ASIC implementation of a hardware-embedded physical unclonable function

Within-die variations in path delays are increasing with scaling. Although higher levels of within-die delay variations are undesirable from a design perspective, they represent a rich source of entropy for applications that make use of ‘secrets’, such as authentication, hardware metering and encryption. Physical unclonable functions or PUFs are a class of circuit primitives that leverage within-die variations as a means of generating random bitstrings for these types of applications. In this study, the authors present test chip results of a hardware-embedded delay PUF (HELP) that extracts entropy from the stability characteristics and within-die variations in path delays. HELP obtains accurate measurements of path delays within core logic macros using an embedded test structure called regional delay behaviour (REBEL). REBEL provides capabilities similar to an off-chip logic analyser, and allows very fast analysis of the temporal behaviour of signals emerging from paths in a core logic macro. Statistical characteristics related to the randomness, reproducibility and uniqueness of the bitstrings produced by HELP are evaluated across industrial-level temperature and supply voltage variations.