Thanks jb that clears up a lot.
j
On Apr 14, 2008, at 6:14 AM, jimmy bahuleyan wrote:
Julian wrote:
My fear is that get a hold of P will allow for someone else to use
it to start a protocol disassembly. For instance anyone could
create a DHE-RSA-AES256-SHA TLS server and use P to listen for
connections, of course if would have to have a cert signed by CA to
proceed even if they have P.
Without certificates (anon-DH), yes someone could do a man-in-the-
middle attack; with certificates they would be hard pressed, since
they wouldn't have the server's private key. As for listening, no
matter what P you use a listener could easily follow the protocol;
but TLS is designed to be resilient, so he couldn't get hold of the
session keys.
The protocol here is TLS where each client is a server, so
shouldn't each client/server have their own DH P?
Or am I looking at this wrong, since I am using distributed PKI,
then exposing P is moot?
P,G are DH parameters which both the server and client need to know.
Normally they are public knowledge; if the server and client don't
share the P,G, then the server sends it to client (DH can't work if
both don't have the same P,G).
So, what happens is
client makes a random value Y which is private.
server makes a random value X which is private.
client uses {P,G} to make public value Y' from Y.
server uses {P,G} to make public value X' from X.
exchanges X',Y' and both arrive at a common value Z.
The security of DH lies in the fact that any attacker given
knowledge of X',Y',G,P cannot derive X or Y (Discrete Logarithm
problem) and hence cannot derive Z. And normally all systems
generate X,Y for each DH exchange.
Hope that helps.
-jb
--
Real computer scientists don't comment their code. The identifiers
are
so long they can't afford the disk space.
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