Radio key (40, 56) to authenticate a user of a vehicle with respect to a door unlocking system of the vehicle and/or a drive starting device of the vehicle, comprising: - a housing (42, 58) in which is disposed a transponder unit, - wherein the transponder unit is designed to receive a request signal transmitted wirelessly from the vehicle and in reaction to the received request signal to emit an authentication signal based on an authentication code via an antenna of the radio key (40, 56), wherein the transponder unit to this end is a system encompassing a receiving electronic for the request signal, a sender electronic for the authentication signal and an authorisation electronic, and the authorisation electronic comprises a processor for processing the request signal and for generating the authentication signal and a storage element for the authentication code, - wherein a region of the housing has a form of an insertion area of a standardised memory card for a smart phone by means of which the memory card can be fixed in standardised manner in an insertion slot of the smart phone, wherein the form is based on a form factor of one of the following memory cards: an SD-memory card, a micro-SD-memory card, a nano-SD memory card, - wherein the transponder unit is designed to carry out a cabled data communication with the smart phone only unidirectionally to the transponder unit, so that it is not possible to read the authentication code from the smart phone in which the radio key (40, 56) is inserted, and - wherein the housing of the radio key (40, 56) has a communication interface according to a standard for a communication interface for the memory card and a control device, which control device is designed to receive via the communication interface from the smart phone codeword data and depending on the received codeword data to activate the transponder unit, whereby it is made possible to lock the radio key (40, 56) and only then to enable the use of the radio key (40, 56) for authenticating a user with respect to the door unlocking system or the drive starting device, when the user inputs a correct codeword using the smart phone. - a housing (42, 58) in which is disposed a transponder unit, - wherein the transponder unit is designed to receive a request signal transmitted wirelessly from the vehicle and in reaction to the received request signal to emit an authentication signal based on an authentication code via an antenna of the radio key (40, 56), wherein the transponder unit to this end is a system encompassing a receiving electronic for the request signal, a sender electronic for the authentication signal and an authorisation electronic, and the authorisation electronic comprises a processor for processing the request signal and for generating the authentication signal and a storage element for the authentication code, - wherein a region of the housing has a form of an insertion area of a standardised memory card for a smart phone by means of which the memory card can be fixed in standardised manner in an insertion slot of the smart phone, wherein the form is based on a form factor of one of the following memory cards: an SD-memory card, a micro-SD-memory card, a nano-SD memory card, - wherein the transponder unit is designed to carry out a cabled data communication with the smart phone only unidirectionally to the transponder unit, so that it is not possible to read the authentication code from the smart phone in which the radio key (40, 56) is inserted, and - wherein the housing of the radio key (40, 56) has a communication interface according to a standard for a communication interface for the memory card and a control device, which control device is designed to receive via the communication interface from the smart phone codeword data and depending on the received codeword data to activate the transponder unit, whereby it is made possible to lock the radio key (40, 56) and only then to enable the use of the radio key (40, 56) for authenticating a user with respect to the door unlocking system or the drive starting device, when the user inputs a correct codeword using the smart phone.