Hiding secrecy leakage in leaky helper data

PUFs provide cryptographic keys for embedded systems without dedicated secure memory. Practical PUF implementations often show a bias in the PUF responses, which leads to secrecy leakage in many key derivation constructions. However, previously proposed mitigation techniques remove the bias at the expense of discarding large numbers of PUF response bits. Instead of removing the bias from the input sequence, this work reduces the secrecy leakage through the helper data. We apply the concept of wiretap coset coding to add randomness to the helper data such that an attacker cannot isolate significant information about the key anymore. Examples demonstrate the effectiveness of coset coding for different bias parameters by computing the exact leakage for short code lengths and applying upper bounds for larger code lengths. In our case study, we compare a secrecy leakage mitigation design with coset coding and Differential Sequence Coding (DSC). It reduces the number of required PUF response bits by 60% compared to state-of-the-art debiasing approaches.