Abstract: A method is disclosed for modifying an iterated block cipher by controlling the operations and transformations that cause diffusion. In one embodiment which is applicable to any iterated block cipher (12), a diffusion function (10), during encryption, is selected based on a parameter which measures the order of permutation of the diffusion function (10) and applies the diffusion function (10) to the encryption routine (12). The user chooses the required amount of diffusion for a given block of plaintext (11). The plaintext (11) is then encrypted using the modified diffusion function (10) to produce a ciphertext (14) which is then sent over a communications channel (16) which may be noisy. At the receiving end (18) of the communications channel (16), the received ciphertext (20), which now may be corrupted by bit errors, is passed through an iterated block cipher decryption routine (22) using the same diffusion function (10) selected earlier during encryption.