A complex linear feedback shift register design for the A5 keystream generator

A5 encryption/decryption algorithm depends on the use of linear feedback shift registers (LFSR) in order to generate the key-stream bits. The A5-1 algorithm uses three different LFSRs with a majority function to add non-linearity, while A5-2 uses four LFSRs. In this paper a new multi-configurable LFSR design variant of the A5 algorithm is proposed, in which the inverse taps of the LFSR are used to generate the output key-stream bits. The new LFSR design, called a complex LFSR, techniques can be effectively applied in software or hardware to a communications stream to provide a low cost and high speed encryption capability