Entropy analysis and new constructions of biometric key generation systems

In this paper, a new, soft two-level approach for the generation of multiple and revocable biometric keys, adapted to the analog nature of biometric signals, is proposed. It consists of a novel randomized soft code-offset construction for the Euclidean metric, at the first level, and a code-redundancy construction for the Hamming metric, possibly based on a Reed-Solomon code, at the second level. The Shannon entropy analysis shows that the new construction achieves optimal security with respect to biometric template protection, whereas the previously proposed constructions for the Euclidean metric are shown to be insecure in the multiple-key setting. In addition, a general code-offset construction for the Hamming metric is analyzed in terms of the Shannon entropy instead of the usual min entropy. This allows a nice characterization of the codes that provide biometric template protection in the multiple-key scenario as well as a further differentiation among these codes with respect to the achievable security level.