| dc.description.abstract | Visual cryptography is a very secure and unique way to protect secrets. Unlike traditional cryptographic schemes, it uses human eyes to recover the secret without any complex decryption algorithms and the aid of computers. Thus, when computers or any other decryption devices are not available, visual cryptography schemes can be very useful.
In the study of visual cryptography, pixel expansion and contrast are two primary issues. The technique of pixel expansion has been largely adopted to construct visual cryptography schemes. However, pixel expansion can result in many problems such as the problem of image distortion, the requirement of more storage space, and the difficulty in carrying shares. Thus, some methods were proposed to cope with the problems of pixel expansion. Many of these methods integrate the probability concept with the ready-made schemes to avoid pixel expansion. In such methods, the contrast is the same with that provided by the underlying schemes. Although some schemes can provide the best contrast for some access structures, there are still many other schemes can not provide the best contrast. Therefore, in this dissertation, a multi-objective linear programming model for general access structures is constructed to deal with the problems of pixel expansion and contrast. Then, the solution space is searched by the modified simplex method, which is a kind of goal programming algorithms. Experimental results will show that, in average, the proposed method has better contrast than Ateniese et al.’s.
It should be noted that the method of visual cryptography would result in contrast loss of the recovered secret images. If the secret image to be encrypted is a gray-level image with a narrow dynamic range in its gray scales, the phenomenon of contrast loss can be a serious problem because the recovered secret image may be difficult to be identified. Therefore, in this dissertation, a block encryption procedure with thresholding techniques is proposed to compensate the losing contrast for gray-level images.
Recently, visual cryptography or its concept is largely adopted to protect the intellectual property rights for digital images. However, some of the methods do not satisfy the security conditions of visual cryptography; thus they can not be used to protect the copyright of the images. Moreover, many methods are not suitable for gray-level or color watermarks. Therefore, in this dissertation, we come up with a solution to deal with the problems of security and color watermarks. In our scheme, the theory and properties of sampling distribution of means are employed to satisfy the requirements of security and robustness. The proposed method does not really embed the watermark into the host image. Instead, an ownership share is generated from the host image as a key to reveal the watermark without resorting to the original image. This property also allows multiple watermarks to be registered for a single host image without destroying other hidden watermarks. Since the host image will not be altered, the proposed scheme is very suitable for the applications where modifications to the image are not allowed. Finally, the experimental results will show that our method can resist many common attacks. | en_US |