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Versions: 00 01 RFC 2537

INTERNET-DRAFT                          RSA/MD5 KEYs and SIGs in the DNS
                                                            October 1998
                                                      Expires April 1999




         RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)
         ------- ---- --- ---- -- --- ------ ---- ------ -----

                         Donald E. Eastlake 3rd



Status of This Document

   This draft, file name draft-ietf-dnssec-rsa-01.txt, is intended to be
   become a Proposed Standard RFC.  Distribution of this document is
   unlimited. Comments should be sent to the DNS security mailing list
   <dns-security@tis.com> or to the author.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months.  Internet-Drafts may be updated, replaced, or obsoleted by
   other documents at any time.  It is not appropriate to use Internet-
   Drafts as reference material or to cite them other than as a
   ``working draft'' or ``work in progress.''

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

   [Changes from the previous draft: change date, update author info,
   add RFC 2119 reference]



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.







Donald E. Eastlake 3rd                                          [Page 1]

INTERNET-DRAFT                                        RSA/MD5 in the DNS


Table of Contents

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

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

      1. Introduction............................................3
      2. RSA Public KEY Resource Records.........................3

      3. RSA/MD5 SIG Resource Records............................4

      4. Performance Considerations..............................5
      5. Security Considerations.................................5

      References.................................................6
      Author's Address...........................................6
      Expiration and File Name...................................6


































Donald E. Eastlake 3rd                                          [Page 2]

INTERNET-DRAFT                                        RSA/MD5 in the DNS


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 [draft-ietf-
   dnssec-secext2-*].  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 [draft-ietf-dnssec-secext2-*].  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.











Donald E. Eastlake 3rd                                          [Page 3]

INTERNET-DRAFT                                        RSA/MD5 in the DNS


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 [draft-ietf-dnssec-secext2-*].  See
   [draft-ietf-dnssec-secext2-*] for fields in the SIG RR RDATA which
   precede the signature itself.

        hash = MD5 ( data )

        signature = ( 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 PKCS1,
   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 one octet shorter than the value of n.

   (The above specifications are identical to the corresponding part of
   Public Key Cryptographic Standard #1 [PKCS1].)

   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.










Donald E. Eastlake 3rd                                          [Page 4]

INTERNET-DRAFT                                        RSA/MD5 in the DNS


4. Performance Considerations

   General signature generation speeds are roughly the same for RSA and
   DSA [RFC xDSA].  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 [draft-ietf-dnssec-
   secext2-*] 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.

















Donald E. Eastlake 3rd                                          [Page 5]

INTERNET-DRAFT                                        RSA/MD5 in the DNS


References

   [NETSEC] - Network Security: PRIVATE Communications in a PUBLIC
   World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall
   Series in Computer Networking and Distributed Communications, 1995.

   [PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc.,
   3 June 1991, Version 1.4. [there is an ID on this and any resulting
   RFC could be substitutes if available in time]

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

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

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

   [draft-ietf-dnssec-secext2-*] - Domain Name System Security
   Extensions, D. Eastlake, C. Kaufman, January 1997.

   [RFC xDSA] - draft-ietf-dnssec-dss-*.txt

   [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
   318 Acton Street
   Carlisle, MA 01741 USA

   Telephone:   +1-978-287-4877
                +1-914-784-7913
   FAX:         +1-978-371-7148
   EMail:       dee3@us.ibm.com



Expiration and File Name

   This draft expires in April 1999.

   Its file name is draft-ietf-dnssec-rsa-01.txt.


Donald E. Eastlake 3rd                                          [Page 6]


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