Research On Optimizing Parameters And Aspect Variant Of Visual Cryptography

Visual cryptography is a new cryptographic scheme introduced by Naor and Shmire in Eurocrypt 94. It is an improved secret sharing scheme different from any traditional cryptography. The scheme is perfectly secure and very easy to implement. And it can be decoded directly by the human visual system. The system is similar to a one time pad in the sense.A "visual" recovery for a set X consists of xeroxing the shares given to the participants in X onto transparencies, and then stacking them. The participants in a qualified set will be able to see the secret image without any knowledge of cryptography and without performing any cryptographic computation. While the participants in any forbidden set can not gain information about the shared image, even though it has access to an infinite computational power.Visual cryptography has two parameters: pixel expansion and contrast. It's very difficult to optimize both of them in theory. On the basis of studying the parameters, (2,n) visual cryptography schemes are proposed based on permutation and combination in this paper. The schemes can balance performance between pixel expansion and contrast, i.e., the parameters are close to optimality.In the process of researching on pixel expansion and contrast, it is found that there exists a problem of aspect ration variant between recovered image and original image in visual cryptography. In this paper, the relationship between two improved schemes, which solved the problem of aspect ratio variant, has been proved. At the same time a scheme of encrypting after increasing the secret image has been provided to encrypt the edge of the image.At last, the design and implementation of visual cryptography system is completed in this paper. The system has two parts: basic visual cryptography system and aspect ratio invariant visual cryptography system.