140 / 2015-10-24 16:55:24

Design of Enhanced Pseudo-Random Sequence Generator usable in GSM Communication

In GSM cellular standard, A5 family of stream ciphers are used for providing users’ security over wireless communication and for this, three standards namely A5/1, A5/2 and A5/3 algorithms are stipulated. Initially, these algorithms were kept secret, but once disclosed, many cryptographic weaknesses were revealed and several cryptanalytic attacks were proposed particularly in A5/1.In this paper, a new enhanced LFSR based stream cipher for A5 family has been proposed that includes variable tapping mechanism, variable feedback polynomial, new clocking scheme and key generation mechanism with increased number of LFSRs and their lengths. The proposed scheme uses four LFSRs of length of 30, 32, 29 and 37 (instead of three as used in other works), two of which are used for mutating of the main back-bone LFSR while the fourth LFSR mutates the final output. The mutation apparently induces variable feedback polynomial effect in the back-bone LFSR. This mutation is further randomized with a new clocking mechanism, where the clocking bit, generated by XOR-ing of two bits taken from two LFSRs, decides whether the LFSR output will stop or go.Results have shown that the randomness of clocking has been improved, i.e., the probability of an LFSR to be clocked is about 50% (better than previous 75%). Overall probability of mutation is nearly 0.35. The proposed algorithm has been simulated using MATLAB and its randomness properties have been tested using ‘Randomness Test Suit’ given by National Institute of Standard and Technology (NIST), USA.It can be concluded that the proposed scheme is robust and resistive to the cryptographic attacks compared to the conventional A5/1 stream cipher. It generates cryptographically better binary sequence than the A5/1 stream cipher of GSM with comparable complexity.