Research On Co-cheating Prevention Visual Cryptography Scheme

Visual Cryptography (VC), which is named after new cryptographic techniques as well as Quantum Cryptography and Network Cryptography, has been a research hotspot for its simplicity of decryption and absolute security in recent years. Nevertheless, there are also cheating problems in VC as in other secret sharing technologies, adversaries or dishonest participators may cheat other honest participators using fake shares. Therefore, the study of cheating prevention VC not only enhances the application security of VC, but also can help advance its practicality. This thesis reviews the research actuality of VC, analyzes the sorts and mechanisms of cheating. Then indicates shortcomings of two main kinds of co-cheating prevention VC schemes: Speculation Prevention Scheme (SPS) and Forgery Prevention Scheme (FPS), and modifies the FPS emphatically. On the basis of above work, a novel co-cheating prevention method without basic matrices, which provides a new way for solving the co-cheating prevention problem, is proposed at last.Firstly, this thesis points out that the complicate verification process will affect execution efficiency of FPS. By means of designing special verification shares for the trusty third party, a novel FPS is proposed. In the scheme, the third party not only holds several normal verification shares which are bijective with each share, but also keeps a special verification share connected with all shares. With the special verification share, the scheme can validate several shares simultaneously, and the checking efficiency is improved greatly.Secondly, in allusion to bad recovery effects of the existing FPS, a new FPS based on XOR operation is designed. The scheme not only has no pixel expansion with the modified secret sharing way, but also makes the verification image be recovered with no distortion because of the XOR operation introduced, whose algebraic structure is group, during the verification process.Based on the above design thoughts, a novel FPS, which considers both validation efficiency and recovery effects eclectically, is proposed with iterative algorithm. In the scheme, both two problems above are improved, since the verification image is shared with special algorithm, recovered by XOR operation, and the verification process is optimized through the iterative algorithm.Lastly, a novel co-cheating prevention VC scheme based on random numbers is proposed. The scheme creates parts of shares with random numbers, and is expanded from (n, n) threshold structure to general access structure by cumulative array. Since avoiding the basic matrix construction, the proposed scheme not only has no pixel expansion, but also prevents collusive cheaters from speculating making use of basic matrices. The contents of this thesis lay the foundation for further research and applications of the co-cheating prevention VC.