| Summary: | 碩士 === 國立政治大學 === 資訊科學學系 === 99 === Visual cryptography is different from the conventional cryptography, because it can be decrypted directly by human visual system without any complex computation. Conventional visual secret sharing is designed for hiding only single secret image, and it is inefficient to hide numerous secret images.
In this thesis, we introduce a new visual secret sharing scheme which requires only two share images, and by shifting one of them up to down or left to right, our new scheme can decrypt more secret images.
In most of the current researches on multiple secrets visual cryptography, the amount of the images that can be hidden is limited by the number of times that a sharing image can be rotated. Although some researches provide ways to hide n secret images where n can be any value, the contrast and the ratio of shape will decrease when n is large. Because the contrast determines the image’s clarity and pixel expansion determines the size of composite image, they are two important indicators of researches on visual secret sharing. The main contributions of this thesis are: Firstly, the amount of secret images to be hidden can be any value n. Secondly, the contrast, pixel expansion and the ratio of shape is independent of n, which means that they will remain the same for different n. Thirdly, when hiding the same amount of secret images, our scheme has the least size of composite image (ie., the size of share B) comparing with other schemes.
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