Study On Cheating And Cheating-Prevention In Visual Cryptography Schemes

Visual Cryptography which provides a way to share a secret black-white image was first introduced by Naor and Shamir at EUROCRYPT '94. The idea is to encode the secret image into shares which are distributed to the participants after printing on transparencies. The secret image will be recognized by human eyes system after stacking all of part of shares without complicated computation.The advantages of Visual Cryptography are as follows.First, each share is formed random by black and white pixels, all shares are indistinguishable. So, Visual Cryptography is perfectly secure.Second, users without any cryptographic knowledge can get the secret information just by stacking shares. So, the decoding operation of Visual Cryptography is simple.Because of above advantages Visual Cryptography is more and more popular.In general, the research of Visual Cryptography mostly focus on optimizing parameter, extending the original VCS (such as Multi-Secret Sharing, circle shares, the general access structure of VCS etc.), shares with meaning, the application of Visual Cryptography, and so on. This thesis concerns about the cheating and cheating-prevention of Visual Cryptography.Three cheating methods, such as cahoots cheating, MP(Malicious Participant) cheating and MO (Malicious Outsider) cheating, are reviewed briefly. The corresponding cheating-prevention methods are studied as well. Then a new cheating-prevention scheme is revealed. The result of Matlab simulation indicates that the new cheating-prevention scheme has lots of new advantages.At last, a new cheating-prevention (2,2)-VCS based Multi-Secret Sharing is proposed. In this scheme, guess becomes difficult because the arrangement of sub-pixels is changed. Furthermore, only the master knows the verified image. So, it is impossible for attackers to produce fake shares. The scheme implemented the cheating-prevention effectively without additional shares.