An improved attack on A5/1

A5/1 is a stream cipher used in GSM to provide over-the-air communication privacy. Biham and Dunkelman proposed an attack on A5/1 with time complexity of 2^(39.91) and data complexity of 2^(21.1) known bits and memory complexity of 32 GB. In this paper, we propose an improvement on their attack. Our improvement is identification and elimination of useless states from the precomputed table. Furthermore, we propose another way for use of table in online phase of attack that causes decreasing in the time complexity to 2^(37.89) and memory complexity decreases to half.