An authentication system for a pool of vehicles, the authentication system comprising an authentication device in the vehicle, a user key for opening and starting the vehicle and an authentication centre, - the authentication device comprising first storing means for storing access information regarding at least one allowed user key for the vehicle, the authentication device comprising means for sending and receiving information to and from the authentication centre, - the user key comprising second storing means for storing at least a user key identity information, - the authentication centre comprising means for sending and receiving information to and from the authentication device, the authentication centre comprising third storing means for storing the user key identity information, characterized in that the authentication device comprises means for creating and/or storing an encryption key being specific for the vehicle, wherein the authentication centre is arranged to communicate with the authentication device for retrieving the encryption key upon request from a user to use a vehicle, wherein the authentication centre 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 together with either a time period information or a time stamp, wherein the authentication device is arranged to either use the time period for allowing user access to the vehicle or arranged to use the time stamp for creating a time period during which the user is allowed access to the vehicle, wherein the authentication device is arranged to store the time limited access information in the first storing means. - the authentication device comprising first storing means for storing access information regarding at least one allowed user key for the vehicle, the authentication device comprising means for sending and receiving information to and from the authentication centre, - the user key comprising second storing means for storing at least a user key identity information, - the authentication centre comprising means for sending and receiving information to and from the authentication device, the authentication centre comprising third storing means for storing the user key identity information, characterized in that the authentication device comprises means for creating and/or storing an encryption key being specific for the vehicle, wherein the authentication centre is arranged to communicate with the authentication device for retrieving the encryption key upon request from a user to use a vehicle, wherein the authentication centre 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 together with either a time period information or a time stamp, wherein the authentication device is arranged to either use the time period for allowing user access to the vehicle or arranged to use the time stamp for creating a time period during which the user is allowed access to the vehicle, wherein the authentication device is arranged to store the time limited access information in the first storing means.