1. Center for Information Security and Technologies, Korea University, 136-713 Seoul, Republic of Korea
2. Institut National de Recherche en Informatique et en Automatique (INRIA), 35042 Rennes, France
3. Department of Ubiquitous Computing, Hallym University, 39 Hallymdaehak-gil, Chuncheon-si, Gangwon-do 200-702, Republic of Korea