AN APPARATUS AND METHOD FOR INHIBITING INSTRUCTION MANIPULATION

An apparatus and method are provided for inhibiting instruction manipulation. The apparatus has execution circuitry for performing data processing operations in response to a sequence of instructions from an instruction set, and decoder circuitry for decoding each instruction in the sequence in order to generate control signals for the execution circuitry. Each instruction comprises a plurality of instruction bits, and the decoder circuitry is arranged to perform a decode operation on each instruction to determine from the value of each instruction bit, and knowledge of the instruction set, the control signals to be issued to the execution circuitry in response to that instruction. An input path to the decoder circuitry comprises a set of wires over which the instruction bits of each instruction are provided. Scrambling circuitry is used to perform a scrambling function on each instruction using a secret scrambling key, such that the wire within the set of wires over which any given instruction bit is provided to the decoder circuitry is dependent on the secret scrambling key. The decode operation performed by the decoder circuitry is then adapted to incorporate a descrambling function using the secret scrambling key to reverse the effect of the scrambling function. As a result, independent of which wire any given instruction bit is provided on, the decode operation is arranged when decoding a given instruction to correctly interpret each instruction bit of that given instruction, based on knowledge of the instruction set, in order to determine from the value of each instruction bit the control signals to be issued to the execution circuitry in response to that given instruction.