Re: Some stuff about Diffie-Hellman (and more :-)

[Hal <hfinney@shell.portal.com>]

Quite a few misconceptions here, I'm afraid:

From: rcain@netcom.com (Robert Cain)
> In the Diffie-Hellman exchange there is a well-known-prime, w, and a
> well-knwon-modulus, m.

w is supposed to be a "generator" of the group of integers mod m.  It does
not have to be prime.  It is supposed to be such that the series w**0, w**1,
w**2,...,w**m-1 does not repeat but goes through all the integers less than m.
Testing for such w's is pretty easy if you know the factorization of m,
involving a few arithmetic tests.

> For those interested that don't know I think
> it then proceeds as follows (don't have notes in front of me so please
> someone correct me if I'm misremembering it) where ** is the power or
> exponentiation operator and % is the modulus operator:
> 
> 	1) Bob generates a one time random prime, b, then computes

b does not have to be prime; it is a random number less than m.

> 		B = (w ** b) % m
> 	   and sends B to Carol.
> 
> 	2) Carol generates a one time random prime, c, then computes

Likewise, c does not have to be prime; it is a random number less than m.

> 		C = (w ** c) % m
> 	   and sends C to Bob.
> 
> 	3) Bob generates a session key:

Carol does this, not Bob.

> 		K = (B ** c) % m
> 
> 	4) Carol generates a session key:

Bob does this, not Carol.

> 		K = (C ** b) % m
>[...]
> Now, the tutorial over :-), the question is; is there a "standard"
> well-known-prime, w, and a "standard" well-known-modulus, m, and if
             ^^^^^-- generator
> not, let's define one.

I don't think there is a need for this.  The two sides need to agree on
a pair but they could just pick it at the beginning.  If everyone uses
the same m,w it would help attackers of the scheme to focus their efforts
on these numbers.  I believe there was some discussion of using well-known
numbers in the Digital Signature Standard (which is based on the same
problem as DH) but I don't know what the resolution was.

> I suppose that PGP uses a well known pair but
> they are big and not easy to hand around without going through media (I
> think.)

PGP does not uses DH and has no well known numbers.

If you do want well known numbers, I really think it will not be that bad
just to put them into the program.  Coming up with an algorithm to choose
and test a generator from scratch is probably going to be larger and
certainly going to be far slower than just hard-wiring the number in.

Hal