Study On Techniques Of Fragile Digital Watermarking And Steganography For Images

The arrival of information age caused a revolution in our society and our daily life. Rapid development and extensive use of computer network technique, communication technique and multimedia technique, especially the rocketing development of Internet, have made the security and protection of network information be a problem, which needs to be urgently resolved. In this situation, information hiding technique used as the main method for copyright protection and covert communication has been focused on the international information security field.Clear focus on protecting the fidelity, integrity and security of digital content, this thesis devotes to researching fragile watermarking technique and digital steganographic technique with image as the carrier.Firstly, the current fragile watermarking algorithm used for precise authentication of image is analysed. In order to enhance the security of this algorithm, the properties of super randomicity and high sensitivity on initial value of chaotic map are utilized, and a secure chaotic fragile watermarking algorithm is proposed. Randomly interlaced distribution of original image content and watermark content generated from original image enhances the security of the proposed algorithm. Experimental results show that the proposed algorithm can precisely detect and localize the malicious tamper of image content; simultaneously, the watermarked image has some merits such as good imperceptibility; the extraction of watermark without the original image.Secondly, from the point of view of protecting the imperceptibility of secret information, image steganographic algorithm used for covert communication is researched, and the conclusion is obtained that the imperceptibility of secret information is the premise condition for protecting the security of the algorithm. Based on HVS (Human Vision System) model, a chaos-based self-adaptive image steganographic algorithm in DCT (Discrete Cosine Transform) domain is proposed. The JND (Just Noticeable Difference) obtained from HVS model is used to adjust the embedding strength and position, which realizes the perfect compromise of imperceptibility and robustness. Experimental results validate the validity of the proposed algorithm.Thirdly, the influence of image statistic caused by F₁ substitution and F_-1 substitution is theoretically analysed, and the ideal embedding ratio relation of F₁ substitution and F_-1 substitution used to against SPA steganalysis is obtained. Hence, the idea of error correction is utilized and a novel image steganographic algorithm against SPA steganalysis is proposed. Expermental results show that the proposed secure algorithm can resist SPA steganalysis effectively. Simultaneously, the less modification and imperceptible stego-image can be obtained.Fourthly, on the basis of the analysis of the current iterative blending image steganographic algorithm, a novel image steganographic algorithm based on wavelet contrast is proposed. The sum of absolute values of wavelet contrasts of each image block is adopted as the measurement of visual sensitivity. Under the direction of this measurement, suitable image blocks are selected and the scrambled gray scale image is embedded into these suitable image blocks by iterative blending method. Simulation results show that the proposed algorithm has good invisibility and large capacity for information hiding.Finally, in order to increase the capacity of image steganographic algorithm, the idea of iterative blending is applied to integer wavelet domain and a novel self-adaptive image steganographic algorithm is presented. The employment of integer wavelet overcomes the boundary difference and rounding difference that exist in traditional image steganographic algorithms. Simultaneously, the HVS model is sufficiently used to adjust the imperceptibility and robustness. Experimental results show that the proposed algorithm has a large capacity, good imperceptibility and strong robustness.