  Stream ciphers play an important role in those applications where high
throughput remains critical and resources are very restricted e.g. in Europe
and North America, A5/1 is widely used stream cipher that ensure
confidentiality of conversations in GSM mobile phones. However careful security
analysis of such cipher is very important due to widespread practical
applicability. The basic building blocks used in the design of A5/1 are linear
feedback shift registers (LFSRs). Algebraic attacks are new and very powerful
tool to cryptanalyse LFSRs based stream ciphers even non-linear combiner are
concerned. In this paper we compared previous attacks on A5/1 as well as an
algebraic attack and a new improved guess and determine attack is proposed.
