Secure message transmission is of extreme importance in today's information-based society: military, diplomatic, and corporate data transmissions must be safeguarded; so also must the account of every individual who has an automatic-teller bank account or whose purchases are subject to point-of-sale, direct account debiting. The only known way to keep all such transactions secret and authentic is by way of cryptographic techniques. But most cryptosystems in use today are not fool-proof-- their "symmetric" nature allows them to be compromised if either the sender's or the receiver's "key"...