To whom correspondence should be addressed. E-mail: xionghu.uestc@ gmail. com Xiong Hu, born in 1982. He received his B. S degree and M.S. degree from University of Electronic Science and Technology of China in 2004 and 2006 respectively. He is now a Ph. D candidate in School of Computer Science and Engineering, University of Electronic Science and Technology of China. His recent research interests include information security and cryptography