An implementation of ciphertext autokey cipher in which the leftmost n bits of the data encryption standard (DES) output are added by the EXCLUSIVE OR operation to N bits of plaintext to produce N bits of ciphertext (where N is the number of bits enciphered at one time), and these N bits of ciphertext are fed back into the algorithm by first shifting the current DES input N bits to the left, and then appending the N bits of ciphertext to the right-hand side of the shifted input to produce a new DES input used for the next iteration of the algorithm.