( mod p ) They exchange these quantities and then each calculates a Z as follows: Zi = Y**i ( mod p ) Zj = X**j ( mod p ) Zi and Zj will both be equal to g**(ij)(mod p) and will be a shared secret between the two parties that an adversary who does not know i or j will not be able to learn from the exchanged messages (unless the adversary can derive i or j by performing a discrete logarithm mod p which is hard for strong p and g). The private key for each party is their secret i (or j). The public key is the pair p and g, which must be the same for the parties, and their individual X (or Y). For further information about Diffie-Hellman and precautions to take in deciding on a p and g, see [RFC 2631]. Donald Eastlake 3rd [Page 4]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS 2. Diffie-Hellman KEY Resource Records Diffie-Hellman keys are stored in the DNS as KEY RRs using algorithm number 2. The structure of the RDATA portion of this RR is as shown below. The first 4 octets, including the flags, protocol, and algorithm fields are common to all KEY RRs as described in [RFC 2535]. The remainder, from prime length through public value is the "public key" part of the 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. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | KEY flags | protocol | algorithm=2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | prime length (or flag) | prime (p) (or special) / +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / prime (p) (variable length) | generator length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | generator (g) (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | public value length | public value (variable length)/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / public value (g^i mod p) (variable length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Prime length is length of the Diffie-Hellman prime (p) in bytes if it is 16 or greater. Prime contains the binary representation of the Diffie-Hellman prime with most significant byte first (i.e., in network order). If "prime length" field is 1 or 2, then the "prime" field is actually an unsigned index into a table of 65,536 prime/generator pairs and the generator length SHOULD be zero. See Appedix A for defined table entries and Section 4 for information on allocating additional table entries. The meaning of a zero or 3 through 15 value for "prime length" is reserved. Generator length is the length of the generator (g) in bytes. Generator is the binary representation of generator with most significant byte first. PublicValueLen is the Length of the Public Value (g**i (mod p)) in bytes. PublicValue is the binary representation of the DH public value with most significant byte first. The corresponding algorithm=2 SIG resource record is not used so no format for it is defined. Donald Eastlake 3rd [Page 5]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS 3. Performance Considerations 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. 4. IANA Considerations Assignment of meaning to Prime Lengths of 0 and 3 through 15 requires an IETF consensus as defined in [RFC 2434]. Well known prime/generator pairs number 0x0000 through 0x07FF can only be assigned by an IETF standards action. RFC 2539, the Proposed Standard predecessor of this document, assigned 0x0001 through 0x0002. This document proposes to assign 0x0003. Pairs number 0s0800 through 0xBFFF can be assigned based on RFC documentation. Pairs number 0xC000 through 0xFFFF are available for private use and are not centrally coordinated. Use of such private pairs outside of a closed environment may result in conflicts. 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 important and dependent on local policy. In addition, the usual Diffie-Hellman key strength considerations apply. (p-1)/2 should also be prime, g should be primitive mod p, p should be "large", etc. [RFC 2631, Schneier] Donald Eastlake 3rd [Page 6]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS References [RFC 1034] - P. Mockapetris, "Domain names - concepts and facilities", November 1987. [RFC 1035] - P. Mockapetris, "Domain names - implementation and specification", November 1987. [RFC 2434] - Guidelines for Writing an IANA Considerations Section in RFCs, T. Narten, H. Alvestrand, October 1998. [RFC 2535] - Domain Name System Security Extensions, D. Eastlake 3rd, March 1999. [RFC 2539] - Storage of Diffie-Hellman Keys in the Domain Name System (DNS), D. Eastlake, March 1999, obsoleted by this RFC. [RFC 2631] - Diffie-Hellman Key Agreement Method, E. Rescorla, June 1999. [RFC 2671] - Extension Mechanisms for DNS (EDNS0), P. Vixie, August 1999. [Schneier] - Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and Source Code in C", 1996, John Wiley and Sons. 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 Expiration and File Name This draft expires in January 2002. Its file name is draft-ietf-dnsext-rfc2539bis-dhk-00.txt. Donald Eastlake 3rd [Page 7]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS Appendix A: Well known prime/generator pairs These numbers are copied from the IPSEC effort where the derivation of these values is more fully explained and additional information is available. Richard Schroeppel performed all the mathematical and computational work for this appendix. A.1. Well-Known Group 1: A 768 bit prime The prime is 2^768 - 2^704 - 1 + 2^64 * { [2^638 pi] + 149686 }. Its decimal value is 155251809230070893513091813125848175563133404943451431320235 119490296623994910210725866945387659164244291000768028886422 915080371891804634263272761303128298374438082089019628850917 0691316593175367469551763119843371637221007210577919 Prime modulus: Length (32 bit words): 24, Data (hex): FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF Generator: Length (32 bit words): 1, Data (hex): 2 A.2. Well-Known Group 2: A 1024 bit prime The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }. Its decimal value is 179769313486231590770839156793787453197860296048756011706444 423684197180216158519368947833795864925541502180565485980503 646440548199239100050792877003355816639229553136239076508735 759914822574862575007425302077447712589550957937778424442426 617334727629299387668709205606050270810842907692932019128194 467627007 Prime modulus: Length (32 bit words): 32, Data (hex): FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381 FFFFFFFF FFFFFFFF Generator: Length (32 bit words): 1, Data (hex): 2 Donald Eastlake 3rd [Page 8]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS A.3. Well-Known Group 3: A 1536 bit prime The prime is 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }. Its decimal value is 241031242692103258855207602219756607485695054850245994265411 694195810883168261222889009385826134161467322714147790401219 650364895705058263194273070680500922306273474534107340669624 601458936165977404102716924945320037872943417032584377865919 814376319377685986952408894019557734611984354530154704374720 774996976375008430892633929555996888245787241299381012913029 459299994792636526405928464720973038494721168143446471443848 8520940127459844288859336526896320919633919 Prime modulus Length (32 bit words): 48, Data (hex): FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1 29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245 E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE45B3D C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F 83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D 670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF Generator: Length (32 bit words): 1, Data (hex): 2 Donald Eastlake 3rd [Page 9]