| In the long river of human history, man has been keeping seeking more efficient ways to communicate. The earliest methods included cave drawings, smoke signals, and drums. Advancements of civilization introduced written language, telegraph, radio/television, and, most recently, electronic mail. And this is only a fraction of what we can expect in the future. As more and more communication is conducted electronically, new needs, issues, and opportunities are born. Covert communication techniques based on information hiding is a wonderful flower, which is in good blossom in recent years.Encrypted communication, namely cryptography, tries to hide the contents of a message, that is, the message is scrambled so that it cannot be understood. In contrast, covert communication, namely steganography,is to conceal the fact that a message is hidden and its sender, its receiver or its very existence, that is, the message is hidden so that it cannot be seen. Covert communication based on information hiding plays an important role in secret communication and, especially, military communication. This thesis focuses on research in covert communications.This thesis involves five aspects of steganographic communication schemes.1. To clarify the basic concepts, sub-disciplines, and terminology in information hiding or steganography discipline.2. To expose the importance of image scrambling in covert communication, to measure the degree of scrambling image, and to design new image scrambling algorithms.3. To seek new steganography or schemes to embed secret information into image for covert communication.4. To estimate the maximum embedding capacity while maintaining the image fidelity?5. To study the robustness of information hiding into image, that is, the anti-jamming performance of image as covert channel.The main works and conclusions in this thesis are as follows:1. Because information hiding or steganography is a new discipline, and there are no standard Chinese terms in the area. To discuss issues expediently, the sub-disciplines about information hiding are introduced. The basic concepts and terms in informationhiding are interpreted and corresponding Chinese terms are presented so that the intact term system is constructed. 2. As one of encryption methods, the important function of image scrambling technique in covert communication is explored. The four definitions of image scrambling degree (ISD) are proposed, and, based on this; several classical geometric scrambling transformations are evaluated. The results show that the given definitions and visual observation are consistent. Ideal image scrambling transformation (IIST) is defined; a theorem is presented which demonstrates the fact that the ISD achieves the maximum when IIST is used.3. Four novel image-scrambling algorithms are proposed, and their performances of scrambling image are analyzed. The computational complexities of these algorithms are O(N2), where N2 represents the number of pixels in the image.(1) A scrambling algorithm based on FFT's idea is proposed, the scrambled image using it and random noise image is indistinguishable using HVS.(2) A scrambling algorithm based on knight-tour transformation is presented, which only scrambles the details of image and keep invariable in the mass, so that it can be applied to hide image details. Comparison of the new algorithm with known techniques for scrambling image shows the advantages of the new approach, both large numbers of secret key and fast encrypting/decrypting. Then the security of image and the information of being hidden into it are increased.(3) A new class of image scrambling technique named sub-affine transformation is presented based on the idea of affine transformation in geometry. The main research pays attention to properties of sub-affine transformation. A necessary condition is given in which an affine transformation is sub-affine transformation . The periodicity of sub-affine transformation of digital image is discussed, and a theorem is acquir...
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