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INTERNET-DRAFT DSA KEYs and SIGs in the DNS
OBSOLETES: RFC 2536 Donald Eastlake 3rd
Motorola
Expires: January 2002 July 2001
DSA KEYs and SIGs in the Domain Name System (DNS)
--- ---- --- ---- -- --- ------ ---- ------ -----
<draft-ietf-dnsext-rfc2536bis-dsa-00.txt>
Donald E. Eastlake 3rd
Status of This Document
This draft is intended to be become a Draft Standard RFC.
Distribution of this document is unlimited. Comments should be sent
to the DNS extensions working group mailing list
<namedroppers@ops.ietf.org> or to the author.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
A standard method for storing US Government Digital Signature
Algorithm keys and signatures in the Domain Name System is described
which utilizes DNS KEY and SIG resource records.
Donald Eastlake 3rd [Page 1]
INTERNET-DRAFT DSA in the DNS
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3
2. DSA KEY Resource Records................................3
3. DSA SIG Resource Records................................4
4. Performance Considerations..............................4
5. Security Considerations.................................5
6. IANA Considerations.....................................5
References.................................................6
Author's Address...........................................6
Expiration and File Name...................................7
Donald Eastlake 3rd [Page 2]
INTERNET-DRAFT DSA 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 [RFC 2535]. Thus
the DNS can now be secured and can be used for secure key
distribution.
This document describes how to store 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 KEY Resource Records
DSA public keys are stored in the DNS as KEY RRs using algorithm
number 3 [RFC 2535]. The structure of the algorithm specific portion
of the RDATA part of this RR is as shown below. These fields, from Q
through Y are the "public key" part of the DSA KEY RR.
The period of key validity is not in the KEY RR but is indicated by
the SIG RR(s) which signs and authenticates the KEY RR(s) at that
domain name.
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 for algorithm 3
if the T octet is greater than 8 is reserved and the remainder of the
RDATA portion 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 so 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
Donald Eastlake 3rd [Page 3]
INTERNET-DRAFT DSA in the DNS
Y = G**X mod P
3. DSA SIG Resource Records
The signature portion of the SIG RR RDATA area, when using the US
Digital Signature Algorithm, is shown below with fields in the order
they occur. See [RFC 2535] for fields in the SIG RR RDATA which
precede the signature itself.
Field Size
----- ----
T 1 octet
R 20 octets
S 20 octets
The data signed is determined as specified in [RFC 2535]. 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 has funcation see [FIPS 180-1] and
[draft-sha1].
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 specified.
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 KEY RRs used in domain name system (DNS) data
Donald Eastlake 3rd [Page 4]
INTERNET-DRAFT DSA in the DNS
authentication.
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 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.
The key size limitation of a maximum of 1024 bits ( T = 8 ) in the
current DSA standard may limit the security of DSA. For particularly
critical 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
manipulated rather than random numbers are used, very 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 requires an IETF standards actions. It is intended
that values unallocated herein be used to cover future extensions of
the DSS standard.
Donald Eastlake 3rd [Page 5]
INTERNET-DRAFT DSA in the DNS
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, January 2000.
[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 1750] - D. Eastlake, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", December 1994.
[RFC 2535] - Domain Name System Security Extensions, D. Eastlake,
March 1999.
[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.
[draft-sha1] - US Secure Hash Algorithm 1 (SHA1), draft-eastlake-
sha1-02.txt, work in progress, D. Eastlake, in IESG queue for
approval as an Informational RFC.
[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
Motorola
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1-508-261-5434(w)
+1-508-634-2066(h)
FAX: +1-508-261-4447(w)
EMail: Donald.Eastlake@motorola.com
Donald Eastlake 3rd [Page 6]
INTERNET-DRAFT DSA in the DNS
Expiration and File Name
This draft expires in January 2002.
Its file name is draft-ietf-dnsext-rfc2536bis-dsa-00.txt.
Donald Eastlake 3rd [Page 7]