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Network Working Group                                        D. EastlakeRequest for Comments: 2537                                           IBMCategory: Standards Track                                     March 1999RSA/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...................................................11. Introduction............................................12. RSA Public KEY Resource Records.........................23. RSA/MD5 SIG Resource Records............................24. Performance Considerations..............................35. Security Considerations.................................4   References.................................................4   Author's Address...........................................5   Full Copyright Statement...................................61. 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   recommended.   The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"   in this document are to be interpreted as described inRFC 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 largerEastlake                    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   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.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   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.comEastlake                    Standards Track                     [Page 5]

RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999Full 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   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.Eastlake                    Standards Track                     [Page 6]