The invention relates to a radio key (54) for authenticating a user of a vehicle (12) with respect to a door unlocking system (14) of the vehicle (12) and/or a drive start device (14) of the vehicle (10). The radio key (54) comprises a transponder unit (44) which is designed to receive a query signal (50) transmitted wirelessly from the vehicle (12) and to transmit, depending on the received query signal (50), an authentication signal (52) based on an authentication code. The transponder unit (44) is arranged in a casing (42). The invention is based on the problem of designing the use of a radio key to be more convenient. An area (54) of the casing (42) has a shape of an insertion area of a standardized memory card for a mobile terminal (10), by means of which the memory card is to be stuck in accordance with the standard in an insertion slot (E) of the mobile terminal (10). The radio key can thus be integrated in a mobile terminal.