Two Trivial Attacks on A5/1:A GSM Stream Cipher

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.