Enhancement of A5/1: Using variable feedback polynomials of LFSR

The Global System for Mobile communication, GSM is the most widely used cellular system in the world, with over a billion customers around the world. GSM was the first cellular system which seriously considered security threats. Previous cellular systems had practically no security, and they were increasingly the subject of criminal activity such as eavesdropping on cellular calls, phone cloning, and call theft. The GSM voice calls are encrypted using a family of algorithms collectively called A5. A5/1 is the stream cipher which encrypts the information transmitted from mobile user. Initially A5 algorithm was kept secret to ensure the security but as algorithm was disclosed many cryptanalytic attacks were proposed and proved the A5 algorithm cryptographically weak. In this paper the modification in A5/1 is proposed to make it robust and resistive to the attacks. Modification is done in two ways (1) feedback tapping mechanism which is enhanced by variable taps for LFSR (Linear Feedback Shift Register) and random shuffling of LFSRs, which increases the complexity of the algorithm without compromising the properties of randomness and (2) clocking rule. The modification has been proposed keeping the ease of implementation in mind. This modified algorithm has been simulated in MATLAB and tested its randomness properties by `Randomness test suit´ given by NIST-National Institute of Standard and Technology and obtained satisfactory results.