matsb@sos.sll.se (Mats Bergstrom) writes: Subj: Clipper Session Key

How do two Clipper/Capstone/Skipjack/Tessera units agree on a session key without compromising it to the bad guys tapping the line? If not known outside of the NSA, what possible solutions are there?

In the case of the MYK-78, the simple Clipper chip to be used in phones and similar devices, that's your problem. You have to do a Diffie-Hellman exchange or RSA or something externally. Or hire a courier. This will cause compatibility problems between different manufacturers' Clipper devices, unless they agree on a standard up front. Probably everyone will clone AT&T Clipper devices, since they are going to be the first to market. If the government buys a large batch, that will set the standard. Without at least a de-facto protocol standard, the system would be almost useless. There is also the problem of adding encryption to the V.x standards for fax, LAPM for modems, etc. Each of these devices will need some kind of key exchange. The Capstone and Tessera (same thing, different package) have the ability to do a modexp operation. From this you can internally do DH exchange or RSA. As I remember, the Capstone does up to 1024 bit modexp, comparable to PGP in security. Skipjack is just a symmetric cipher like DES, except it has an 80-bit key and has (according to the review team) all of DES's problems fixed. --- Mike