| Summary: | 碩士 === 中興大學 === 資訊管理學系所 === 99 === Visual cryptography (VC) is a kind of cryptosystems. The main function of visual cryptography is to protect secret information. Difference from other techniques, i visual cryptography needs neither complicated computation nor any knowledge of cryptography. Instead, it only needs human visual system to get secret information in the process of decryption. Therefore, visual cryptography can decrypt effectively even in a situation without any technological tools.
In 1995, Naor and Shamir proposed visual cryptography and demonstrated the model of visual secret sharing (VSS) scheme for the first time. In the encryption process, one secret image is encoded into two or more share images, which made up of disorganized black and white pixels, and each share image does not reveal any secret information and distributed to the participants. In the decryption process, the secret information is revealed when all the share images that belong to one qualified set are stacked together; otherwise the secret information won’t be recovered.
In the early years, most researches are devoted to solve the problem of pixel expansion and enhance the contrast and quality of the recovered image in visual cryptography field. Later, some scholars found that the disorganized shared images may arouse people’s suspicion and destruction. Thus, in order to improve the security of sharing process, scholars proposed the extended visual secret sharing scheme by adding the cover images to protect the shared images that possess some confidential information.
This thesis first proposes a grayscale visual cryptography scheme without expansion. The main ideas are to preserve the original black secret pixel and to recover clearer and high quality secret image after decryption. Further, this thesis proposes a color extended visual secret sharing scheme based on the above-mentioned encoding model. This scheme reserves information of the cover images to construct the vivid and meaningful shares in the condition that keeping the quality of recovered image and is suitable for both natural and text secret image. In this way, the scheme enhances the security of sharing process and the share images can be managed easily and efficiently.
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