Securing FPGA-Based Obsolete Component Replacement for Legacy Systems

Component-aging is unavoidable in legacy systems. Although re-designing the system typically results in a high cost, the need to replace aged components for legacy systems is an urgent priority. Unfortunately, the aged components are likely to be obsolete and not available on the current market. Obsolete component replacement with field-programmable gate array (FPGA) devices is emerging as a feasible option to extend the lifetime of legacy systems. While replacing the aged component, we traditionally only focus on matching the functionality and neglect the potential security threats from FPGA replacement. However, recent literature demonstrates that FPGA devices may contain hardware Trojans, which are induced during FPGA device fabrication or bitstream generation time. To prevent the Trojans on FPGA from receiving external inputs or leaking sensitive information, we propose a Runtime Pin Grounding (RPG) scheme to ground the unused pins and check the pin status at every clock cycle. Furthermore, we exploit the principle of moving target defense (MTD) and propose a hardware MTD (HMTD) method. In our method, the aged obsolete unit is replicated to multiple copies in the FPGA device, and two of the replicas are randomly selected for output comparison and thus Trojan detection. We successfully implemented the proposed RPG and HMTD methods on a Nexys-3 FPGA board. Our case study shows that the proposed RPG scheme increases the FPGA utilization rate by less than 0.1%. On average, our HMTD method reduces the hardware Trojan bypass rate by 61% over the existing method.