Study on Linear Complexity of Sequences Generated Using Modified A5/1 Algorithm

GSM technology is widely used to provide voice communication for the mobile users. A5/1 is the stream cipher used for encryption in GSM communication system. Initially A5 algorithm was kept secret to ensure security but when algorithm was disclosed many cryptanalytic attacks were proposed and proved that A5 algorithm is cryptographically weak. In this paper the modification in A5/1 is proposed, major improvement in clocking unit and addition of non linear combining function for the output to improve the Linear Complexity of the output bit sequence generated. The Linear Complexity (LC) of binary sequence so generated are computed and result are discussed using Berleykamp-Massey algorithm.