Prediction of Plaintext in GSM Network Using the SACCH Logical Channel

The GSM cellular standard is still widely used worldwide. In this standard, the A5 ciphering algorithms are employed for protecting user data. A5/1 and A5/3 are two variants of A5 ciphering algorithms that are proven to be very powerful. Most known attacks on these ciphering algorithms assume some known plaintext data. In this paper, for the first time, a method of plaintext prediction is proposed for the SACCH logical channel. The sequence of downlink SACCH messages is modeled by a first-order Markov chain. Experiments on a real-world network show a 99% success rate. Moreover, the average position of correct plaintext in all predicted plaintexts is equal to 3.21. So, the speed of cipher cracking is around one-third of the speed of an ideal plaintext prediction system.