
INTERNET-DRAFT                                DSA Information in the DNS
OBSOLETES: RFC 2536                               Donald E. Eastlake 3rd
                                                   Motorola Laboratories
Expires: September 2006                                       March 2006


            DSA Keying and Signature Information in the DNS
            --- ------ --- --------- ----------- -- --- ---
               <draft-ietf-dnsext-rfc2536bis-dsa-07.txt>
                         Donald E. Eastlake 3rd


Status of This Document

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Abstract

   The standard method of encoding US Government Digital Signature
   Algorithm keying and signature information for use in the Domain Name
   System is specified.









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Table of Contents

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

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

      1. Introduction............................................3
      2. DSA Keying Information..................................3
      3. DSA Signature Information...............................4
      4. Performance Considerations..............................4
      5. Security Considerations.................................5
      6. IANA Considerations.....................................5
      Copyright, Disclaimer, and Additional IPR Provisions.......5

      Normative References.......................................7
      Informative References.....................................7

      Author's Address...........................................8
      Expiration and File Name...................................8
































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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 [RFC 1034, 1035]. The DNS has been extended to
   include digital signatures and cryptographic keys as described in
   [RFC 4033, 4034, 4035] and additional work is underway which would
   require the storage of keying and signature information in the DNS.

   This document describes how to encode US Government Digital Signature
   Algorithm (DSA) keys and signatures in the DNS.  Familiarity with the
   US Digital Signature Algorithm is assumed [FIPS 186-2, Schneier].



2. DSA Keying Information

   When DSA public keys are stored in the DNS, the structure of the
   relevant part of the RDATA part of the RR being used is the fields
   listed below in the order given.

   The period of key validity is not included in this data but is
   indicated separately, for example by an RR such as RRSIG which signs
   and authenticates the RR containing the keying information.

        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 186-2] and [Schneier], T is a key size
   parameter chosen such that 0 <= T <= 8.  (The meaning if the T octet
   is greater than 8 is reserved and the remainder of the data 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. Thus 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 186-2] key generation algorithm [Schneier].  P is in the range
   2**(511+64T) < P < 2**(512+64T) and thus is 64 + 8*T octets long.  G
   and Y are quantities modulo 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
   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



D. Eastlake 3rd                                                 [Page 3]


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        Y = G**X mod P



3. DSA Signature Information

   The portion of the RDATA area used for US Digital Signature Algorithm
   signature information is shown below with fields in the order they
   are listed and the contents of each multi-octet field in "big-endian"
   network order.

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

   First, the data signed must be determined.  Then the following steps
   are taken, as specified in [FIPS 186-2], where Q, P, G, and Y are as
   specified in the public key [Schneier]:

        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

   For information on the SHA-1 hash function see [FIPS 180-2] and [RFC
   3174].

   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 standardized.



4. Performance Considerations

   General signature generation speeds are roughly the same for RSA [RFC
   3110] 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
   recommended for some applications.


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   Current DNS implementations are optimized for small transfers,
   typically less than 512 bytes including DNS overhead.  Larger
   transfers will perform correctly and extensions have been
   standardized [RFC 2671] to make larger transfers more efficient, it
   is still advisable at this time to make reasonable efforts to
   minimize the size of RR sets containing keying and/or signature
   inforamtion consistent with adequate security.



5. Security Considerations

   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 particular
   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 [random] for guidance.  DSA is designed so that if
   biased rather than random numbers are used, 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 (i.e., > 8 ) requires an IETF standards actions.  It
   is intended that values unallocated herein be used to cover future
   extensions of the DSS standard.



Copyright, Disclaimer, and Additional IPR Provisions

   Copyright (C) The Internet Society (2006).  This document is subject to
   the rights, licenses and restrictions contained in BCP 78, and except
   as set forth therein, the authors retain all their rights.


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   This document and the information contained herein are provided on an
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Normative References

   [FIPS 186-2] - U.S. Federal Information Processing Standard: Digital
   Signature Standard, 27 January 2000.

   [RFC 4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "Resource Records for the DNS Security Extensions", RFC 4034,
   March 2005.



Informative References

   [RFC 1034] - "Domain names - concepts and facilities", P.
   Mockapetris, 11/01/1987.

   [RFC 1035] - "Domain names - implementation and specification", P.
   Mockapetris, 11/01/1987.

   [RFC 2671] - "Extension Mechanisms for DNS (EDNS0)", P. Vixie, August
   1999.

   [RFC 3110] - "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System
   (DNS)", D.  Eastlake 3rd. May 2001.

   [RFC 3174] - "US Secure Hash Algorithm 1 (SHA1)", D. Eastlake, P.
   Jones, September 2001.

   [RFC 4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "DNS Security Introduction and Requirements", RFC 4033, March
   2005.

   [RFC 4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "Protocol Modifications for the DNS Security Extensions", RFC
   4035, March 2005.

   [RFC 4086] - Eastlake, D., 3rd, Schiller, J., and S. Crocker,
   "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.

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










D. Eastlake 3rd                                                 [Page 7]


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Author's Address

   Donald E. Eastlake 3rd
   Motorola Labortories
   155 Beaver Street
   Milford, MA 01757 USA

   Telephone:   +1-508-786-7554(w)
   EMail:       Donald.Eastlake@motorola.com



Expiration and File Name

   This draft expires in September 2006.

   Its file name is draft-ietf-dnsext-rfc2536bis-dsa-07.txt.



































D. Eastlake 3rd                                                 [Page 8]

