1. A cryptographic apparatus for deterring unauthorized execution of computer programs of instructions, the apparatus comprising: table means for storing a plurality of digital addresses and a corresponding plurality of multiple-bit digital words; means for determining whether a memory address matches one of the digital addresses in said table means; means for enciphering a plurality of instructions or portions of instructions in a selected program to produce enciphered instructions to be stored at memory locations specified by memory addresses, the enciphering being performed on an instruction or portion thereof if said determining means determines that the memory address of the instruction matches one of the digital addresses in said table means, the enciphering being a function of the digital word in said table means corresponding to the matching digital address; and means for deciphering one of said enciphered instructions whenever the instruction is fetched for execution from a memory location specified by a memory address matching one of the digital addresses in said table means, the deciphering being performed as a function of the digital word in said table means corresponding to the digital address that matches the memory address of the instruction, thereby restoring enciphered instructions to unenciphered form during execution of said selected program.