Research And Optimize On Pixel Unexpanded And Threshold Visual Cryptography Scheme

Visual cryptography is a new cryptographic technology, in which the secret information is directly reconstructed by human visual system. It is similar to "one time one pad" cryptographic system in the intensity of security. From the viewpoint that the size of the image reconstructed is changed or not, visual cryptography schemes can be calculated into pixel expanded schemes and pixel unexpanded schemes.On the basis of security, pixel expanded schemes are valued by two parameters: pixel expansion and relative difference, and to optimize schemes is just to optimize their two parameters. Through research on the relationship between constant weight code and (2, n) threshold schemes, considering the tradeoff among codeword space, codeword length and minimum distance, the dissertation has proposed a (2, n) visual cryptography scheme. The scheme is based on the construction of constant weight code, and it can achieve a better balance between pixel expansion and relative difference.Pixel unexpanded visual cryptography schemes are constructed based on pixel expanded visual cryptography schemes, and to optimize pixel unexpanded schemes is just to optimize their operating mode during sharing process. Researching on close relationship between reconstructing principle of the scheme and human visual system, combing multi-line scan and connected region shared independently, the dissertation has proposed a new pixel unexpanded scheme, applying which can make granularity of the reconstructed image smaller, and make borderline between regions clearer.Based on theoretical research of the dissertation, we has designed and realized a visual cryptography simulation system, which includes sharing module, reconstructing module and setting module.