Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China;
visual cryptography; secret sharing scheme; general access structure;