INTERNET-DRAFT Diffie-Hellman Keys in the DNS
November 1998
Expires May 1999
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-03.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
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
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[Changes from previous draft: add IANA considerations section, update
author info, update file name and dates, add specific well known
groups]
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
Tom Markson
Hemma Prafullchandra
In addition, the following person provided useful comments that have
been incorporated:
Ran Atkinson
Thomas Narten
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
1.1 About This Document....................................4
1.2 About Diffie-Hellman...................................4
2. Diffie-Hellman KEY Resource Records.....................5
3. Performance Considerations..............................6
4. IANA Considerations.....................................6
5. Security Considerations.................................6
References.................................................7
Author's Address...........................................7
Expiration and File Name...................................7
Appendix A: Well known prime/generator pairs...............8
A.1. Well-Known Group 1: A 768 bit prime..................8
A.2. Well-Known Group 2: A 1024 bit prime.................8
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.
1.1 About This Document
This document describes how to store Diffie-Hellman keys in the DNS.
Familiarity with the Diffie-Hellman key exchange algorithm is assumed
[Schneier].
1.2 About Diffie-Hellman
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 65,536
prime/generator pairs and the generator length SHOULD be zero. See
Appedix A for defined table entries and Section 4 for information on
allocating additional table entries. 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 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. IANA Considerations
Assignment of meaning to Prime Lengths of 0 and 3 through 15 requires
an IETF consensus.
Well known prime/generator pairs number 0x0000 through 0x07FF can
only be assigned by an IETF standards action and this Proposed
Standard assigns 0x0001 through 0x0002. Pairs number 0s0800 through
0xBFFF can be assigned based on RFC documentation. Pairs number
0xC000 through 0xFFFF are available for private use and are not
centrally coordinated. Use of such private pairs outside of a closed
environment may result in conflicts.
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 important
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
IBM
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1-978-287-4877
+1-914-784-7913
FAX: +1-978-371-7148
EMail: dee3@us.ibm.com
Expiration and File Name
This draft expires in April 1999.
Its file name is draft-ietf-dnssec-dhk-03.txt.
Donald E. Eastlake 3rd [Page 7]
INTERNET-DRAFT Diffie-Hellman Keys in the DNS
Appendix A: Well known prime/generator pairs
These numbers are copied from the IPSEC effort where the derivation of
these values is more fully explained and additional information is available.
Richard Schroeppel performed all the mathematical and computational
work for this appendix.
A.1. Well-Known Group 1: A 768 bit prime
The prime is 2^768 - 2^704 - 1 + 2^64 * { [2^638 pi] + 149686 }. Its
decimal value is
155251809230070893513091813125848175563133404943451431320235
119490296623994910210725866945387659164244291000768028886422
915080371891804634263272761303128298374438082089019628850917
0691316593175367469551763119843371637221007210577919
Prime modulus: Length (32 bit words): 24, Data (hex):
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF
Generator: Length (32 bit words): 1, Data (hex): 2
A.2. Well-Known Group 2: A 1024 bit prime
The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
Its decimal value is
179769313486231590770839156793787453197860296048756011706444
423684197180216158519368947833795864925541502180565485980503
646440548199239100050792877003355816639229553136239076508735
759914822574862575007425302077447712589550957937778424442426
617334727629299387668709205606050270810842907692932019128194
467627007
Prime modulus: Length (32 bit words): 32, Data (hex):
FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A637ED6B 0BFF5CB6 F406B7ED
EE386BFB 5A899FA5 AE9F2411 7C4B1FE6 49286651 ECE65381
FFFFFFFF FFFFFFFF
Generator: Length (32 bit words): 1, Data (hex): 2
Donald E. Eastlake 3rd [Page 8]