This application relates to transfer of microphone data from a microphone (101, 102, 102a, 201) to a processing module, such as voice biometric authentication module (111) in a secure manner, such that the receiving module can trust that the received audio is genuine. An authentication module (203) is configured to receive microphone data (DM) representative of an audio signal received at the microphone (201), and generate from the microphone data, authentication data (DA) for certifying that the microphone data did pass via the authentication module. The first authentication data (DA) comprises information relating to distinguishing characteristics of the audio content of the microphone data and may, for instance be an acoustic fingerprint of the audio content. The authentication data, may be cryptographically signed or encrypted and sent with the microphone audio to allow a receiver to verify that the audio is genuine and the content has not been substantially altered. The process may be robust to processing of the data that results in imperceptible changes in the audio content.