1. Department of Computer Engineering, Konkuk University, 268 Chungwondaero, Chungcheongbukdo, Chungju 380-701, Republic of Korea
2. Information Assurance Research Group, Advanced Computing Research Centre, University of South Australia, Mawson Lakes, SA 5095, Australia
3. Department of Computer Engineering, Sungkyunkwan University, 2066 Seoburo, Gyeonggido, Suwon 440-746, Republic of Korea