Method and apparatus for performing strong encryption or decryption data using special encryption functions

Abstract

A method and apparatus for decrypting an input block encrypted under a predetermined key in a cryptographic system having a cryptographic facility providing cryptographic functions for transforming blocks of data. The cryptographic functions include an encryption function for encrypting a block under a predetermined key and a transformation function for transforming a block encrypted under a first key to the same block encrypted under a second key. The cryptographic functions have at least one key pair with the property that successive encryption of a block under the keys of the pair regenerates the block in clear form. The input block is first transformed into an intermediate block encrypted under one of the key pair using the transformation function. The intermediate block is then further encrypted under the other of the key pair using the encryption function to generate an output block successively encrypted under the keys of pair, thereby to regenerate the input block in clear form. The invention is useful in cryptographic systems in which the decryption function being emulated by the transformation and encryption functions is unavailable for export control or other reasons.