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Unfortunately, Diffie-Hellman *is* patented, and I'm pretty sure Public Key Partners
(closely related to RSA) holds the patent, just as they hold RSA's.
To quote Steve Bellovin:
U.S. Patent Number: 4200770
Title: Cryptographic Apparatus and Method
Inventors: Hellman, Diffie, Merkle
Assignee: Stanford University
Filed: September 6, 1977
Granted: April 29, 1980
[Expires: April 28, 1997]
So we're stuck with it being patented until 1997.
Too bad - I was starting to think along the same lines about doing a D-H-based mailer.
It's non-trivial, if you have to worry about active eavesdroppers swapping
mail messages on you, and it's easier to do if there's a trusted Key Distribution
Center, and if you think about all the cases carefully you tend to re-create
either Needham-Schroeder or the Everhart-Osborn Bell Labs patent (~1980++),
but you can certainly do it for the common case that says the Bad Guys are
only listening to your mail and not tampering with it.
Bill Stewart, [email protected]