In biometrics, a human being needs to be identified based on some characteristic physiological parameters. Often this recognition is part of some security system. Secure storage of reference data (i.e., user templates) of individuals is a key concern. It is undesirable that a dishonest verifier can misuse parameters that he obtains before or during a recognition process. We propose a method that allows a verifier to check the authenticity of the prover in a way that the verifier does not learn any information about the biometrics of the prover, unless the prover willingly releases these parameters. To this end, we introduce the concept of a delta-contracting and epsilon-revealing function which executes preprocessing in the biometric authentication scheme. It is believed that this concept can become a building block of a public infrastructure for biometric authentication that nonetheless preserves privacy of the participants.