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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.
+
+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...................................................1
+   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
+   References.................................................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.
+
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+Eastlake                    Standards Track                     [Page 1]
+
+RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999
+
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+   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 [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)
+
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+
+Eastlake                    Standards Track                     [Page 2]
+
+RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999
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+   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
+
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+
+Eastlake                    Standards Track                     [Page 3]
+
+RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999
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+   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.
+
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+Eastlake                    Standards Track                     [Page 4]
+
+RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999
+
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+   [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.com
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+Eastlake                    Standards Track                     [Page 5]
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+RFC 2537            RSA/MD5 KEYs and SIGs in the DNS          March 1999
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+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
+   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.
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+Eastlake                    Standards Track                     [Page 6]
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