Enhancement of GSM stream cipher security using variable taps mechanism and nonlinear combination functions on linear feedback shift registers

Abstract

With the advance wireless communication, data security became a significant concern. The GSM standard hardware level encryption technique uses A5/1 algorithm circuit which embedded in the Mobile Equipment. A5/1 algorithm uses Linear Feedback Shift Register (LFSR) to produce a key streams for encode the information sent between the mobile station and the base station. It is a secure cipher among all the versions of ciphers using in GSM. However, latest research studies demonstrate that A5/1 can be subjected to several attacks owing to feeble clocking mechanism which results in a low rate of linear complexity. To overcome from these issues, we introduce a feedback tap mechanism enhanced by variable taps and four nonlinear combination functions. Analysis shows that the proposed method has a high algebraic degree of correlation immunity against basic correlation attack, mathematical attack, linear estimate attack and Berlekamp-Massey attack.