From: collins@newton.apple.com (Scott Collins)
Date: Tue, 8 Feb 94 17:42:12 PST
To: rcain@netcom.com (Robert Cain)
Subject: Re: Some stuff about Diffie-Hellman (and more :-)
Message-ID: <9402090138.AA04905@newton.apple.com>
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>What you say seems to indicate that D-H as we know and
>love it has been rendered obsolete because it depends on the modulus
>being known.  What am I missing?

DH exchange doesn't require a known modulus.  Most people implement it with
a common alpha and small set of generators so that they don't have to
invent a distribution or agreement protocol.  Authenticated DH exchanges
(e.g., station-to-station protocol) can include these parameters as part of
each parties signature; and provide an agreement policy (e.g., initiator's
parameters unless receiver thinks they are weak).

After each party has generated and exchanged an exponent, each verifies the
`certificate' of the other, and the signature of the other over the
exponent pair.

The shared knowledge that makes this possible in this case is, of course,
foreknowledge of the public key of the other party.  If you don't know it
before you start the protocol, you can't really know who you're talking to.
 Other protocols can be designed with other choices of shared knowledge.

STS is immently practical, any other practical and fair scheme is likely to
be similar, i.e., involve shared knowledge, independently generated random
input from both parties, a mechanism for securely (but expensively)
transmitting the random data (typically based on the shared knowledge),
combining the disjoint random data symmetrically so that each party shares
in a fresh secret session key, and finally authentication based in part on
the original shared knowledge.

Hope this helps,


Scott Collins         | "Few people realize what tremendous power there
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