A CPU is provided with an ability to modify its operation in accordance with an encryption key. When a program is compiled, the program is modified in order that execution may be performed with the CPU with its operation modified. In order to execute program instructions, the buffer interdependencies must match that expected by the compiler. This makes analysis of the program operation extremely difficult. The instruction buffer on a keyed microprocessor contains logic which is able to route a subset of the instruction bits on the microprocessor. This selects destination logic gates in the microprocessor which eventually reach a programmable instruction decoder and an instruction buffer interdependency checking logic block.