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    Title: 有意義且不擴展分享影像之漸進式視覺密碼
    Other Titles: Friendly and unexpanded progressive visual cryptography
    Authors: 官振宇;Quan, Zen-yu
    Contributors: 淡江大學資訊管理學系碩士班
    侯永昌;Hou, Young-chang
    Keywords: 視覺密碼;漸進式視覺密碼;機密資訊分享;不擴展分享影像;有意義的分享影像;Visual Cryptography;Progressive Visual Cryptography;Secret Sharing;Unexpaned Share;Meaningful (Friendly) Share
    Date: 2009
    Issue Date: 2010-01-11 04:55:33 (UTC+8)
    Abstract: 視覺密碼是一種資料加密的方法,其作法是將機密資訊分散為 n 張雜亂的 (noise-like) 分享影像 (shares),讓每一個參與機密分享的人都分別持有一張分享影像,其目的是希望透過分享影像間的疊合,並藉由人類的視覺系統即可直接對加密資訊進行解讀。當要還原機密影像時,假如超過 k (k ≦ n) 位參與者共同參與即可解譯機密資訊,否則無法進行解密,這就稱為 (k, n)-threshold 視覺機密分享 (visual secret sharing, VSS) 機制。漸進式視覺密碼不同於傳統視覺密碼,它的概念是透過疊合 2 張以上的分享影像,即可逐步地還原機密資訊,當疊合的分享影像愈多,所還原的機密資訊會愈來愈清晰。
    在現行的漸進式視覺密碼研究中,雖然分享影像的內容已經由雜訊式轉變為有意義的偽裝影像,不過其分享方法仍是以像素擴展為基礎,因此分享影像的大小會擴展為機密影像的 m 倍。當分享影像疊合時,機密影像上的黑點部分,在還原影像也不保證是產生全黑,這將會造成影像的還原品質不佳。此外,在分享影像上的偽裝樣式只有一種,管理者無法得知有哪些參與者共同參與解密,這將會造成管理上的問題。
    為了解決上述的問題,本研究將以漸進式視覺密碼為基礎,提出一個像素不擴展且有意義分享影像的視覺密碼分享方法,並且將分享影像的樣式擴充為多張偽裝影像。隨著疊合的分享影像增加,機密資訊的輪廓將愈來愈清晰,並且機密影像上的黑點部分,在還原影像上一定會是全黑,以產生較佳的色差對比和視覺品質。在分享影像與還原影像的品質上,每一張分享影像和疊合的還原影像上分別有 (i-1)/(n+i) 與 (n-1)/(n+i) 的色差對比,可以清楚地顯示出偽裝影像與機密影像的內容。此外,本研究的分享矩陣設計是可以擴充與變動的,使用者可以根據需要來調整分享與還原影像的色差對比。
    Visual cryptography is a kind of data encoding method, which is encoding secret image into n pieces of noise-like shares and distributed them to every participator, and through stacking these shares to decode it directly by human eyes. As long as there are over k participators, the secrets are able to be decoded, while less than k can not. This is called (k, n)-threshold visual secret sharing (VSS) scheme. Yet progressive visual cryptography is dfferent from traditional VSS, which is decoding secret gradually by superimposing 2 or more pieces of shares. When more shares are being stacked, the outline of the secret will get clearer.
    Although the content of progressive visual cryptography studies have transferred from noise-like shares to meaningful stego ones, the sharing way is still based on pixel-expansion, therefore the sizes of shares expand to m times compared to original image. While recovering a secret image, it causes a bad quality because black pixel cannot be sure being reconstructed totally. Besides, due to there is only one sort of stego form, supervisors cannot know which participators have actually involved in, and it could bring about management problems.
    In order to solve the above problems, our research is in compliance with the theory of progressive visual cryptography to propose an image sharing method with pixel non-expansion as well as meaningful shares; in addition, the content of meaningful shares expands from only single image to many different disguised images. Following the numbers of shares is adding, the contour of the secret image will be more obvious, also the recovered black pixels can be ensured to be totally black and can produce a better quality of color contrast in both stego shares and secret image, which represent (i-1)/(n+i) and (n-1)/(n+i), and able to appears the contents clearly. Moreover, sharing matrix is designed to be expandable and variable in our study to adjust color contrast of both shares and recovered image based on user needs.
    Appears in Collections:[資訊管理學系暨研究所] 學位論文

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