Although SRAM programmed Field Programmable Gate Arrays (FPGA's) have come to dominate the industry due to their density and performance advantages over non-volatile technologies they have a serious weakness in that they are vulnerable to piracy and reverse engineering of the user design. This is becoming increasingly important as the size of chips - and hence the value of customer designs - increases. FPGA's are now being used in consumer products where piracy is more common. Further, reconfiguration of FPGA's in the field is becoming increasingly popular particularly in networking applications and it is vital to provide security against malicious parties interfering with equipment functionality through this mechanism.