Enhanced A5/1 cipher with improved linear complexity

Global system for mobile communication (GSM) uses A5/1 stream cipher to encrypt the information transmitted between subscribers mobile and the base station. Recent research studies show that A5/1 cipher has some limitations, due to which it is susceptible to various cryptographic attacks. It has weak clocking mechanism and output bit sequence of A5/1 has low value of linear complexity. In this paper, we present an enhanced scheme of A5/1 encryption algorithm, in which the clocking mechanism is improved and linear combining function of A5/1 is replaced with two cryptographically better nonlinear functions to strengthen the cipher. It has been observed that the enhanced scheme has much better and more irregular clocking. It is shown that the proposed scheme provides improvement in the value of linear complexity of output bit sequence generated. Both the designs are coded in VHDL and synthesized on FPGA device.