INTERNET-DRAFT Diffie-Hellman Keys in the DNS
March 1998
Expires September 1998
Storage of Diffie-Hellman Keys 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-dhk-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,
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Donald E. Eastlake 3rd [Page 1]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
Abstract
A standard method for storing Diffie-Hellman keys in the Domain Name
System is described which utilizes DNS KEY resource records.
Acknowledgements
Part of the format for Diffie-Hellman keys and the description
thereof was taken from an Internet draft by:
Ashar Aziz <ashar.aziz@eng.sun.com>
Tom Markson <markson@incog.com>
Hemma Prafullchandra <hemma@eng.sun.com>
In addition, the following person provided useful comments that have
been incorporated:
Ran Atkinson <rja@inet.org>
Donald E. Eastlake 3rd [Page 2]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgements...........................................2
Table of Contents..........................................3
1. Introduction............................................4
2. Diffie-Hellman KEY Resource Records.....................5
3. Performance Considerations..............................6
4. Security Considerations.................................6
References.................................................7
Author's Address...........................................7
Expiration and File Name...................................7
Donald E. Eastlake 3rd [Page 3]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
1. Introduction
The Domain Name System (DNS) is the current global hierarchical
replicated distributed database system for Internet addressing, mail
proxy, and similar information. The DNS has been extended to include
digital signatures and cryptographic keys as described in [draft-
ietf-dnssec-secext2-*.txt]. Thus the DNS can now be used for secure
key distribution.
This document describes how to store Diffie-Hellman keys in the DNS.
Familiarity with the Diffie-Hellman key exchange algorithm is assumed
[Schneier].
Diffie-Hellman requires two parties to interact to derive keying
information which can then be used for authentication. Since DNS SIG
RRs are primarily used as stored authenticators of zone information
for many different resolvers, no Diffie-Hellman algorithm SIG RR is
defined. For example, assume that two parties have local secrets "i"
and "j". Assume they each respectively calculate X and Y as follows:
X = g**i ( mod p )
Y = g**j ( mod p )
They exchange these quantities and then each calculates a Z as
follows:
Zi = Y**i ( mod p )
Zj = X**j ( mod p )
Zi and Zj will both be equal to g**(ij)(mod p) and will be a shared
secret between the two parties that an adversary who does not know i
or j will not be able to learn from the exchanged messages (unless
the adversary can derive i or j by performing a discrete logarithm
mod p which is hard for strong p and g).
The private key for each party is their secret i (or j). The public
key is the pair p and g which must be the same for the parties and
their individual X (or Y).
Donald E. Eastlake 3rd [Page 4]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
2. Diffie-Hellman KEY Resource Records
Diffie-Hellman keys are stored in the DNS as KEY RRs using algorithm
number 2. 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 as described in [draft-
ietf-dnssec-secext2-*.txt]. The remainder, from prime length through
public value 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| KEY flags | protocol | algorithm=2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| prime length (or flag) | prime (p) (or special) /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ prime (p) (variable length) | generator length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| generator (g) (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| public value length | public value (variable length)/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ public value (g^i mod p) (variable length) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prime length is length of the Diffie-Hellman prime (p) in bytes if it
is 16 or greater. Prime contains the binary representation of the
Diffie-Hellman prime with most significant byte first (i.e., in
network order). If "prime length" field is 1 or 2, then the "prime"
field is actually an unsigned index into a table of up to 65,536
predefined prime/generator pairs to be defined in which case the
generator length should be zero. The meaning of a zero or 3 through
15 value for "prime length" is reserved.
Generator length is the length of the generator (g) in bytes.
Generator is the binary representation of generator with most
significant byte first. PublicValueLen is the Length of the Public
Value (g**i (mod p)) in bytes. PublicValue is the binary
representation of the DH public value with most significant byte
first.
The corresponding algorithm=2 SIG resource record is not used so no
format for it is defined.
Donald E. Eastlake 3rd [Page 5]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
3. Performance Considerations
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.
4. 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.
In addition, the usual Diffie-Hellman key strength considerations
apply. (p-1)/2 should also be prime, g should be primitive mod p, p
should be "large", etc. [Schneier]
Donald E. Eastlake 3rd [Page 6]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
References
[RFC 1034] - P. Mockapetris, "Domain names - concepts and
facilities", 11/01/1987.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and
specification", 11/01/1987.
[draft-ietf-dnssec-secext2-*.txt] - Domain Name System Security
Extensions, D. Eastlake.
[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: +1 978 287 4877
+1 703 620-4200 (main office, Reston, VA)
FAX: +1 978 371 7148
EMail: dee@cybercash.com
Expiration and File Name
This draft expires in September 1998.
Its file name is draft-ietf-dnssec-dhk-02.txt.
Donald E. Eastlake 3rd [Page 7]
