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Versions: 00 01 02 03 RFC 2539

INTERNET-DRAFT                            Diffie-Hellman Keys in the DNS
                                                           November 1998
                                                        Expires May 1999




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

                         Donald E. Eastlake 3rd



Status of This Document

   This draft, file name draft-ietf-dnssec-dhk-03.txt, is intended to be
   become a Proposed 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 view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

   [Changes from previous draft: add IANA considerations section, update
   author info, update file name and dates, add specific well known
   groups]














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>
        Thomas Narten <narten@raleigh.ibm.com>































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
      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




























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 [draft-
   ietf-dnssec-secext2-*.txt].  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).






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 [draft-
   ietf-dnssec-secext2-*.txt].  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 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 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 [draft-ietf-dnssec-
   secext2-*] 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]











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.

   [draft-ietf-dnssec-secext2-*.txt] - Domain Name System Security
   Extensions, D. Eastlake.

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




Author's Address

   Donald E. Eastlake 3rd
   IBM
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1-978-287-4877
                +1-914-784-7913
   FAX:         +1-978-371-7148
   EMail:       dee3@us.ibm.com



Expiration and File Name

   This draft expires in April 1999.

   Its file name is draft-ietf-dnssec-dhk-03.txt.
















Donald E. 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 E. Eastlake 3rd                                          [Page 8]


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