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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.


 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

 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
 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.

Eastlake Standards Track [Page 1] RFC 2537 RSA/MD5 KEYs and SIGs in the DNS March 1999

 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
 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)

Eastlake Standards Track [Page 2] RFC 2537 RSA/MD5 KEYs and SIGs in the DNS March 1999

 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

Eastlake Standards Track [Page 3] RFC 2537 RSA/MD5 KEYs and SIGs in the DNS March 1999

 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.
 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.


 [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.

Eastlake Standards Track [Page 4] RFC 2537 RSA/MD5 KEYs and SIGs in the DNS 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
 65 Shindegan Hill Road, RR #1
 Carmel, NY 10512
 Phone:   +1-914-276-2668(h)
 Fax:     +1-914-784-3833(w)

Eastlake Standards Track [Page 5] RFC 2537 RSA/MD5 KEYs and SIGs 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
 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

Eastlake Standards Track [Page 6]

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