Research On Two Kinds Of Visual Cryptography Schemes

In 1994,Naor and Shamir first introduced the concept of visual cryptography scheme(VCS).This scheme gradually became an attractive research point and absorbs the attentions of many scholars due to the simplified recovery.VCS is a branch of secret sharing,in which a secret image is encrypted into n noise-like shares.In the recovering phase,the secret image can be reconstructed by stacking sufficient shares.Based on the previous excellent work of visual cryptography scheme,this thesis makes a further research on the contrast of recovered image and cheating immune scheme.In Chapter 1,we briefly introduce the origin,evolution and research status of VCS,and sketch the development of VCS in detail.We summarize our contributions at the end of this part.In Chapter 2,some corresponding preliminaries are given,including some mathematic knowledge,the principle of modern cryptography,the basic concept of access structure and secret sharing,and the foundational knowledge of visual cryptography scheme.In Chapter 3,we develop a novel contrast improved OR and XOR based(k,n)-VCS without pixel expansion.Significantly,we give a general simplified calculation formula to compute the theoretical contrast of the proposed scheme,which further perfects the theoretical research.In addition,if there are no computing devices,then the secret image can be recovered by stacking the shares directly.Meanwhile,we obtain a higher quality of the reconstructed secret image by performing XOR operation when computing devices are available.Since the proposed scheme is based on the parity basis matrices,our scheme has no pixel expansion.Finally,we give theoretical analysis and experimental results.In Chapter 4,we generate n original shares by using random grid based visual cryptography scheme(RG-based VCS),and stamp authentication patterns on original shares to obtain verifiable shares,where the authentication patterns are three adjacent and non-intersect concentric solid black rings.In authentication phase,the authentication patterns can be revealed respectively by stacking any two verifiable shares in several ways,including rotating one of the two shares by 90°,180° and 270° counterclockwise.The main contribution of this scheme is that we propose a novel cheating immune VCS without the deficiencies of the existing schemes.Since the proposed scheme is based on random grid,our scheme has no pixel expansion.Theoretical analysis and experimental results are given at the end of chapter.In Chapter 5,we improve Renet al.’s scheme[60].More precisely,in order to facilitate the certification process,we embed meaningful characters into the random chosen authentication patterns in each divided blocks.Furthermore,we fix the security vulnerability in the stacked results of share Sg and verification Verg,where 1≤g≤n.Since the improved scheme encrypts the secret image by random grids，the generated shares have no pixel expansion.Finally,theoretical analysis and experimental results are conducted to evaluate the efficiency and security of the proposed scheme.Finally,in Chapter 6，we conclude the thesis and briefly introduce other problems to be considered in future work.