Improving RO-PUF quality on FPGAs by incorporating design-dependent frequency biases

Physically unclonable functions (PUFs) based on ring oscillators (ROs) are a popular primitive in hardware security, meant to enable the unambiguous and tamper-proof identification of computer chips. This is achieved by exploiting different signal delays on each chip stemming from uncontrollable variations during the manufacturing process. Thus, the relation between RO frequencies on an individual chip can be used as the chip´s unique PUF signature. In this work, we show how ROs implemented on a larger number of Altera Cyclone IV FPGAs are biased towards slower or faster frequencies in non-uniform ways depending on the FPGA´s programming with different design; even though the ROs are placed and routed equally. Without considering these biases, inter-device uniqueness of the PUF signatures is degraded. We demonstrate that subtracting the mean frequency of each RO - derived using only a small training set of devices - from the sampled frequencies overcomes this disadvantage; i.e. the uniqueness is increased drastically while maintaining reliability.