A processor, circuit and method provide for fast decryption of encrypted program instructions for execution by the processor. A programmable look-up coding is used to decode a field within the instructions. The decoded field for the instructions are recombined with the remaining portion of the same instructions to yield the decoded instructions. The programmable look-up coding can be programmed and controlled by a process executing at a higher privilege level than the program represented by the instructions, so that security against code-modifying attacks is enhanced.