Cryptanalysis of GSM Encryption Algorithm A5/1

The A5/1 algorithm is one of the most famous stream cipher algorithms used for over-the-air communication privacy in GSM. The purpose of this paper is to analyze several weaknesses of A5/1, including an improvement to an attack and investigation of the A5/1 state transition. Biham and Dunkelman proposed an attack on A5/1 with a time and data complexity of 2^39:91and 2^21:1, respectively. In this paper, we propose a method for identification and elimination of useless states from the pre-computed tables and a new approach to access the table in the online phase of the attack which reduces the time complexity to 237:89 and the required memory in half. Furthermore, we discuss another weakness of A5/1 by investigating its internal state transition and its keystream sequence period. Consequently, the internal states are divided into two classes, initially periodic and ultimately periodic. The presented model is verified using a variety of simulations which are consistent with the theoretical results.