The data address portions of a set of computer instructions are scrambled in accordance with a predetermined cipher key before the instructions are loaded into the instruction memory. The instructions with the scrambled data address portions are then loaded into the instruction memory at addresses corresponding to the original program sequence. The data involved in the program is loaded into a separate data memory at the addresses specified in the original unscrambled program. An unscrambler circuit which operates in accordance with the cipher key is coupled in series with the data memory address input conductors and is mounted on the data memory unit and is encapsulated therewith to prevent anyone from examining the unscrambler to determine the cipher key or from electrically interrogating the unscrambler in such a manner as to determine the cipher key. The scrambled data addresses do not interfere with normal computer operations or maintenance procedures but do prevent anyone from reproducing the original unscrambled program with the information available in the instruction and data memories.