On the entropy of Physically Unclonable Functions

A physically unclonable function (PUF) is a hardware device that can generate intrinsic responses from challenges. The responses serve as unique identifiers and it is required that they be as little predictable as possible. A loop-PUF is an architecture where n single-bit delay elements are chained. Each PUF generates one bit response per challenge. We model the relationship between responses and challenges in a loop-PUF using Gaussian random variables and give a closed-form expression of the total entropy of the responses. It is shown that n bits of entropy can be obtained with n challenges if and only if the challenges constitute a Hadamard code. Contrary to a previous belief, it is shown that adding more challenges results in an entropy strictly greater than n bits. A greedy code construction is provided for this purpose.