Department of Electronics and Information Technology, Politecnico di Milano, Milano, Italy;Department of Electrical and Computer Engineering,, University of Massachusetts, Amherst, MA, Italy;Department of Electrical and Computer Engineering,, École normale supÉrieure, Équipe de cryptographie, 45 rue d′Ulm, Paris cedex 05, MA, France;Institute for Computer Science, University of Innsbruck, 45 rue d′Ulm, Innsbruck, MA, Austria