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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 <>

Eastlake Standards Track [Page 1] 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 )

Eastlake Standards Track [Page 2] RFC 2539 Diffie-Hellman Keys in the DNS March 1999

 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.

Eastlake Standards Track [Page 3] RFC 2539 Diffie-Hellman Keys in the DNS March 1999

 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]

Eastlake Standards Track [Page 4] 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)

Eastlake Standards Track [Page 5] 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

Eastlake Standards Track [Page 6] RFC 2539 Diffie-Hellman Keys in the DNS March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 copyrights defined in the Internet Standards process must be
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 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Eastlake Standards Track [Page 7]

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