Study Of Visual Cryptography Technology

Visual Cryptography (VC) is a new secret sharing technology. It improves the decryption complexity of the secret sharing system to decrypt depended upon human's vision. Compared with the traditional cryptology, VC has some outstanding advantages such as confidentiality, security and easy recovery. It mainly applied in high security areas. This dissertation investigates the visual cryptography technology, with emphasis on some important research directions and is to propose safer and high efficient visual cryptography schemes. The dissertation obtains main results as follows:1. A perfect contrast Visual Cryptography Scheme (VCS) is proposed for the features of the large pixel-expansion and low contrast in VC. This scheme simultaneously applies the basic Boolean operations and simple shifting to keep the consistency between the reconstructed image and the original secret image without pixel expansion. Compared with the schemes proposed in literatures, this scheme has the advantages of perfect contrast, compatibility, low computational complexity and few rounds to realize simply Boolean operations.2. The design of above scheme is extended to the so-called "general access structure", a general reconstructed scheme is proposed for the existing visual cryptography schemes. The scheme has two advantages: First, the scheme reaches the perfect contrast for general access structure. Second, compared with the scheme in literature, the proposed scheme computes 3 times"OR"operation and 1 time"NOT"operation fewer each time in computation program. And the auxiliary image selected with black pixels primarily, has stronger confidentiality and security.3. To make different users with different security levels obtain different information, we introduce the concept of hierarchical participants and present a multilevel-based VCS for general access structure. In this scheme, the super-participants can reveal the original secret directly by superimposing their sharing images, but the sub-participants cannot obtain the secret image unless they combine with the super-participants'images.4. A cheating detected VCS is put forward based on both (k,n) and (k+1,n) threshold schemes for the participants'cheating in VC. In virtue of the third trusted party, any participants can be active or passive required to verify their sharing images by superimposing enough sharing images to recover corresponding secret images. Compared with the scheme proposed in literature, experiments shows our scheme has the better reconstructed image effect. 5. We present a VCS based on message authentication model by means of the public key infrastructure based on message authentication model for the dishonest distribution existing in VC. Verify the authenticity of issued sharing images and detect the distributions'honest or not though superimposing the public and the private sharing images.6. A VCS based on the digital signature is presented for discrimination and management for single-user to massive sharing images, applying the digital signature and the time-stamp technology in VC. The scheme solves the problem on the storage and management of the sharing images, increases the verification of image contents, and enhances the validity of storage and management without security effect.