It is now a decade since the appearance of W. Diffie and M. E. Hellmann's startling paper, "New Directions in Cryptography." This paper not only established the new field of public-key cryptography but also awakened scientific interest in secret-key cryptography, a field that had been the almost exclusive domain of secret agencies and mathematical hobbyist. A number of ex cellent books on the science of cryptography have appeared since 1976. In the main, these books thoroughly treat both public-key systems and block ciphers (i. e. secret-key ciphers with no memo ry in the enciphering...