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.
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)
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 precomputation, 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
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 MessageDigest 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.
[Schneier] Bruce Schneier, "Applied Cryptography Second Edition:
protocols, algorithms, and source code in C", 1996, John
Wiley and Sons, ISBN 0471117099.
Author's Address
Donald E. Eastlake 3rd
IBM
65 Shindegan Hill Road, RR #1
Carmel, NY 10512
Phone: +19142762668(h)
+19147847913(w)
Fax: +19147843833(w)
EMail: dee3@us.ibm.com
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