re: A.Walker's query about expiration date of DH patent. (I'm not a lawyer: This is just my opinion. Get advice from a professional for making your business decisions.) A couple of years ago, as part of the legislation implementing the GATT treaty, Congress changed the patent lifetime. The old rule was 17 years from grant date; the new rule is 20 years from filing date. There were various transition rules; one was that holders of current patents could select either rule. Presumably they will choose the rule that gives the later expiration date. In the case of the DHM patent #4200770, this gives then a few more months, to Sept. 6 1997. Roger Schlafly has made a reasonable case that the DHM patent is invalid because the invention was "published" more than a year before the patent was filed. (He dug up a copy of a preprint they were circulating with a stamped received-date of ?May 1976?.) There is also a patent by Hellman & Merkle that seems to deal with the (busted) knapsack cryptosystem. It's number 4218582, filed Oct 6, 1977, granted Aug 19, 1980. My cursory reading shows that it includes the math both for knapsack crypto and for doing modular exponentiation. It might be a good idea to look this over to make sure it doesn't affect your plans. Rich Schroeppel rcs@cs.arizona.edu