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RFC 2536 - DSA KEYs and SIGs in the Domain Name System (DNS)

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Network Working Group                                        D. EastLake
Request for Comments: 2536                                           IBM
Category: Standards Track                                     March 1999

           DSA KEYs and SIGs 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 US Government Digital Signature
   Algorithm keys and signatures in the Domain Name System is described
   which utilizes DNS KEY and SIG resource records.

Table of Contents

   1. Introduction............................................1
   2. DSA KEY Resource Records................................2
   3. DSA SIG Resource Records................................3
   4. Performance Considerations..............................3
   5. Security Considerations.................................4
   6. IANA Considerations.....................................4
   Author's Address...........................................5
   Full Copyright Statement...................................6

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 [RFC 2535].  Thus
   the DNS can now be secured and can be used for secure key

   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 [RFC 2535].  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
   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 [RFC 2535] 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 [RFC 2535].  Then the
   following steps are taken, as specified in [FIPS 186], 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

   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
   2537] 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 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 [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 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.


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

   [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 1750]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness
                Recommendations for Security", RFC 1750, December 1994.

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

   [RFC 2537]   Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
                System (DNS)", RFC 2537, March 1999.

   [Schneier]   Schneier, B., "Applied Cryptography Second Edition:
                protocols, algorithms, and source code in C", 1996.

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)
   EMail:   dee3@us.ibm.com

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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