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INTERNET-DRAFT DSA Information in the DNS
OBSOLETES: RFC 2536 Donald E. Eastlake 3rd
Motorola Laboratories
Expires: February 2005 August 2004
DSA Keying and Signature Information in the DNS
--- ------ --- --------- ----------- -- --- ---
<draft-ietf-dnsext-rfc2536bis-dsa-04.txt>
Donald E. Eastlake 3rd
Status of This Document
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
or will be disclosed, and any of which I become aware will be
disclosed, in accordance with RFC 3668.
Distribution of this document is unlimited. Comments should be sent
to the DNS extensions working group mailing list
<namedroppers@ops.ietf.org>.
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The list of current Internet-Drafts can be accessed at
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Abstract
The standard method of encoding US Government Digital Signature
Algorithm keying and signature information for use in the Domain Name
System is specified.
Copyright Notice
Copyright (C) The Internet Society 2004. All Rights Reserved.
D. Eastlake 3rd [Page 1]
INTERNET-DRAFT DSA Information in the DNS
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Copyright Notice...........................................1
Table of Contents..........................................2
1. Introduction............................................3
2. DSA Keying Information..................................3
3. DSA Signature Information...............................4
4. Performance Considerations..............................4
5. Security Considerations.................................5
6. IANA Considerations.....................................5
Copyright and Disclaimer...................................5
Normative References.......................................7
Informative References.....................................7
Authors Address............................................8
Expiration and File Name...................................8
D. Eastlake 3rd [Page 2]
INTERNET-DRAFT DSA Information 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 [RFC 1034, 1035]. The DNS has been extended to
include digital signatures and cryptographic keys as described in
[RFC intro, proto, records] and additional work is underway which
would require the storage of keying and signature information in the
DNS.
This document describes how to encode US Government Digital Signature
Algorithm (DSA) keys and signatures in the DNS. Familiarity with the
US Digital Signature Algorithm is assumed [FIPS 186-2, Schneier].
2. DSA Keying Information
When DSA public keys are stored in the DNS, the structure of the
relevant part of the RDATA part of the RR being used is as shown
below.
The period of key validity is not included in this data but is
indicated separately, for example by an RR which signs and
authenticates the RR containing the keying information.
Field Size
----- ----
T 1 octet
Q 20 octets
P 64 + T*8 octets
G 64 + T*8 octets
Y 64 + T*8 octets
As described in [FIPS 186-2] and [Schneier], T is a key size
parameter chosen such that 0 <= T <= 8. (The meaning if the T octet
is greater than 8 is reserved and the remainder of the data may have
a different format in that case.) Q is a prime number selected at
key generation time such that 2**159 < Q < 2**160 so Q is always 20
octets long and, as with all other fields, is stored in "big-endian"
network order. P, G, and Y are calculated as directed by the [FIPS
186-2] key generation algorithm [Schneier]. P is in the range
2**(511+64T) < P < 2**(512+64T) and thus is 64 + 8*T octets long. G
and Y are quantities modulo P and so can be up to the same length as
P and are allocated fixed size fields with the same number of octets
as P.
During the key generation process, a random number X must be
generated such that 1 <= X <= Q-1. X is the private key and is used
in the final step of public key generation where Y is computed as
D. Eastlake 3rd [Page 3]
INTERNET-DRAFT DSA Information in the DNS
Y = G**X mod P
3. DSA Signature Information
The portion of the RDATA area used for US Digital Signature Algorithm
signature information is shown below with fields in the order they
occur.
Field Size
----- ----
T 1 octet
R 20 octets
S 20 octets
The data signed must be determined. Then the following steps are
taken, as specified in [FIPS 186-2], where Q, P, G, and Y are as
specified in the public key [Schneier]:
hash = SHA-1 ( data )
Generate a random K such that 0 < K < Q.
R = ( G**K mod P ) mod Q
S = ( K**(-1) * (hash + X*R) ) mod Q
For infromation on the SHA-1 hash function see [FIPS 180-1] and [RFC
3174].
Since Q is 160 bits long, R and S can not be larger than 20 octets,
which is the space allocated.
T is copied from the public key. It is not logically necessary in
the SIG but is present so that values of T > 8 can more conveniently
be used as an escape for extended versions of DSA or other algorithms
as later standardized.
