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Versions: 00 01 02 RFC 3110

INTERNET-DRAFT                              RSA SIGs and KEYs in the DNS
OBSOLETES RFC 2537                                           August 2000
                                                   Expires February 2001

      RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)
      --------- ---- --- --- ---- -- --- ------ ---- ------ -----
                         D. Eastlake 3rd

Status of This Document

   This draft is intended to be become a Proposed Standard RFC.
   Distribution of this document is unlimited. Comments should be sent
   to the DNS extensions mailing list <namedroppers@ops.ietf.org> or to
   the author.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC 2026.  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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   Since the adoption of a Proposed Standard for RSA signatures in the
   DNS [RFC 2537], advances in hashing have been made.  A new DNS
   signature algorithm is defined to make these advances available in
   SIG resource records (RRs).  The use of the previously specified
   weaker mechanism is deprecated.  The algorithm number of the RSA KEY
   RR is changed to correspond to this new SIG algorithm.  No other
   changes are made to DNS security.

D. Eastlake 3rd                                                 [Page 1]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS

Table of Contents

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

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

      1. Introduction............................................3
      2. RSA Public KEY Resource Records.........................3
      3. RSA/SHA1 SIG Resource Records...........................4
      4. Performance Considerations..............................5
      5. IANA Considerations.....................................6
      6. Security Considerations.................................6


      Author's Address...........................................8
      Expiration and File Name...................................8

D. Eastlake 3rd                                                 [Page 2]

INTERNET-DRAFT                                       RSA/SHA1 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 [RFC 1034, 1035, etc.]. 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

   Familiarity with the RSA and SHA-1 algorithms is assumed [Schneier,
   FIP180] in this document.

   [RFC 2537] described how to store RSA keys and RSA/MD5 based
   signatures in the DNS.  However, since the adoption of [RFC 2537],
   continued cryptographic research has revealed hints of weakness in
   the MD5 [RFC 1321] algorithm used in [RFC 2537]. The SHA1 Secure Hash
   Algorithm [FIP180], which produces a larger hash, has been developed.
   By now there has been sufficient experience with SHA1 that it is
   generally acknowledged to be stronger than MD5.  While this stronger
   hash is probably not needed today in most secure DNS zones, critical
   zones such a root and most TLDs are sufficiently valuable targets
   that it would be negligent not to provide what are generally agreed
   to be stronger mechanisms. Furthermore, future advances in
   cryptanalysis and/or computer speeds may require a stronger hash
   everywhere.  In addition, the additional computation required by SHA1
   above that required by MD5 is insignificant compared with the
   computational effort required by the RSA modular exponentiation.

   This document describes how to produce RSA/SHA1 SIG RRs in Section 3
   and, so as to completely replace [RFC 2537], describes how to produce
   RSA KEY RRs in Section 2.

   Implementation of the RSA algorithm in DNS with SHA1 is MANDATORY for
   DNSSEC.  The generation of RSA/MD5 SIG RRs as described in [RFC 2537]

   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT
   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 (TBD, suggest 5) [RFC 2535].  The structure of the algorithm
   specific portion of the RDATA part of such RRs is as shown below.

D. Eastlake 3rd                                                 [Page 3]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS

        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 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.

   Note: KEY RRs for use with RSA/SHA1 DNS signatures MUST use this
   algorithm number (rather than the algorithm number specified in the
   obsoleted [RFC 2537]).

   [Note: This changes the algorithm number for RSA KEY RRs to be the
   same as the new algorithm number for RSA/SHA1 SIGs.  Or, from another
   point of view, adds a new KEY RR with a new algorithm number that is
   otherwise identical to the RSA KEY RR defined in RFC 2537.  In fact,
   RSA KEYs do not depend on selection of hash algorihtm.  An
   alternative would be to leave RSA KEY RRs unchanged and indicated by
   algorithm 1.]

3. RSA/SHA1 SIG Resource Records

   RSA/SHA1 signatures are stored in the DNS using SIG resource records
   (RRs) with algorithm number (TBD, 5 suggested).

   The signature portion of the SIG RR RDATA area, when using the
   RSA/SHA1 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 = SHA1 ( data )

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

   where SHA1 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 SHA1 algorithm designator prefix required in PKCS1 [RFC
   2437], that is,

D. Eastlake 3rd                                                 [Page 4]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS

     hex (to be supplied).

   This prefix is included to make it easier to use standard
   cryptographic libraries.  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 [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.  These are protocol limits.  For a discussion of key size see
   [RFC 2541].

   Leading zero bytes are permitted in the RSA/SHA1 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 DNS overhead.  Larger
   transfers will perform correctly and extensions have been
   standardized [RFC 2671] 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.

D. Eastlake 3rd                                                 [Page 5]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS

5. IANA Considerations

   The DNSSEC algorithm number (TBD, 5 suggested) is allocated for

6. 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 particularly critical applications,
   implementers are encouraged to consider the range of available
   algorithms and key sizes.  See also [RFC 2541], "DNS Security
   Operational Considerations".

D. Eastlake 3rd                                                 [Page 6]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS


   [FIP180] -U.S. Department of Commerce, "Secure Hash Standard", FIPS
   PUB 180-1, 17 Apr 1995.

   [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.

   [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

   [RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", March 1997.

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

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

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

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

   [RDC 2541] - D. Eastlake, "DNS Security Operational Considerations",
   March 1999.

   [RFC 2671] - P. Vixie, "Extension Mechanisms for DNS (EDNS0)", August

   [Schneier] - Bruce Schneier, "Applied Cryptography Second Edition:
   protocols, algorithms, and source code in C", 1996, John Wiley and
   Sons, ISBN 0-471-11709-9.

D. Eastlake 3rd                                                 [Page 7]

INTERNET-DRAFT                                       RSA/SHA1 in the DNS

Author's Address

   Donald E. Eastlake 3rd
   140 Forest Avenue
   Hudson, MA 01749 USA

   Telephone:   +1-978-562-2827 (h)
                +1-508-261-5434 (w)
   FAX:         +1-508-261-4777 (w)
   EMail:       Donald.Eastlake@motorola.com

Expiration and File Name

   This draft expires in February 2001.

   Its file name is draft-eastlake-dnsext-rsa-00.txt.

D. Eastlake 3rd                                                 [Page 8]

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