Radio key for a vehicle

Radio key (54, 56) 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), comprising:

• 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, and
• a casing (42, 58) in which the transponder unit (44) is arranged, wherein an area (54) of the housing (42) has a shape of an insertion area of a standardized memory card for a smartphone (10), by means of which the memory card is to be stuck in accordance with the standard in an insertion slot (E) of the smartphone (10), wherein the shape is based on a form factor of one of the following memory cards:
• a SD memory card, a micro SD memory card, a nano SD memory card, characterized in that the transponder unit (44) is designed
• a) to carry out a wired data exchange with the smartphone (10) either only unidirectionally towards the transponder unit (44) or not at all, so that it is not possible to read out the authentication code from the smartphone (10) in which the radio key (54, 56) is inserted, and
• b) to be operated autonomously, so that no communication of authorization data to the smartphone (10) occurs, and in that
• the radio key (40, 56) comprises a communication interface according to a standard for a communication interface for the memory card, and the radio key (40, 56) comprises a control device which is designed to receive code word data and/or identification data of the smartphone (10) from the smartphone (10) via the communication interface and to activate the transponder unit (44) depending on the received data.