1. Center for Information Security Technologies (CIST), Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea
2. Department of Mathematics, University of Seoul, Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, Republic of Korea
3. Department of Computer Science and Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 139-743, Republic of Korea
4. Electronics and Telecommunication Research Institute, 218 Gajeong-ro, Yuseong-gu, Daejeon 305-700, Republic of Korea