An Improved Cryptanalysis of the A5/2 Algorithm for Mobile Communications

S. Petrović and A. Fúster-Sabater (Spain)


Cryptanalysis, GSM system, Linearization,Boolean functions, Stream ciphers.


In this paper, an improved cryptanalysis of the A5/2 algo rithm for data protection in GSM mobile communications is described. Due to certain design weaknesses such as frequent reinitializations, bad distribution of the feedback taps of the LFSRs or small number of skipped bits during the process of reinitialization, the linear relations among the output bits of this algorithm can be efficiently recon structed. The attack is, essentially, a procedure which determines such relations. The vast majority of the un known output bits that appear after four known frames of output bits can be reconstructed. In the worst case, the time complexity of the attack is 217 . By performing some precomputation, a significant improvement of the performances of this reconstruction method is achieved.

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