視覺機密分享(Visual Secret Sharing, VSS)機制是一種簡易且安全的影像加密技術。一般的VSS機制主要是將一個二元的機密影像加密至多張影像。每張影像看似雜訊並稱為share。最重要的是從單一share的外觀上絕對無法察覺到任何與機密影像有關的訊息。當要取回機密影像的訊息時，收集一定張數或以上的share，並且將這些share疊合後可取得疊合影像，疊合影像中存在著不同亮度(白色像素不同比例)的區域，即可透過肉眼辨識出機密影像的訊息。反之，如果疊合小於一定張數的share，則無法取得任何機密影像的訊息。
在一般的VSS機制有一個特點，即是對於參與者各自所持有的share，除了收集一定張數的share才能取回機密影像，且每張share所能夠還原機密影像的能力皆相同。換句話說，無論任何share到齊所還原的機密影像其清晰程度皆一致。事實上，在一些情況時參與者的優先權程度是不一樣的，代表參與者所持有的share，其能夠還原機密影像的能力並不相同。在本文中，分別提出兩個不同的優先權式的VSS (Priority VSS, PVSS)機制，在提出的PVSS機制中滿足以下特點：第一，每張share的優先權是需要可被調整的，對於應用才有足夠的彈性；第二，每張share除了無法透漏與機密影像有關的訊息，也不能夠洩漏share本身的優先權程度，有完整的安全性；最後，每張share所還原機密的能力是與各自優先權有關。
另一個一般的VSS機制特點，是所產生的share外觀上皆是無意義的雜訊影像，雖有良好的安全性，但也造成管理上的不便，倘若不經意混淆了所持有的share，將可能造成還原機密影像的困難。在本文中，分別提出兩個不同的友善的VSS (Friendly VSS)機制，在第一個方法中，首要目的是讓share的外觀上呈現掩護影像的訊息，並且在視覺上呈現多階層的掩護影像內容。在第二個機制中，除了讓share的外觀上是有意義的訊息，還讓所還原的機密影像，在視覺上呈現多階層的機密影像內容，讓所還原的機密影像的視覺品質相當良好。
最後，本研究提出一種基於VSS的醫療影像浮水印機制。這方法有以下特性：(a)不需要修改欲保護影像，從而保持其醫療影像的品質，並不會因為有任何失真而產生醫療糾紛；(b)根據影像的特徵與VSS將產生所有權分享影像和主要分享影像，並且通過將所有權分享影像和主要分享影像相結合來提取隱藏的浮水印；(c)所有權分享影像的外觀上是呈現掩護影像的內容，使得所有權分享影像在管理上較為便利；(d)浮水印的尺寸與欲保護影像無關。
總體而言，在本文中所提出的機制，由各章節對於各個機制的實驗結果與分析可知，對於本文方法的效能上，與先前學者的方法比較起來皆來的好，讓不同視覺密碼機制的方法與應用上，可以更為精進。

Visual secret sharing (VSS) is a simple and secure image–encryption technology. Conventional VSS schemes primarily encrypt a given binary image into multiple images, each one of them resembling noise and being referred to as a share. The most important feature of VSS is that it is absolutely impossible to discern any secret image–related information by using the exterior appearance of a single share alone. When trying to retrieve secret–image–related information, a minimal number of shares should be collected and then superimposed to form a stacked image, in which there are regions showing different levels of brightness (with different white–pixel ratios), and the secret image related information can be discerned by visually observing the regions with the human naked eye. However, if the number of stacked shares is less than a threshold value, it is impossible to retrieve any secret image-related information.
Conventional VSS schemes have a feature in common that each share, held by a different participant, has the same ability to recover a given secret image. Additionally, only after a certain number of shares are collected it is possible to retrieve the secret image. In other words, a combination of the same number of randomly selected shares, in sufficient quantity, will make the recovered images have the same clarity. In some circumstances, the priorities may be different among the participants, meaning that the shares held by them may have different abilities to recover a given secret image. This study proposes two different types of priority VSS schemes, in which the following requirements are fulfilled: first, the priority of each share can be adjusted when necessary, providing sufficient flexibility in the use of shares. Second, each share is unable to reveal either secret image-related information or the priority weighting of the share, which ensures an integrated security. Finally, the ability of each share for recovering the secret is associated with the priority weighting of the share.
Another feature of the conventional VSS schemes is that all the generated shares are images with noise–like exterior appearance. Despite providing good security, this would result in management inconvenience if a share is mistaken for another one, likely making it difficult to recover the secret image. This study proposes two different friendly VSS schemes. The first scheme primarily aims at not only presenting cover-image information in the exterior appearance of shares but also visually presenting multi–level contents of the cover images. The second scheme, on top of allowing each share to present meaningful information in the exterior appearance, also allows the multi–level contents of the secret image to be visually presented, generating good visual quality in the recovered image.
Finally, this study proposes a VSS based medical image watermarking scheme, which has the following characteristics: (a) it does not require any modification of the image that is to be protected, thereby maintaining the quality of the medical image and avoiding medical disputes due to any image distortion; (b) it generates an ownership share image and a master share image, according to the image characteristics and VSS, and extracts the hidden watermarks by combining the two images; (c) cover image content is depicted in the appearance of the ownership share image, thereby making the management of ownership share image convenient; (d) the size of the watermark is independent of the image to be protected.
Overall, the schemes presented in this manuscript, as demonstrated through the experimental results and data analysis described in each Chapter and Section, outperform earlier schemes proposed by other researchers, and would further enhance the development of methods and their application to different visual encryption schemes

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