1. A cryptographic apparatus for executing a computer program of instructions stored in enciphered form as a plurality of addressable blocks of enciphered program information, each block being stored at a location specified by a digital address, the apparatus comprising: means for storing an encryption key; means for specifying a first digital address of a block of said enciphered program information, thereby addressing the block; product block deciphering means for transforming in a plurality of steps said addressed block of enciphered program information to produce a block of deciphered information including an executable instruction and a second digital address, at least one step in said plurality of steps being controlled by said first digital address, and at least one step in said plurality of steps being controlled by said encryption key; means for decoding and executing an instruction included in a block of said deciphered information; and means for specifying the second digital address in said block of deciphered information so as to address a successor block of said enciphered program information which includes a successor instruction in enciphered form for deciphering by said deciphering means.