Factoring is only one method. Case in point: solving the game of Go has been proven to take exponential time *and* exponential space. An encryption algorithm based on it could potentially be very hard, although there are subtleties about trap doors and the cost of key generation and use... Paul E. Baclace peb@procase.com P.S.: I'm installing procmail over here...get version v2.91 (I fixed a segv in v2.61 [on SunOS] yesterday and should have just picked up the newer version first.)