Cryptanalysis
KeeLoq was first cryptanalyzed by Andrey Bogdanov using sliding techniques and efficient linear approximations. Nicolas Courtois attacked KeeLoq using sliding and algebraic methods. The attacks by Bogdanov and Courtois do not pose any threat to the actual implementations that seem to be much more vulnerable to simple brute-force of the key space that is reduced in all the code-hopping implementations of the cipher known to date. Individual "code hopping" implementations are also often vulnerable to a replay attack exploited by jamming the channel while intercepting the code, since code hopping is done by incrementing the IV on each use instead of using the current time. It made KeeLoq "code grabbers" quite popular among most car thieves, although some of them use FPGA-based devices to break KeeLoq-based keys by brute force within about two weeks thanks to the reduced key length in the real world implementations.
In 2007, researchers in the COSIC group at the university at Leuven, Belgium, (K.U.Leuven) in cooperation with colleagues from Israel found a new attack against the system. Using the details of the algorithm that were leaked in 2006, the researchers started to analyze the weaknesses. After determining the part of the key common to cars of a specific model, the unique bits of the key can be cracked with only sniffed communication between the key and the car.
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