Abstract: A digital computer system adapted for executing a set of instructions including at least one encrypted instruction. The system includes a main memory for storing the instructions, a cache memory for storing selected instructions with a relatively fast access time, a selectively operable decryption system for decrypting selected encrypted instructions from the main memory, and a central processing unit. The system is adapted so that the program stored in the main memory may be executed by the central processing unit. To this end, the decrypted instructions are decrypted only during execution when those instructions are transferred from the main memory to the cache memory so that plaintext versions of those encrypted instructions exist only in the cache memory in response to requests by the central processing unit while executing the program.