有意義且不擴展分享影像之漸進式視覺密碼

Friendly and Unexpanded Progressive Visual Cryptography

10.6382/JIM.201007.0131

侯永昌(Young-Chang Hou)；官振宇(Zen-Yu Quan)

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

資訊管理學報

17卷3期（2010 / 07 / 01）

131 - 154

繁體中文

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，假如有超過k(k≦n)位參與者共同參與即可解譯機密資訊，否則就無法在疊合影像上察覺任何機密資訊，這就稱為(k, n)門檻機制的視覺機密分享(visual secret sharing, VSS)。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，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，在疊合影像不保證是全黑，白點的部份也不保證是半黑半白，這將會造成疊合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，在疊合影像上一定會是全黑，白點的部份也一定是半黑半白，可以產生較佳的色差對比和視覺品質。在分享影像與疊合影像的品質上，每一張分享影像和疊合後的還原影像上分別有(i-1)/(n+i)與(i-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疊合影像的色差對比。

Visual cryptography (VC) is a kind of data encoding method, which is encoding a secret image into n pieces of noise-like shares and distributing them to n participants. As long as we gather over k participants's share, the secret content can be revealed by human eyes through stacking those shares, while less than k participants cannot get any information about the secrets. This is called (k, n) -threshold visual secret sharing (VSS) scheme. Progressive VC (PVC) is dfferent from traditional VSS, which recovers the secret gradually. That's means, as more shares are being stacked, the outline of the secret image will be getting more clearer. The previous studies of PVC were all based on pixel-expansion, therefore the size of shares are m times larger than the original ones. While recovering a secret image, they would cause a poor visual quality because the black pixels on the restored image might not be fully reconstructed to be black and white pixels might not be 50% of black. Besides, they used one kind of the camouflage image to produce all shares, supervisors might be able to know which participants had actually involved in, and it might bring about some management problems. In this paper, we propose a novel PVC scheme to produce non-expansion and meaningful shares; and the number of the camouflage images can be extended from single-meaningful image to multiple-meaningful images. As the number of stacking shares increases, the contour of the secret image will be more obvious on the stacked shares. Our research can ensure the black pixels on the restored image to be fully black, and white pixels to be 50% black and 50% white which will cause better contrast, (i-1) / (n + i) and (n-1) / (n + i), on both shares and restored image, respectively, which can display the content of the camouflage and secret images more clearly. Moreover, our dispatching matrices are flexible which can be used to adjust the contrast based on user needs.

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