Variable length data words are processed segment-by-segment together with corresponding segments of an enciphering key. As a function of the contents of a data segment, one of several modification modes is chosen. In accordance with the chosen modification mode, the enciphering key is changed following each enciphering operation of a segment. The described steps are preferably carried out three times for the full data word. The result of the third cycle represents the enciphered data word. Deciphering is effected in the same manner.