INTERNET-DRAFT DSA KEYs and SIGs in the DNS
January 1998
Expires July 1998
DSA KEYs and SIGs in the Domain Name System (DNS)
--- ---- --- ---- -- --- ------ ---- ------ -----
Donald E. Eastlake 3rd
Status of This Document
This draft, file name draft-ietf-dnssec-dss-02.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.
This document is an Internet-Draft. 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. Internet-Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet-
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
To learn the current status of any Internet-Draft, please check the
1id-abstracts.txt listing contained in the Internet-Drafts Shadow
Directories on ds.internic.net (East USA), ftp.isi.edu (West USA),
nic.nordu.net (North Europe), ftp.nis.garr.it (South Europe),
munnari.oz.au (Pacific Rim), or ftp.is.co.za (Africa).
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 E. 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................................4
3. DSA SIG Resource Records................................5
4. Performance Considerations..............................6
5. Security Considerations.................................6
References.................................................7
Author's Address...........................................7
Expiration and File Name...................................7
Donald E. 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 [draft-ietf-
dnssec-secext2-*]. 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 [Schneier]. Implementation
of DSA is mandatory for DNS security.
Donald E. Eastlake 3rd [Page 3]
INTERNET-DRAFT DSA in the DNS
2. DSA KEY Resource Records
DSA public keys are stored in the DNS as KEY RRs using algorithm
number 3 [draft-ietf-dnssec-secext2-*]. 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] 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 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 modulus 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
Y = G**X mod P
Donald E. Eastlake 3rd [Page 4]
INTERNET-DRAFT DSA in the DNS
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 [draft-ietf-dnssec-secext2-*] 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 [draft-ietf-dnssec-
secext2-*]. Then the following steps are taken, as specified in
[FIPS 186], 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
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.
Donald E. Eastlake 3rd [Page 5]
INTERNET-DRAFT DSA in the DNS
4. Performance Considerations
General signature generation speeds are roughly the same for RSA [RFC
xRSA] 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
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.
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.
Donald E. Eastlake 3rd [Page 6]
INTERNET-DRAFT DSA in the DNS
References
[FIPS 186] - U.S. Federal Information Processing Standard: Digital
Signature Standard.
[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", 12/29/1994.
[draft-ietf-dnssec-secext2-*] - Domain Name System Security
Extensions, D. Eastlake, C. Kaufman, January 1997.
[RFC xRSA] - draft-ietf-dnssec-rsa-*.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-dss-02.txt.
Donald E. Eastlake 3rd [Page 7]
