Network Working Group                                        D. Eastlake
Request for Comments: 2539                                           IBM
Category: Standards Track                                     March 1999

     Storage of Diffie-Hellman Keys in the Domain Name System (DNS)

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.


   A standard method for storing Diffie-Hellman keys in the Domain Name
   System is described which utilizes DNS KEY resource records.


   Part of the format for Diffie-Hellman keys and the description
   thereof was taken from a work in progress by:

      Ashar Aziz <>
      Tom Markson <>
      Hemma Prafullchandra <>

   In addition, the following person provided useful comments that have
   been incorporated:

      Ran Atkinson <>
      Thomas Narten <>

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RFC 2539             Diffie-Hellman Keys in the DNS           March 1999

Table of Contents

   1. Introduction............................................2
   1.1 About This Document....................................2
   1.2 About Diffie-Hellman...................................2
   2. Diffie-Hellman KEY Resource Records.....................3
   3. Performance Considerations..............................4
   4. IANA Considerations.....................................4
   5. Security Considerations.................................4
   Author's Address...........................................5
   Appendix A: Well known prime/generator pairs...............6
   A.1. Well-Known Group 1:  A 768 bit prime..................6
   A.2. Well-Known Group 2:  A 1024 bit prime.................6
   Full Copyright Notice......................................7

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

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

                Zi = Y**i ( mod p ) Zj = X**j ( mod p )

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   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).

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.

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   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

   The corresponding algorithm=2 SIG resource record is not used so no
   format for it is defined.

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 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. IANA Considerations

   Assignment of meaning to Prime Lengths of 0 and 3 through 15 requires
   an IETF consensus.

   Well known prime/generator pairs number 0x0000 through 0x07FF can
   only be assigned by an IETF standards action and this Proposed
   Standard assigns 0x0001 through 0x0002. 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.  [Schneier]

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RFC 2539             Diffie-Hellman Keys in the DNS           March 1999


   [RFC 1034]   Mockapetris, P., "Domain Names - Concepts and
                Facilities", STD 13, RFC 1034, November 1987.

   [RFC 1035]   Mockapetris, P., "Domain Names - Implementation and
                Specification", STD 13, RFC 1035, November 1987.

   [RFC 2535]   Eastlake, D., "Domain Name System Security Extensions",
                RFC 2535, March 1999.

   [Schneier]   Bruce Schneier, "Applied Cryptography: Protocols,
                Algorithms, and Source Code in C", 1996, John Wiley and

Author's Address

   Donald E. Eastlake 3rd
   65 Shindegan Hill Road, RR #1
   Carmel, NY 10512

   Phone:   +1-914-276-2668(h)
   Fax:     +1-914-784-3833(w)

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RFC 2539             Diffie-Hellman Keys in the DNS           March 1999

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

   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

   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

   Generator: Length (32 bit words):  1, Data (hex): 2

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Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

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   This document and the information contained herein is provided on an

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