INTERNET-DRAFT Diffie-Hellman Keys in the DNS November 1997 Expires May 1998 Storage of Diffie-Hellman Keys in the Domain Name System ------- -- -------------- ---- -- --- ------ ---- ------ Donald E. Eastlake 3rd Status of This Document This draft, file name draft-ietf-dnssec-dhk-01.txt, is intended to be become a Proposed or Experimental 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). Donald E. 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 an Internet draft by: Ashar Aziz <ashar.aziz@eng.sun.com> Tom Markson <markson@incog.com> Hemma Prafullchandra <hemma@eng.sun.com> In addition, the following person provided useful comments that have been incorporated: Ran Atkinson <rja@inet.org> Donald E. 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 2. Diffie-Hellman KEY Resource Records.....................5 3. Performance Considerations..............................6 4. Security Considerations.................................6 References.................................................7 Author's Address...........................................7 Expiration and File Name...................................7 Appendix A. Predefined Prime/Generator Pairs...............8 Donald E. 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 2065. Thus the DNS can now be used for secure key distribution. This document describes how to store Diffie-Hellman keys in the DNS. Familiarity with the Diffie-Hellman key exchange algorithm is assumed [Schneier]. 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). Donald E. 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 2065. 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. If "prime length" field is 1, then the "prime" field is actually an index into a table of 256 predefined prime/generator pairs (Appendix A) in which case the generator length should be zero. The meaning of a zero or 2 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 E. 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 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, an authenticating SIG RR will also be returned. 4. Security Considerations Many of the security consideration in RFC 2065 apply. Of course, the Diffie-Hellman key stored in the DNS for an entity should not be trusted unless it has been obtain via a trusted DNS resolver that vouches for its security or unless the application using the key has done a similar authentication. 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. [Schneier] Donald E. Eastlake 3rd [Page 6]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS References [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 2065] - Domain Name System Security Extensions, D. Eastlake, C. Kaufman, January 1997. [Schneier] - Bruce Schneier, "Applied Cryptography: Protocols, Algorithms, and Source Code in C", 1996, John Wiley and Sons 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, VA) FAX: +1 978 371 7148 EMail: dee@cybercash.com Expiration and File Name This draft expires in May 1998. Its file name is draft-ietf-dnssec-dhk-01.txt. Donald E. Eastlake 3rd [Page 7]

INTERNET-DRAFT Diffie-Hellman Keys in the DNS Appendix A. Predefined Prime/Generator Pairs TBD. Donald E. Eastlake 3rd [Page 8]