International Association
for Cryptologic Research

This paper belongs to a sequence of manuscripts that discuss generic and easy-to-apply security metrics for Strong Physical Unclonable Functions (PUFs). These metrics cannot and shall not fully replace in-depth machine learning (ML) studies in the security assessment of Strong PUF candidates. But they can complement the latter, serve in initial complexity analyses, and allow simple iterative design optimization. Moreover, they are computationally more efficient and far easier to standardize than typical ML-studies. This manuscript treats one very natural, but also very impactful metric, and investigates the effects that the alteration of single challenge bits has on the associated PUF-responses. We define several concrete metric scores based on this idea, and demonstrate their predictive power by applying them to various popular Strong PUF design families as test cases. This includes XOR Arbiter PUFs, XOR Bistable Ring PUFs, and Feed-Forward Arbiter PUFs, whose practical security is particularly well known after two decades of intense research. In passing, our manuscript also suggests techniques for representing our metric scores graphically, and for interpreting them in a meaningful manner. Our work demonstrates that if comparable methods had existed earlier, various Strong PUF candidates deemed secure and broken later could have been recognized and winnowed early on.