Cryptanalysis of the Megamos Crypto Automotive Immobilizer

The Megamos Crypto key fob is used in one of the most widely deployed automotive electronic immobilizers. Such an anti-theft device is designed to prevent hot-wiring of the vehicle. We have reverse-engineered all proprietary security mechanisms of the key fob and have found several weaknesses in the cipher and also in their usage and configuration by carmakers. We exploit these weaknesses in three practical attacks that recover the 96-bit key fob secret key. We end our article with suggestions to mitigate some of our attacks, something that knowledgeable users can do themselves.