Patent ID: 7020282

Claim:
In a protocol for cryptographic communication via a communication channel “I” in which a sending cryptographic unit “S” transmits onto the communication channel I an encrypted cyphertext message “M” obtained by supplying both a plaintext message “P” and a cryptographic key “K” to a first cryptographic device, and in which a receiving cryptographic unit “R” receives the cyphertext message M from the communication channel I and by supplying the cyphertext message M together with the key K to a second cryptographic device decrypts the plaintext message P therefrom, a method by which the units S and R mutually establish a cryptographic key K by first exchanging messages before the sending unit S transmits the cyphertext message M comprising the steps of: a. the receiving unit R transmitting for storage in a publicly accessible repository a plurality of public quantities; b. the sending unit S: i. retrieving the plurality of public quantities from the publicly accessible repository; and ii. using at least some of the plurality of public quantities, computing and transmitting to the receiving unit R a plurality of sender's quantities; c. the receiving unit R, using at least some of the plurality of public quantities and at least one of the plurality of sender's quantities received from the sending unit S: i. computing and transmitting to the sending unit S at least one receiver's quantity; and ii. computing the key K; and d. the sending unit S, using at least some of the plurality of public quantities and the receiver's quantity received from the receiving unit R, computing the key K.