Method for Authenticating a User for Using a Rental Vehicle Reserved by the User, and Computer Program Product and Authentication System

The disclosure relates to a method for authenticating a user for using a reserved rental vehicle. In one embodiment, radio-signal-based primary authentication information is transmitted to an electronic input unit of the user and the user can be authenticated on the basis of a check of the primary authentication information using a server system outside the vehicle. Secondary authentication information based on optical image processing is transmitted, in addition to the primary authentication information, to the electronic input unit of the user and to an authentication unit of the rental vehicle. If the user could not be successfully authenticated for using the reserved rental vehicle using the primary authentication information, the user is authenticated for using the reserved rental vehicle on the basis of a check of the secondary authentication information using the authentication unit of the rental vehicle.