4. Performance Considerations
General signature generation speeds are roughly the same for RSA [RFC
3110] and DSA. 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 than
RSA when the RSA public exponent is chosen to be small, as is
recommended for some applications.
D. Eastlake 3rd [Page 4]
INTERNET-DRAFT DSA Information in the DNS
Current DNS implementations are optimized for small transfers,
typically less than 512 bytes including DNS overhead. Larger
transfers will perform correctly and extensions have been
standardized [RFC 2671] to make larger transfers more efficient, it
is still advisable at this time to make reasonable efforts to
minimize the size of RR sets containing keying and/or signature
inforamtion consistent with adequate security.
5. Security Considerations
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.
The key size limitation of a maximum of 1024 bits ( T = 8 ) in the
current DSA standard may limit the security of DSA. For particular
applications, implementors are encouraged to consider the range of
available algorithms and key sizes.
DSA assumes the ability to frequently generate high quality random
numbers. See [RFC 1750] for guidance. DSA is designed so that if
biased rather than random numbers are used, high bandwidth covert
channels are possible. See [Schneier] and more recent research. The
leakage of an entire DSA private key in only two DSA signatures has
been demonstrated. DSA provides security only if trusted
implementations, including trusted random number generation, are
used.
6. IANA Considerations
Allocation of meaning to values of the T parameter that are not
defined herein (i.e., > 8 ) requires an IETF standards actions. It
is intended that values unallocated herein be used to cover future
extensions of the DSS standard.
Copyright and Disclaimer
Copyright (C) The Internet Society 2004. This document is subject to
the rights, licenses and restrictions contained in BCP 78 and except
as set forth therein, the authors retain all their rights.
D. Eastlake 3rd [Page 5]
INTERNET-DRAFT DSA Information in the DNS
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
D. Eastlake 3rd [Page 6]
INTERNET-DRAFT DSA Information in the DNS
Normative References
[FIPS 180-1] - U.S. Federal Information Processing Standard: Secure
Hash Standard, April 1995.
[FIPS 186-2] - U.S. Federal Information Processing Standard: Digital
Signature Standard, 27 January 2000.
[RFC records] - "Resource Records for the DNS Security Extensions",
R. Arends, R. Austein, M. Larson, D. Massey, S. Rose, work in
progress, draft-ietf-dnsext-dnssec-records- *.txt.
Informative References
[RFC 1034] - "Domain names - concepts and facilities", P.
Mockapetris, 11/01/1987.
[RFC 1035] - "Domain names - implementation and specification", P.
Mockapetris, 11/01/1987.
[RFC 1750] - "Randomness Recommendations for Security", D. Eastlake,
S. Crocker, J. Schiller, December 1994.
[RFC intro] - "DNS Security Introduction and Requirements", R.
Arends, M. Larson, R. Austein, D. Massey, S. Rose, work in progress,
draft-ietf-dnsext-dnssec-intro-*.txt.
[RFC protocol] - "Protocol Modifications for the DNS Security
Extensions", R. Arends, M. Larson, R. Austein, D. Massey, S. Rose,
work in progress, draft-ietf-dnsext-dnssec-protocol-*.txt.
[RFC 2671] - "Extension Mechanisms for DNS (EDNS0)", P. Vixie, August
1999.
[RFC 3110] - "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System
(DNS)", D. Eastlake 3rd. May 2001.
[RFC 3174] - "US Secure Hash Algorithm 1 (SHA1)", D. Eastlake, P.
Jones, September 2001.
[Schneier] - "Applied Cryptography Second Edition: protocols,
algorithms, and source code in C" (second edition), Bruce Schneier,
1996, John Wiley and Sons, ISBN 0-471-11709-9.
D. Eastlake 3rd [Page 7]
INTERNET-DRAFT DSA Information in the DNS
Authors Address
Donald E. Eastlake 3rd
Motorola Labortories
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1-508-786-7554(w)
+1-508-634-2066(h)
EMail: Donald.Eastlake@motorola.com
Expiration and File Name
This draft expires in February 2005.
Its file name is draft-ietf-dnsext-rfc2536bis-dsa-04.txt.
D. Eastlake 3rd [Page 8]
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