| Summary: | 碩士 === 國立中興大學 === 資訊管理學系所 === 98 === Visual secret sharing was proposed by Naor and Shamir in 1995. Visual secret sharing schemes encode a secret image into two or more share images, and single share image can not obtain any information about the secret image. The manager distributes these share images to different participants. The secret image can be recovered by stacking the qualified set of share images. On the contrary, nothing can be obtained if the set is unqualified. The main issues of visual secret sharing are how to reduce expansion rate and raise revealing contrast of the recovered secret image. Therefore, this thesis proposes two visual secret sharing schemes without pixel expansion and provides better quality of the recovered secret image. The first scheme is based on pixel swapping technique. This scheme embeds a main secret and an extra confidential image into two cover images. Both of the generated share images are meaningful. People can obtain the main secret image by stacking the share images together, and the extra confidential image can be revealed by shifting one of the share images for certain units for authentication. Except for sharing binary secret image, this scheme can also apply to share color secret image. The second scheme encodes a grayscale secret image into two share images with six kinds of gray levels. This scheme provides better visual quality of recovered secret image. In decoding process, the properties of non-expansion and high recovered image quality preserve the characteristics of original secret image; so that the recovered secret information can be recognized correctly, correctness is raised. Furthermore, the meaningful share images in the first scheme and high quality of share images do not attract to attackers and easy for management, security is enhanced.
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