RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)
RFC 2537
Document  Type 
RFC  Proposed Standard
(March 1999; No errata)
Obsoleted by RFC 3110



Last updated  20130302  
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IESG  IESG state  RFC 2537 (Proposed Standard)  
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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. 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 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) 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. nShow full document text