DNSSEC RSA: RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)

Network Working Group                                    D. Eastlake 3rd
Request for Comments: 3110                                      Motorola
Obsoletes: 2537                                                 May 2001
Category: Standards Track


      RSA/SHA-1 SIGs and RSA KEYs 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 (2001).  All Rights Reserved.

Abstract

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

   Since the adoption of a Proposed Standard for RSA signatures in the
   DNS (Domain Name Space), advances in hashing have been made.  A new
   DNS signature algorithm is defined to make these advances available
   in SIG 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.

Acknowledgements

   Material and comments from the following have been incorporated and
   are gratefully acknowledged:

      Olafur Gudmundsson

      The IESG

      Charlie Kaufman

      Steve Wang

Table of Contents

   1. Introduction................................................... 2
   2. RSA Public KEY Resource Records................................ 3
   3. RSA/SHA1 SIG Resource Records.................................. 3
   4. Performance Considerations..................................... 4
   5. IANA Considerations............................................ 5
   6. Security Considerations........................................ 5
   References........................................................ 5
   Author's Address.................................................. 6
   Full Copyright Statement.......................................... 7

1. Introduction

   The Domain Name System (DNS) is the global hierarchical replicated
   distributed database system for Internet addressing, mail proxy, and
   other information [RFC1034, 1035, etc.].  The DNS has been extended
   to include digital signatures and cryptographic keys as described in
   [RFC2535].  Thus the DNS can now be secured and used for secure key
   distribution.

   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
   [RFC1321] 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, most top level domains, and some second and third level
   domains, 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
   is NOT RECOMMENDED.

   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 5 [RFC2535].  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 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.

3. RSA/SHA1 SIG Resource Records

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

   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 [FIP180],
   "|" 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
   [RFC2437], that is,

         hex 30 21 30 09 06 05 2B 0E 03 02 1A 05 00 04 14

   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 parts of
   Public Key Cryptographic Standard #1 [RFC2437].)

   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.

4. Performance Considerations

   General signature generation speeds are roughly the same for RSA and
   DSA [RFC2536].  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.

   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.  If a key is known to be used only for
   authentication, as is the case with DNSSEC, then an exponent of 3 is
   acceptable.  However other applications in the future may wish to
   leverage DNS distributed keys for applications that do require
   confidentiality.  For keys which might have such other uses, a more
   conservative choice would be 65537 (F4, the fourth fermat number).

   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 [RFC2671] 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. IANA Considerations

   The DNSSEC algorithm number 5 is allocated for RSA/SHA1 SIG RRs and
   RSA KEY RRs.

6. Security Considerations

   Many of the general security considerations 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".

References

   [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.

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, November 1987.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2437]  Kaliski, B. and J. Staddon, "PKCS #1: RSA Cryptography
              Specifications Version 2.0", RFC 2437, October 1998.

   [RFC2535]  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

   [RFC2536]  Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
              (DNS)", RFC 2536, March 1999.

   [RFC2537]  Eastlake, D., "RSA/MD5 KEYs and SIGs in the Domain Name
              System (DNS)", RFC 2537, March 1999.

   [RFC2541]  Eastlake, D., "DNS Security Operational Considerations",
              RFC 2541, March 1999.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
              2671, August 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
   Motorola
   155 Beaver Street
   Milford, MA 01757 USA

   Phone:   +1-508-261-5434 (w)
            +1-508-634-2066 (h)
   Fax      +1-508-261-4777 (w)
   EMail:   Donald.Eastlake@motorola.com

Full Copyright Statement

   Copyright (C) The Internet Society (2001).  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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.