Dismantling Megamos Crypto: Wirelessly Lockpicking A Vehicle Immobilizer

The Megamos Crypto transponder is used in one of the most widely deployed electronic vehicle immobilizers. It is used among others in most Audi, Fiat, Honda, Volkswagen and Volvo cars. Such an immobilizer is an anti-theft device which prevents the engine of the vehicle from starting when the corresponding transponder is not present. This transponder is a passive RFID tag which is embedded in the key of the vehicle. In this paper, the authors have reverse-engineered all proprietary security mechanisms of the transponder, including the cipher and the authentication protocol which we publish here in full detail. This article reveals several weaknesses in the design of the cipher, the authentication protocol and also in their implementation.