| Summary: | 碩士 === 國立東華大學 === 資訊工程學系 === 103 === A visual cryptography scheme (VCS) shares a secret image into some shadow images (referred to as shadows) in a way that requires the shadows in a certain privileged coalitions for the secret reconstruction. However, the secret image cannot be revealed if they are not combined in the prescribed way. VCS has a novel stacking-to-see property where decoding requires neither knowledge of cryptography nor computer. VCS is often used as a (k, n)-threshold scheme, where k is the threshold value to reveal the secret and n is the number of shadows. One can reconstruct a secret image by k or more shadows, while he cannot gain any information from less than k shadows.
VCS can also be applied on colored secret images. Bot shadows and secret image of colored VCS are color. Recently, De Prisco and De Santis introduced a new notion of colored-black-and-white VCS (known as CBW-VCS). Same as the CVCS, the CBW-VCS has colored pixels in shadows but it shares the black-and-white secret image not the colored image. When compared with the black-and-white VCS, the CBW-VCS can reduce the pixel expansion and enhance the contrast due to using colored pixels in shadows. The conventional VCS has noise-like shadows, which are suspected and difficult for identification and management. Therefore, a so-called extended VCS (EVCS) is proposed. De Prisco and De Santis’s CBW-VCS also has noise-like shadows. To address this problem, we study the (k, n)-CBW-EVCS that is a (k, n)-CBW-VCS with the extended ability revealing meaningful shadows.
In this thesis, we propose three CBW-EVCSs: (2, 2)-CBW-EVCS, (2, n)-CBW-EVCS and (k, n)-PB-CBW-EVCS. Besides, we define the security, contrast and cover image conditions of (k, n)-CBW-VCS. Also, we theoretically prove that all schemes satisfy the above three conditions.
Keywords: Visual secret sharing, Visual cryptography, Extended visual cryptography, Black and white secret image, Color shadow image.
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