I missed the context because my mailbox overflowed, but Eric mentioned Diffie-Hellman key exchange. If the need is for a one-shot quick-n-dirty session key exchange (as for the audio talk program John Gilmore mentioned) an alternative is to do a quick PGP keygen, send the session key across using the PGP key, then destroy the PGP key. The only reason I mention this is that it can be done in a couple of minutes with existing tools tomorrow, if you need it. Eric is right that if you are designing something from scratch DH is often more appropriate (although PK's allow for authentication if you have a trusted signature, preventing spoofing attacks). Hal