## Cryptology ePrint Archive: Report 2007/055

Cryptanalysis of the KeeLoq block cipher

Andrey Bogdanov

Abstract: KeeLoq is a block cipher used in numerous widespread passive entry and remote keyless entry systems as well as in various component identification applications. The KeeLoq algorithm has a 64-bit key and operates on 32-bit blocks. It is based on an NLFSR with a nonlinear feedback function of 5 variables.

In this paper a key recovery attack with complexity of about $2^{52}$ steps is proposed (one step is equivalent to a single KeeLoq encryption operation). In our attack we use the techniques of guess-and-determine, slide, and distinguishing attacks. Several real-world applications are vulnerable to the attack. To our best knowledge this is the first paper to describe and cryptanalyze the KeeLoq block cipher.

Category / Keywords: secret-key cryptography / secret-key cryptography, block ciphers, cryptanalysis, linear cryptanalysis, boolean functions, authentication codes, applications