CRYPTOGRAPHIC METHOD AND APPARATUS FOR PUBLIC KEY EXCHANGE WITH AUTHENTICATION

A technique for use in a public key exchange cryptographic system, in which two user devices establish a common session key by exchanging information over an insecure communication channel, and in which each user can authenticate the identity of the other, without the need for a key distribution center. Each device has a previously stored unique random number Xi, and a previously stored composite quantity that is formed by transforming Xi to Yi using a transformation of which the inverse in computationally infeasible; then concatenating Yi with a publicly known device identifier, and digitally signing the quantity. Before a communication session is established, two user devices exchange their signed composite quantities, transform them to unsigned form, and authenticate the identity of the other user. Then each device generates the same session key by transforming the received Y value with its own X value. For further security, each device also generates another random number X'i, which is transformed to a corresponding number Y'i. These Y'i values are also exchanged, and the session key is generated in each device, using a transformation that involves the device's own Xi and X'i number and the Yi and Y' i numbers received from the other device.