Cryptographic method and apparatus

Abstract

Cryptographic techniques for enciphering computer messages or the like include a unit for generating a cipher program for enciphering message data or plaintext by using a plurality of kinds of basic involution processing programs and desired key data, and a unit for enciphering the message data into ciphertext by executing the generated cipher program for the message data. A part of the involution processing program includes a substitution transformation portion and a permutation transformation portion. In accordance with the bit pattern of the above key data, a sequence for executing each of the involution processing programs and a sequence for executing the substitution transformation portion and the permutation transformation portion are determined. The above plurality of transformation processes include an operation of circular shifting to the right or left by X bits and an operation of circular shifting to the right or left by Y bits, with the X and Y being mutually different numbers. An indication of detailed numerical values of the X and Y and the right or left direction for a circular shift is given by key data. According to the above structure, a part of the functions for structuring the encipherment algorithms and the sequence for executing the functions are changed by key data. With the above arrangement, a large amount of algorithm transformation patterns are generated by the product of a number of changes of a part of the functions and a sum of combinations of permutation in the sequence of executing the functions, thus making it extremely difficult to crack the algorithms.