Abstract: A portable authentication apparatus, in particular for use with a motor vehicle, includes a transmission unit and a receiving unit for the wireless transmission of signals between the authentication apparatus and a control apparatus which is built into the motor vehicle and which is intended for controlling the authorization of access and/or the authorization of use for the motor vehicle. The authentication apparatus further has an encoding and decoding unit for decoding the received signals and for encoding the transmitted signals. The authentication apparatus includes an inertial detector and/or a motion detector.