A Reconfigurable SRAM Based CMOS PUF With Challenge to Response Pairs

This paper presents a reconfigurable SRAM-based physically unclonable function (PUF) topology with multiple challenge-response pairs (CRPs) per cell. The proposed PUF structure enables a very large CRP space by connecting additional pull-up and pull-down paths to each SRAM cell. These alternate pathways to the supply rail and ground are activated by the challenge inputs, which effectively reconfigure the transfer characteristics of each cross-coupled inverter. A newly proposed response instability detector improves bit error rate (BER) performance by discarding unstable response. In addition, the proposed PUF adds indirect challenges by scrambling the responses using a Galois linear feedback shift register (LFSR). The proposed PUF can be applied to a wider range of applications as a CRP PUF because it has multiple CRPs in addition to the small area and fast operating speed, which are the advantages of the conventional SRAM structure. In order to verify the performance of the proposed architecture, a $32\times 32$ -bit reconfigurable SRAM PUF array with 32-bit challenge is implemented in a 65 nm CMOS process. Experimental results show a core area of $88.867~\mathrm {\mu } \mathrm {m^{2}/bit}$ , energy efficiency of 0.082 pJ/bit, and inter-chip Hamming distance (HD) of 48.93% across 40 chips. By applying an unstable bit discard (UBD) scheme, BER is improved from 13.7% to 0.9%. Compared to the state-of-the-art, the proposed PUF is shown to be highly competitive in area, throughput and energy efficiency.