A Compact TRNG Design for FPGA Based on the Metastability of RO-driven Shift Registers

True random number generators (TRNGs), as an important component of security systems, have received a lot of attention for their related research. The previous researches have provided a large number of TRNG solutions, however, they still failed to reach an excellent tradeoff in various performance metrics. This article presents a shift-registers metastability-based TRNG, which is implemented by compact reference units and comparison units. By forcing the D flip-flops in the shift-registers into the metastable state, it optimizes the problem that the conventional metastability entropy sources consume excessive hardware resources. And a new method of metastable randomness extraction is used to reduce the bias of metastable output. The proposed TRNG is implemented in Xilinx Spartan-6 and Virtex-6 FPGAs, which generate random sequences that pass the NIST SP800-22, NIST SP800-90B tests and show excellent robustness to voltage and temperature variations. This TRNG can consume only 3 slices of the FPGA, but it has a high throughput rate of 25 Mbit/s. In comparison with state-of-the-art FPGA-compatible TRNGs, the proposed TRNG achieves the highest figure of merit FOM, which means that the proposed TRNG significantly outperforms previous researches in terms of hardware resources, throughput rate, and operating frequency tradeoffs.