A wireless security module (20) adapted to authenticate a mobile device (16), such as a mobile telephone, thereby to allow communication between the mobile device and a terminal (12), such as a self-service terminal. The wireless security module (20) is paired with the mobile device (16) and is adapted to send an authentication signal to the mobile device (16) or the terminal (12) using a wireless communication path, such as a telecommunications network or an IR link or a Bluetooth connection.