RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)

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Abstract

Table of Contents

1. Introduction

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

        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

  hash = MD5 ( data )

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

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

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 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

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

Author's Address

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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