Randomness Analysis of A5/1 Stream Cipher for Secure Mobile Communication

Abstract

Information Security over mobile communication networks is vital for secure communication. The GSM voice calls are encrypted using a family of algorithms cooperatively called A5. Global System for Mobile communication (GSM) uses A5/1 stream cipher in order to provide privacy on air communication. A5/1 algorithm uses LFSR to generate key stream. Longer shift registers with the help of feedback, generate patterns so long that they look like pseudo random pattern.A5/1 stream cipher generates key which encrypt the information transmitted between subscribers mobile and the base station. It is strong encryption algorithm among all cryptographic algorithms used in GSM but recent research studies show that A5/1 cipher is cryptanalized by a number of attacks..It has feeble clocking mechanism and output bit sequence of A5/1 has low rate of linear complexity. To overcome these problems we convert LFSR based stream cipher into NLFSR based stream cipher using nonlinear combinational generator. It has been observed that the enhanced scheme has much better and more strengthen. NIST Statistical test package approximate the randomness performance of output bit streams of two ciphers. The randomness results confirm that the output bit-stream generated by the proposed stream cipher has improved the randomness performance.