INTERNET-DRAFT DSA KEYs and SIGs in the DNS
September 1997
Expires March 1998
DSA KEYs and SIGs in the Domain Name System
--- ---- --- ---- -- --- ------ ---- ------
Donald E. Eastlake 3rd
Status of This Document
This draft, file name draft-ietf-dnssec-dss-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
<dnssec@tis.com> or to the author.
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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.
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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................................5
4. Performance Considerations..............................6
5. Security Considerations.................................6
References.................................................7
Author's Address...........................................7
Expiration and File Name...................................7
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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 2065. Thus the
DNS can now be secured and 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].
2. DSA KEY Resource Records
DSA public keys are stored in the DNS as KEY RRs using algorithm
number 3 (see RFC 2065). The structure of the RDATA portion of this
RR is as shown below. The first 4 octets, including the flags,
protocol, and algorithm fields are common to all KEY RRs. The
remainder, from Q through Y is the "public key" part of the 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.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| flags | protocol | algorithm=3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T | |
|-+-+-+-+-+-+-+-+ Q (20 octets) |
| .../
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| P (64+T*8 octets) /
| .../
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| G (64+8*T octets) /
| .../
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Y (64+8*T 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
Donald E. Eastlake 3rd [Page 3]
INTERNET-DRAFT DSA in the DNS
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
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3. DSA SIG Resource Records
The signature portion of the SIG RR RDATA area, when using the US
Digital Signature Algorithm, is shown below. See RFC 2065 for fields
in the SIG RR RDATA which precede the signature itself.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| T | |
|-+-+-+-+-+-+-+-+ R, 20 octets |
| .../
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| S, 20 octets |
| .../
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The data signed is determined as specified in RFC 2065. 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 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.
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4. Performance Considerations
Signature generation speeds are roughly the same for RSA and DSA.
Key generation is faster for DSA. Signature verification is an order
of magnitude slower than RSA.
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, an authenticating SIG RR will also be returned.
5. Security Considerations
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 [Schneier]. The leakage of an entire
DSA private key in only two DSA signatures has been demonstrated.
Thus DSA provides security only if trusted implementations, including
trusted random number generation, are used.
The key size limitation of a maximum of 1024 bits ( T = 8 ) limits
the security of DSA. For particularly critical high level keys,
sizes of 2048 and larger are now used in RSA.
Many of the general security consideration in RFC 2065 apply. Of
course, the DSA key stored in the DNS for an entity should not be
trusted unless it has been obtain via a trusted DNS resolver that
vouches for its security or unless the application using the key has
done a similar authentication.
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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.
[RFC 2065] - Domain Name System Security Extensions, D. Eastlake, C.
Kaufman, January 1997.
[Schneier] - Bruce Schneier, "Applied Cryptography: Protocols,
Algorithms, and Source Code in C", 1996, John Wiley and Sons
Author's Address
Donald E. Eastlake 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 508 287 4877
+1 703 620-4200 (main office, Reston, VA)
FAX: +1 508 371 7148
EMail: dee@cybercash.com
Expiration and File Name
This draft expires in March 1998.
Its file name is draft-ietf-dnssec-dss-00.txt.
Donald E. Eastlake 3rd [Page 7]
