Research On The Method Of Visual Cryptography

Secret sharing is a cryptography technique that breaks secret information into pieces and assigns them to different participants. Such mechanism can not only prevent the secret information being highly centralized, but also guarantee the security and completeness of the secret, reducing the risk of secret missing. Visual cryptography is a new kind of secret sharing scheme. Because of the easiness during decoding phase, without any complex computation and high quality of security, visual cryptography has been attract many researchers’ attention.This thesis focuses on solving the pixel expansion problem and enhancing the contrast of decoding secret image. The main contributions can be summarized as follows:1. To solve the pixel expansion problem, we proposed a new visual cryptography scheme base on probabilistic selection for color image, which can generate share images without pixel expansion. We partition the secret image into blocks. For each block, we construct two basic matrices and an extended matrix containing the cover images’ pixel information. Then a selection principle based on probabilistic is proposed to decide the encryption matrix. After the encoding of each block, we use the error diffusion algorithm and propose an optimization method based on reducing the total perceived error to generate and optimize share images. Experiments demonstrate that the proposed method can generate share images with high visual quality without pixel expansion and outperforms the other schemes.2. Inspired by the (k, n) threshold visual cryptography scheme, we proposed a contrast maximum model, which can obtain better visual effect of the decoding secret image. Theoretically we prove the security of the model. Experiments results demonstrate the security and feasibility of the proposed method.3. Region incrementing visual cryptography scheme based on random grids is proposed in this part. Secret image is firstly divided into different regions with different security levels. For each region, we apply different sharing structure during the encryption processing, i.e. using the sharing structure (i+1, n+1) to encrypt the region with security level i. Then we propose two methods to generate share images. One is superposition-based method and the other is spliced-based method. Finally we proposed another novel method based on the ring structure, which can make the applications of region incrementing visual cryptography more diverse. Experimental results show that the proposed three methods can achieve region incrementing effect, and has the obvious advantage compared to other methods.