Paper Title
LFSR Based Stream Cipher (Enhanced A5/1)
Abstract
In GSM cellular standard, A5 family of stream ciphers are used for providing user 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. The major weaknesses in A5/1 are due to the use of fixed feedback polynomial
based LFSR, weak clocking mechanism, insignificant taps of register structure, non-invertible clocking mechanism and
frequent resetting. In this paper, a new enhanced LFSR based stream cipher for A5 family has been proposed that includes
variable tapping mechanism, variable feedback polynomial and new clocking scheme with increased number of LFSRs and
increased length of each LFSR. The proposed scheme uses four LFSRs of length 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 be included in final output or not. The proposed algorithm has been
simulated by using MATLAB and its randomness properties have been tested using ‘Randomness Test Suit’ given by
National Institute of Standard and Technology (NIST), USA. The results shows that the proposed scheme is robust and
resistive to the cryptographic attacks as 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.
Keywords- Stream cipher, A5/1, GSM standard, LFSR, Mutation, Randomness Test Suit