INTERNET-DRAFT Diffie-Hellman Information in the DNS
OBSOLETES: RFC 2539 Donald E. Eastlake 3rd
Motorola Laboratories
Expires: January 2004 July 2003
Storage of Diffie-Hellman Keying Information in the DNS
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Status of This Document
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This document is an Internet Draft and is in full conformance with
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Abstract
A standard method for encoding Diffie-Hellman keys in the Domain Name
System is described.
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Acknowledgements
Part of the format for Diffie-Hellman keys and the description
thereof was taken from a work in progress by Ashar Aziz, Tom Markson,
and Hemma Prafullchandra. In addition, the following persons
provided useful comments that were incorporated into the predecessor
of this document: Ran Atkinson, Thomas Narten.
Table of Contents
Status of This Document....................................1
Abstract...................................................1
Acknowledgements...........................................2
Table of Contents..........................................2
1. Introduction............................................3
1.1 About This Document....................................3
1.2 About Diffie-Hellman...................................3
2. Encoding Diffie-Hellman Keying Information..............4
3. Performance Considerations..............................5
4. IANA Considerations.....................................5
5. Security Considerations.................................5
Normative References.......................................6
Informative Refences.......................................6
Author's Address...........................................6
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
A.3. Well-Known Group 3: A 1536 bit prime.................9
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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
similar information [RFC 1034, 1035]. The DNS has been extended to
include digital signatures and cryptographic keys as described in
[RFC 2535] and additonal work is underway which would require the
storage of keying and signature information in the DNS.
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, RFC 2631].
1.2 About Diffie-Hellman
Diffie-Hellman requires two parties to interact to derive keying
information which can then be used for authentication. Thus Diffie-
Hellman is inherently a key agreement algorithm. As a result, no
format is defined for Diffie-Hellman "signature information". 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**(i*j)(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).
For further information about Diffie-Hellman and precautions to take
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in deciding on a p and g, see [RFC 2631].
2. Encoding Diffie-Hellman Keying Information
When Diffie-Hellman keys appear within the RDATA portion of a RR,
they are encoded as shown below.
The period of key validity is not included in this data but is
indicated separately, for example by an RR which signs and
authenticates the RR containing the keying information.
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.
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3. Performance Considerations
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 RR sets containing keying information consistent
with adequate security.
4. IANA Considerations
Assignment of meaning to Prime Lengths of 0 and 3 through 15 requires
an IETF consensus as defined in [RFC 2434].
Well known prime/generator pairs number 0x0000 through 0x07FF can
only be assigned by an IETF standards action. RFC 2539, the Proposed
Standard predecessor of this document, assigned 0x0001 through
0x0002. This document assigns 0x0003. 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
Keying information retrieved from the DNS should not be trusted
unless (1) it has 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 security 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. [RFC 2631, Schneier]
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INTERNET-DRAFT Diffie-Hellman Information in the DNS
Normative References
[RFC 2631] - "Diffie-Hellman Key Agreement Method", E. Rescorla, June
1999.
[RFC 2434] - Guidelines for Writing an IANA Considerations Section in
RFCs, T. Narten, H. Alvestrand, October 1998.
Informative Refences
[RFC 1034] - P. Mockapetris, "Domain names - concepts and
facilities", November 1987.
[RFC 1035] - P. Mockapetris, "Domain names - implementation and
specification", November 1987.
[RFC 2535] - Domain Name System Security Extensions, D. Eastlake 3rd,
March 1999.
[RFC 2539] - Storage of Diffie-Hellman Keys in the Domain Name System
(DNS), D. Eastlake, March 1999, obsoleted by this RFC.
[RFC 2671] - Extension Mechanisms for DNS (EDNS0), P. Vixie, August
1999.
[Schneier] - Bruce Schneier, "Applied Cryptography: Protocols,
Algorithms, and Source Code in C" (Second Edition), 1996, John Wiley
and Sons.
Author's Address
Donald E. Eastlake 3rd
Motorola Laboratories
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1-508-851-8280 (w)
+1-508-634-2066 (h)
EMail: Donald.Eastlake@motorola.com
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Expiration and File Name
This draft expires in January 2004.
Its file name is draft-ietf-dnsext-rfc2539bis-dhk-03.txt.
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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
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A.3. Well-Known Group 3: A 1536 bit prime
The prime is 2^1536 - 2^1472 - 1 + 2^64 * { [2^1406 pi] + 741804 }.
Its decimal value is
241031242692103258855207602219756607485695054850245994265411
694195810883168261222889009385826134161467322714147790401219
650364895705058263194273070680500922306273474534107340669624
601458936165977404102716924945320037872943417032584377865919
814376319377685986952408894019557734611984354530154704374720
774996976375008430892633929555996888245787241299381012913029
459299994792636526405928464720973038494721168143446471443848
8520940127459844288859336526896320919633919
Prime modulus Length (32 bit words): 48, 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 ECE45B3D
C2007CB8 A163BF05 98DA4836 1C55D39A 69163FA8 FD24CF5F
83655D23 DCA3AD96 1C62F356 208552BB 9ED52907 7096966D
670C354E 4ABC9804 F1746C08 CA237327 FFFFFFFF FFFFFFFF
Generator: Length (32 bit words): 1, Data (hex): 2
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