Systemic Frequency Biases in Ring Oscillator PUFs on FPGAs

Abstract

Physically unclonable functions (PUFs) are an emerging primitive in hardware security, enabling the identification of computer-chips. A promising type particularly for FPGA implementations is the Ring Oscillator (RO) PUF, where signal delays-stemming from uncontrollable variations in the manufacturing process-are used as device-specific characteristics. Based on experimental results gathered with 38 identical Altera FPGAs, we show the existence of non-device-specific i.e., systemic RO frequency biases, traced back to (1) the internal routing within the RO's look-up tables, (2) the RO locations on the FPGAs, or (3) the non-PUF payload activity. As these biases are the same for all devices, the result is poor inter-device uniqueness and unreliable signatures under changing payloads. After characterizing these biases with a newly developed set of metrics, we suggest a method to overcome them: Using only a small sample of devices, the average bias over all devices for each RO is predicted and the relative differences caused by systemic biases are nullified. We demonstrate the viability of this method by determining the sufficient random sample sizes and showing that the inter-device uniqueness is drastically increased and the PUF signatures become reliable even under changing payload activities.