Secure interleaving - physical layer built-in security enhancement of CDMA systems

Generally the physical layer of a communication system does not possess built-in security features except the spread spectrum systems. However, the physical layer built-in in-formation privacy of the conventional CDMA system, provided by pseudo-random scrambling, is far from adequate and can be improved by applying cryptographic techniques in the scrambling process. Motivated by the fact that after scrambling, chips spread from one symbol still cluster together and could be fragile to strong burst errors and fading effects, in this paper, a chip-level secure interleaving procedure is proposed to improve the system performance while enhancing the security measure. More specifically, the AES algorithm is combined with block interleaving. It should be noted that interleaving is in fact a special case of scrambling. Security analysis is provided to demonstrate the effectiveness of the proposed secure interleaving scheme under exhaustive search attack. Simulation examples are presented to illustrate the robustness of chip-level interleaving over channels with severe fading or strong burst errors.