INTERNET-DRAFT RSA/MD5 KEYs and SIGs in the DNS
January 1998
Expires July 1998
RSA/MD5 KEYs and SIGs in the Domain Name System (DNS)
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Donald E. Eastlake 3rd
Status of This Document
This draft, file name draft-ietf-dnssec-rsa-00.txt, is intended to be
become a Proposed Standard RFC. Distribution of this document is
unlimited. Comments should be sent to the DNS security mailing list
<dns-security@tis.com> or to the author.
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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.
Donald E. Eastlake 3rd [Page 1]
INTERNET-DRAFT RSA/MD5 in the DNS
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3
2. RSA Public KEY Resource Records.........................3
3. RSA/MD5 SIG Resource Records............................4
4. Performance Considerations..............................5
5. Security Considerations.................................5
References.................................................6
Author's Address...........................................6
Expiration and File Name...................................6
Donald E. Eastlake 3rd [Page 2]
INTERNET-DRAFT RSA/MD5 in the DNS
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 [draft-ietf-
dnssec-secext2-*]. 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.
2. RSA Public KEY Resource Records
RSA public keys are stored in the DNS as KEY RRs using algorithm
number 1 [draft-ietf-dnssec-secext2-*]. 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.
Donald E. Eastlake 3rd [Page 3]
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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 [draft-ietf-dnssec-secext2-*]. See
[draft-ietf-dnssec-secext2-*] for fields in the SIG RR RDATA which
precede the signature itself.
hash = MD5 ( data )
signature = ( 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 PKCS1,
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 one octet shorter than the value of n.
(The above specifications are identical to the corresponding part of
Public Key Cryptographic Standard #1 [PKCS1].)
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.
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4. Performance Considerations
General signature generation speeds are roughly the same for RSA and
DSA [RFC xDSA]. 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
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 [draft-ietf-dnssec-
secext2-*] 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.
Donald E. Eastlake 3rd [Page 5]
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References
[NETSEC] - Network Security: PRIVATE Communications in a PUBLIC
World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall
Series in Computer Networking and Distributed Communications, 1995.
[PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc.,
3 June 1991, Version 1.4.
[RFC 1034] - P. Mockapetris, "Domain names - concepts and
facilities", 11/01/1987.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and
specification", 11/01/1987.
[RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April
1992.
[draft-ietf-dnssec-secext2-*] - Domain Name System Security
Extensions, D. Eastlake, C. Kaufman, January 1997.
[RFC xDSA] - draft-ietf-dnssec-dss-*.txt
[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
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 978 287 4877
+1 703 620-4200 (main office, Reston, Virginia)
FAX: +1 978 371 7148
EMail: dee@cybercash.com
Expiration and File Name
This draft expires in July 1998.
Its file name is draft-ietf-dnssec-rsa-00.txt.
Donald E. Eastlake 3rd [Page 6]
