This work deals with the security challenges in authentication protocols employing volatile biometric features, where the authentication is indeed a comparison between a fresh biometric template and that enrolled during the enrollment phase. We propose a security model for biometric-based authentication protocols by assuming that the biometric features to be public. Extra attention is paid to the privacy issues related to the sensitive relationship between a biometric feature and the relevant identity. Relying on the Goldwasser-Micali encryption scheme, we introduce a protocol for biometric-based authentication and prove its security in our security model. Keywords. Authentication, biometrics, privacy.