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DSA KEYs and SIGs in the Domain Name System (DNS)

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 2536.
Author Donald E. Eastlake 3rd
Last updated 2013-03-02 (Latest revision 1998-10-14)
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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Document shepherd (None)
IESG IESG state Became RFC 2536 (Proposed Standard)
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INTERNET-DRAFT                              DSA KEYs and SIGs in the DNS
                                                            October 1998
                                                      Expires April 1999

           DSA KEYs and SIGs in the Domain Name System (DNS)
           --- ---- --- ---- -- --- ------ ---- ------ -----

                         Donald E. Eastlake 3rd

Status of This Document

   This draft, file name draft-ietf-dnssec-dss-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
   <> 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 (Africa), (Northern
   Europe), (Southern Europe), (Pacific
   Rim), (US East Coast), or (US West Coast).

   [Changes from previous draft: change dates, update author info, add
   IANA Considerations]


   A standard method for storing US Government Digital Signature
   Algorithm keys and signatures in the Domain Name System is described
   which utilizes DNS KEY and SIG resource records.

Donald E. Eastlake 3rd                                          [Page 1]

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Table of Contents

      Status of This Document....................................1

      Table of Contents..........................................2

      1. Introduction............................................3
      2. DSA KEY Resource Records................................3
      3. DSA SIG Resource Records................................4
      4. Performance Considerations..............................4
      5. Security Considerations.................................5
      6. IANA Considerations.....................................5

      Author's Address...........................................6
      Expiration and File Name...................................6

Donald E. Eastlake 3rd                                          [Page 2]

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1. Introduction

   The Domain Name System (DNS) is the global hierarchical replicated
   distributed database system for Internet addressing, mail proxy, and
   other information. The DNS has been extended to include digital
   signatures and cryptographic keys as described in [draft-ietf-
   dnssec-secext2-*].  Thus the DNS can now be secured and can be used
   for secure key distribution.

   This document describes how to store US Government Digital Signature
   Algorithm (DSA) keys and signatures in the DNS.  Familiarity with the
   US Digital Signature Algorithm is assumed [Schneier].  Implementation
   of DSA is mandatory for DNS security.

2. DSA KEY Resource Records

   DSA public keys are stored in the DNS as KEY RRs using algorithm
   number 3 [draft-ietf-dnssec-secext2-*].  The structure of the
   algorithm specific portion of the RDATA part of this RR is as shown
   below.  These fields, from Q through Y are the "public key" part of
   the DSA 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.

        Field     Size
        -----     ----
         T         1  octet
         Q        20  octets
         P        64 + T*8  octets
         G        64 + T*8  octets
         Y        64 + T*8  octets

   As described in [FIPS 186] and [Schneier]:  T is a key size parameter
   chosen such that 0 <= T <= 8.  (The meaning for algorithm 3 if the T
   octet is greater than 8 is reserved and the remainder of the RDATA
   portion may have a different format in that case.)  Q is a prime
   number selected at key generation time such that 2**159 < Q < 2**160
   so Q is always 20 octets long and, as with all other fields, is
   stored in "big-endian" network order.  P, G, and Y are calculated as
   directed by the FIPS 186 key generation algorithm [Schneier].  P is
   in the range 2**(511+64T) < P < 2**(512+64T) and so is 64 + 8*T
   octets long.  G and Y are quantities modulus P and so can be up to
   the same length as P and are allocated fixed size fields with the
   same number of octets as P.

   During the key generation process, a random number X must be

Donald E. Eastlake 3rd                                          [Page 3]

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   generated such that 1 <= X <= Q-1.  X is the private key and is used
   in the final step of public key generation where Y is computed as

        Y = G**X mod P

3. DSA SIG Resource Records

   The signature portion of the SIG RR RDATA area, when using the US
   Digital Signature Algorithm, is shown below with fields in the order
   they occur.  See [draft-ietf-dnssec-secext2-*] for fields in the SIG
   RR RDATA which precede the signature itself.

        Field     Size
        -----     ----
         T         1 octet
         R        20 octets
         S        20 octets

   The data signed is determined as specified in [draft-ietf-dnssec-
   secext2-*].  Then the following steps are taken, as specified in
   [FIPS 186], where Q, P, G, and Y are as specified in the public key

        hash = SHA-1 ( data )

        Generate a random K such that 0 < K < Q.

        R = ( G**K mod P ) mod Q

        S = ( K**(-1) * (hash + X*R) ) mod Q

   Since Q is 160 bits long, R and S can not be larger than 20 octets,
   which is the space allocated.

   T is copied from the public key.  It is not logically necessary in
   the SIG but is present so that values of T > 8 can more conveniently
   be used as an escape for extended versions of DSA or other algorithms
   as later specified.

4. Performance Considerations

   General signature generation speeds are roughly the same for RSA [RFC
   xRSA] and DSA.  With sufficient pre-computation, signature generation
   with DSA is faster than RSA.  Key generation is also faster for DSA.
   However, signature verification is an order of magnitude slower than
   RSA when the RSA public exponent is chosen to be small as is

Donald E. Eastlake 3rd                                          [Page 4]

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   recommended for KEY RRs used in domain name system (DNS) data

   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, at least one authenticating SIG RR will also be

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 essential
   and dependent on local policy.

   The key size limitation of a maximum of 1024 bits ( T = 8 ) in the
   current DSA standard may limit the security of DSA.  For particularly
   critical applications, implementors are encouraged to consider the
   range of available algorithms and key sizes.

   DSA assumes the ability to frequently generate high quality random
   numbers.  See [RFC 1750] for guidance.  DSA is designed so that if
   manipulated rather than random numbers are used, very high bandwidth
   covert channels are possible.  See [Schneier] and more recent
   research.  The leakage of an entire DSA private key in only two DSA
   signatures has been demonstrated.  DSA provides security only if
   trusted implementations, including trusted random number generation,
   are used.

6. IANA Considerations

   Allocation of meaning to values of the T parameter that are not
   defined herein requires an IETF standards actions.  It is intended
   that values unallocated herein be used to cover future extensions of
   the DSS standard.

Donald E. Eastlake 3rd                                          [Page 5]

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   [FIPS 186] - U.S. Federal Information Processing Standard: Digital
   Signature Standard.

   [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 1750] - D. Eastlake, S. Crocker, J. Schiller, "Randomness
   Recommendations for Security", 12/29/1994.

   [draft-ietf-dnssec-secext2-*] - Domain Name System Security
   Extensions, D. Eastlake, C. Kaufman, January 1997.

   [RFC xRSA] - draft-ietf-dnssec-rsa-*.txt - RSA/MD5 KEYs and SIGs in
   the Domain Name System (DNS), D. Eastlake.

   [Schneier] - Bruce Schneier, "Applied Cryptography Second Edition:
   protocols, algorithms, and source code in C", 1996, John Wiley and
   Sons, ISBN 0-471-11709-9.

Author's Address

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

   Telephone:   +1-978-287-4877
   FAX:         +1-978-371-7148

Expiration and File Name

   This draft expires in April 1999.

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

Donald E. Eastlake 3rd                                          [Page 6]