Cars of a car rental system are made operable by having a renter present a digital key issued from the car rental system. The digital key specifies the starting date and time of a given rental transaction, and the identification of the car the key is for. The digital key is further signed by the car...http://www.google.de/patents/US7366677?utm_source=gb-gplus-sharePatent US7366677 - Access control for rental cars
