| Summary: | 博士 === 國立交通大學 === 資訊科學與工程研究所 === 97 === In this dissertation, we propose several techniques to protect important data and private images in a transmission or storage system. For a confidential image or secret data, two hiding methods are proposed to conceal the existence of the hidden data in the cover images. Because people often compress digital images to reduce the waiting time of the receiver before transmitting the images, the first method uses the search-order coding to embed secret data in the index file of the vector quantization compression result. The proposed method causes no distortion to the VQ version of the image, and the receiver end can obtain both the hidden data and the VQ image. To embed a bigger confidential image, the MSOC scheme in the second method utilizes the feature of high correlation among adjacent pixels (i.e. neighboring pixels are often with similar gray-values) to encode the important image. An adjustable threshold T is used in the MSOC; and this T directly controls the quality of the extracted image. In the embedding part, we use a variance-based criterion to estimate the hiding capacity of each pixel in the cover image. Then the MSOC code is embedded in the cover image using two sets of modulus function. Experimental results show that the quality of both the stego-images and extracted important images are competitive to those obtained in many existing steganography methods reported recently.
To protect important images in public environment, we develop two image authentication methods along with different recovery abilities. The first method is used to protect a single image. In the method, the authentication data for each block is generated using some related information within the block, and then embedding it into the block to serve as the attestation for the integrity of the image. Meanwhile, the recovery data obtained by vector quantization technique are shared by using an (r, n)-threshold sharing method, and then scattered all over the image. The proposed method can not only detect whether malicious manipulations have occurred, but also self-recover the tampered parts. In the transmission of multiple images, it is possible that the network connection is unstable; and hence, some images at the receiver end are lost. To solve this problem, we develop an image authentication method with cross-recovery ability to protect a group of images. In the method, a two-layer sharing scheme is designed to preserve the recovery data of all images. The proposed method can not only verify the integrity of each member of the image group, but also reconstruct those lost images by the mutual support of the surviving members.
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