Network Working Group                                        D. Eastlake
Request for Comments: 2537                                           IBM
Category: Standards Track                                     March 1999


         RSA/MD5 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 RSA keys and and RSA/MD5 based
   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. RSA Public KEY Resource Records.........................2
   3. RSA/MD5 SIG Resource Records............................2
   4. Performance Considerations..............................3
   5. Security Considerations.................................4
   References.................................................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 used for secure key distribution.

   This document describes how to store RSA keys and and RSA/MD5 based
   signatures in the DNS.  Familiarity with the RSA algorithm is assumed
   [Schneier].  Implementation of the RSA algorithm in DNS is
   recommended.

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
   in this document are to be interpreted as described in RFC 2119.

2. RSA Public KEY Resource Records

   RSA public keys are stored in the DNS as KEY RRs using algorithm
   number 1 [RFC 2535].  The structure of the algorithm specific portion
   of the RDATA part of such RRs is as shown below.

           Field             Size
           -----             ----
           exponent length   1 or 3 octets (see text)
           exponent          as specified by length field
           modulus           remaining space

   For interoperability, the exponent and modulus are each currently
   limited to 4096 bits in length.  The public key exponent is a
   variable length unsigned integer.  Its length in octets is
   represented as one octet if it is in the range of 1 to 255 and by a
   zero octet followed by a two octet unsigned length if it is longer
   than 255 bytes.  The public key modulus field is a multiprecision
   unsigned integer.  The length of the modulus can be determined from
   the RDLENGTH and the preceding RDATA fields including the exponent.
   Leading zero octets are prohibited in the exponent and modulus.

3. RSA/MD5 SIG Resource Records

   The signature portion of the SIG RR RDATA area, when using the
   RSA/MD5 algorithm, is calculated as shown below.  The data signed is
   determined as specified in [RFC 2535].  See [RFC 2535] for fields in
   the SIG RR RDATA which precede the signature itself.


     hash = MD5 ( data )

     signature = ( 00 | 01 | FF* | 00 | prefix | hash ) ** e (mod n)

   where MD5 is the message digest algorithm documented in [RFC 1321],
   "|" is concatenation, "e" is the private key exponent of the signer,
   and "n" is the modulus of the signer's public key.  01, FF, and 00
   are fixed octets of the corresponding hexadecimal value. "prefix" is
   the ASN.1 BER MD5 algorithm designator prefix specified in [RFC
   2437], that is,

      hex 3020300c06082a864886f70d020505000410 [NETSEC].

   This prefix is included to make it easier to use RSAREF (or similar
   packages such as EuroRef).  The FF octet MUST be repeated the maximum
   number of times such that the value of the quantity being
   exponentiated is the same length in octets as the value of n.

   (The above specifications are identical to the corresponding part of
   Public Key Cryptographic Standard #1 [RFC 2437].)

   The size of n, including most and least significant bits (which will
   be 1) MUST be not less than 512 bits and not more than 4096 bits.  n
   and e SHOULD be chosen such that the public exponent is small.

   Leading zero bytes are permitted in the RSA/MD5 algorithm signature.

   A public exponent of 3 minimizes the effort needed to verify a
   signature.  Use of 3 as the public exponent is weak for
   confidentiality uses since, if the same data can be collected
   encrypted under three different keys with an exponent of 3 then,
   using the Chinese Remainder Theorem [NETSEC], the original plain text
   can be easily recovered.  This weakness is not significant for DNS
   security because we seek only authentication, not confidentiality.

4. Performance Considerations

   General signature generation speeds are roughly the same for RSA and
   DSA [RFC 2536].  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 with DSA 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
   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.

   For interoperability, the RSA key size is limited to 4096 bits.  For
   particularly critical applications, implementors are encouraged to
   consider the range of available algorithms and key sizes.

References

   [NETSEC]     Kaufman, C., Perlman, R. and M. Speciner, "Network
                Security: PRIVATE Communications in a PUBLIC World",
                Series in Computer Networking and Distributed
                Communications, 1995.

   [RFC 2437]   Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
                Specifications Version 2.0", RFC 2437, October 1998.

   [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 1321]   Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321
                April 1992.

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

   [RFC 2536]   EastLake, D., "DSA KEYs and SIGs in the Domain Name
                System (DNS)", RFC 2536, March 1999.

   [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
   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

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