Silicon based Physical Unclonable Functions (PUFs) are circuits which measure random variations of the manufacturing process to generate a secret. The quality of a PUF predominantly depends on its predictability and reliability. Available quality measures are not always sufficient and do not show the root cause for high predictability. This paper suggests entropy and joint entropy of certain views on PUFs and interprets the measures to show different design issues to a designer and to give a global measure for PUF quality. Simulation results for Arbiter PUF, Ring Oscillator PUF, and SRAM PUF are evaluated by the new measures and results for Ring Oscillator and Arbiter PUFs show how the measures can be used to guide designers.
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