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rfc:rfc2536

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.

Abstract

 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

 Abstract...................................................1
 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
 References.................................................5
 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
 distribution.

Eastlake Standards Track [Page 1] RFC 2536 DSA in the DNS March 1999

 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

Eastlake Standards Track [Page 2] RFC 2536 DSA in the DNS March 1999

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
 authentication.
 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

Eastlake Standards Track [Page 3] RFC 2536 DSA in the DNS March 1999

 the DNS consistent with adequate security.  Keep in mind that in a
 secure zone, at least one authenticating SIG RR will also be
 returned.

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.

Eastlake Standards Track [Page 4] RFC 2536 DSA in the DNS March 1999

References

 [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
 IBM
 65 Shindegan Hill Road, RR #1
 Carmel, NY 10512
 Phone:   +1-914-276-2668(h)
          +1-914-784-7913(w)
 Fax:     +1-914-784-3833(w)
 EMail:   dee3@us.ibm.com

Eastlake Standards Track [Page 5] RFC 2536 DSA 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
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 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
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Eastlake Standards Track [Page 6]

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