The paper analyzes and proposes some enhancements of Ring-Oscillators-based Physical Unclonable Functions (PUFs). PUFs are\r\nused to extract a unique signature of an integrated circuit in order to authenticate a device and/or to generate a key.We show that\r\ndesigners of RO PUFs implemented in FPGAs need a precise control of placement and routing and an appropriate selection of\r\nROs pairs to get independent bits in the PUF response. We provide a method to identify which comparisons are suitable when\r\nselecting pairs of ROs. Dealing with power consumption, we propose a simple improvement that reduces the consumption of the\r\nPUF published by Suh et al. in 2007 by up to 96.6%. Last but not least, we point out that ring oscillators significantly influence one\r\nanother and can even be locked. This questions the reliability of the PUF and should be taken into account during the design.
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