Authentication system and method for a pool of vehicles

The invention relates to an authentication system (1) and a method for a pool of vehicles (2). The authentication system (1) comprises an authentication device (3) in the vehicle (2), a user key (4) for opening and starting the vehicle (2) and an authentication centre (5). The authentication device (3) comprises first storing means (6) for storing access information regarding at least one allowed user key (4) for the vehicle. The authentication device (3) communicates with the authentication centre (5) when a user wants to use a vehicle, by sending an encryption key to the authentication centre (5). The authentication centre (5) is arranged to create a time limited access information by encrypting the user key identity information by use of the encryption key and then sending the access information to the authentication device (3) together with either a time period information or a time stamp. The user has access to the vehicle during the time period.