pubkeys for p and g

[martin f krafft]

> I'm not certain I understand your questions, but here are some
> answers (I think).

To clear this up:

I am well aware how DH works, and what the mathematical properties
of p and g are and have to be.

My point was that some commercial vendors (Check Point and others)
claim, that if two partners want to perform a DH key exchange, they
may use their two public keys for g and p. This, in effect, would
mean that g and p were not globally known, but that the public keys
are used in their place.

I am well aware that p and g are globally known as defined in the
chosen DH Group. However, I am wondering how Check Point (and
others) can claim that public keys may well be used in place,
thereby invalidating the need for a globally constant p and g pair.
These public keys are independent of the public keys exchanged as
part of DH, which are simply calculated by the g^x mod p formula of
DH, from the private keys.

Thus every communication party would have a key pair, aA and bB,
where the capital letter is the public key. Then, the following
happens:

  let g = A and p = B
  let A' = g^a mod p and B' = g^b mod p
         = A^a mod B        = A^b mod B

and off you go, doing DH with g = A, p = B, and the keypairs aA' and
bB' on either side.

This would, in my opinion, only be possible if:

  - there would be a rule to decide which public key is p and which
    is g.
  - all public keys (RSA in this case) are primes.
  - all public keys are good generators mod p.

We are writing a book and simply want to have some backup. I am
almost sure that Check Point is bullshitting (wouldn't be the first
time), so unless anyone has actually heard of this possibility, I am
going to write this down and influence a thousand people, basically
claiming that Check Point is wrong.

Does it make sense now?

-- 
martin;              (greetings from the heart of the sun.)
  \____ echo mailto: !#^."<*>"|tr "<*> mailto:" net at madduck
 
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experience is what causes a person
to make new mistakes
instead of old ones.
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