In biometric authentication protocols, a user is authenticated or granted access to a service if her fresh biometric trait matches the reference biometric template stored on the service provider. This matching process is usually based on a suitable distance which measures the similarities between the two biometric templates. In this talk, we prove that, when the matching process is performed using a specific family of distances, then information about the reference template is leaked. We show how it is possible to perform a template recovery attack even in privacy-preserving biometric authentication protocols and we formalise this “leakage of information” in a mathematical framework.