由Naor與Shamir兩位學者所提出的視覺密碼 (Visual Cryptography) 是一種用於分享與保護機密影像的極佳技術。該視覺密碼技術在解碼辨認機密影像時，只須使用人類的視覺系統 (Visual System)，不必用到任何的計算裝置；由於此一不必使用計算裝置就能辨認機密影像的特性，特別適用於一些沒有計算裝置的特殊場所，也因該特殊的性質，有須多研究人員投入視覺密碼的相關研究。蹤觀這些研究，有極大部分是在發展新的機制，例如擴張式視覺密碼機制 (Extended Visual Cryptography)主要在研究如何加入辨別圖案到分享圖 (Shares) 上，一般化存取結構視覺密碼機制 (General Access Structure Visual Cryptography) 則發展出一種可以限制存取人員的機制，其它如彩色視覺密碼、半色調 (Halftone) 視覺密碼等，都與發展新的機制有關，這些機制如同當初Naor與Shamir所提出的視覺密碼概念，只用於分享一個機密影像，且都有像素擴張 (Pixel Expansion)及低對比 (Low Contrast)的問題存在。近來有些學者，提出多機密影像的視覺密碼機制，但他們的研究是值基於原先的視覺密碼機制，當分享的機密影像數目增多時，原有的像素擴張及低對比的問題不但存在且更加嚴重，除此之外，這些機制必須重新設計密碼本 (Codebook)。
本論文提出一個多影像的視覺密碼機制，旨在用於分享多機密影像時，可以達到降低像素擴張、提升對比及排除須要重新設計密碼本等效能，所提出的機制採用分離(Separation)、黏貼 (Sticking) 與偽裝 (Camouflaging) 叁種程序，達成將兩張機密影像藏入在兩張分享圖上，並以直接疊合 (Superimposing)方式及人類的視覺系統達到解密功能，而且其加密方法完全不同於傳統視覺密碼的方法，本論文所提出的機制，是值基於分離、黏貼及偽裝三種程序，不但不會造成像素擴張，而且不須重新設計密碼本，最值得一提的是，疊合分享圖重建機密影像時，可以得到更高的對比；由實驗結果顯示，其效能優於其它多影像視覺機密分享機制(Visual Secret Sharing Scheme for Multiple secrets， VSSM)。
其次，本論文提出另一種多區域式視覺密碼機制 (Multiple Regions Visual Cryptography，MRVC)，原先由學者所提出的區域漸進式視覺密碼機制 (Regions Incrementing Visual Cryptography，RIVC) 是一種新穎的概念，但其架構仍採用傳統視覺密碼的方法，即須要建立密碼本，因此像素擴張的問題必然存在其機制中；由於RIVC的新穎性，值得研究如何排除這兩項問題。本論文提出一種結合區域漸進式視覺密碼機制的概念與機率式視覺密碼模式 (Probabilistic Visual Cryptography Approach) 的機制，可以以區域範圍模式逐步揭露機密資訊，對於無權得知機密內容之參與者，分享圖上的區域仍然會保持會亂碼的格式，以達到保護該區域機密的效能；實驗結果顯示，該機制的效能優於原先的RIVC機制。

Visual Cryptography (VC), which proposed by Naor and Shamir is a perfect technique to share and protect secret image. During the decryption process, the Visual Cryptography scheme can recognizes the secret image by using the visual system of human being only; no need of any other computational devices. Because this special property doesn’t need any computational devices to recognize the secret image during the decryption process, Visual Cryptography is suited to be used in the environment which has no computational device. There are many researchers who have devoted themselves to study the related issues of Visual Cryptography because of its special property. Overview of these studies, most of these studies are related to the developing of a new Visual Cryptography scheme. For example, Extended Visual Cryptography scheme is made to understand how to add a recognizing picture on each share. General Access Structures Visual Cryptography is made to develop a scheme which can constraint the user groups to reveal the secret images. And the Color Visual Cryptography, Halftone Visual Cryptography etc. are related to the issue of developing a new scheme organization. These schemes can only share one secret image such as the original Visual Cryptography proposed by Naor and Shamir, encountered with Pixel Expansion and Low Contrast which have existed in the Visual Cryptography scheme since the scheme was proposed. Recently, some researchers proposed the schemes based on the Visual Cryptograph. Because these schemes are based on the Visual Cryptography, therefore, as the number of shared secret increased, the problems of Pixel Expansion and Contrast also became more serious. They need to redesign the Codebook.
In this dissertation, a novel Visual Cryptography scheme for sharing multiple secret images is proposed. The objective is to decrease the Pixel Expansion, enhance Contrast and eliminate the redesigning codebook when the scheme shares multiple secret images. The proposed scheme uses Separating, Sticking and Camouflaging processes to hide and share two secret images with two noise shares. To decrypt two secret images, it needs to superimpose two shares directly and recognize the secret images by visual system. The encryption process of the proposed scheme is different from all previous proposed schemes. The proposed scheme in this dissertation, which is based on separating, sticking and camouflaging processes, not only can generate Pixel Expansion free shares but also obtains higher contrast on recovered images without the need to redesign the codebook. Experimental results show that the proposed scheme has achieved better performance than all of the existing Visual Secret Sharing scheme for multiple secrets (VSSM) schemes.
Next, this dissertation proposes a Multiple Regions Visual Cryptography scheme. It is related to the Region Incrementing Visual Cryptography (RIVC) which proposed by Wang. The RIVC scheme is a novel concept and based on the concept of the conventional VC, therefore, it needs a complicated codebook and encounters more serious pixel expansion problem than the conventional Visual Cryptography scheme. Because of the novelty of RIC, it is valuable to study how to solve these two problems which exist in all schemes based on the conventional VC scheme. In the second study of this dissertation, a MRIVC scheme which combined the concept of RIVC and the probabilistic visual cryptograph model is proposed to disclose the secret region by region with free pixel expansion and the lacking of codebook. For those participants who have no authority to know the secrets hidden in those regions, some regions on superimposed shares the remaining noise format to protect the secret on those regions. Furthermore, according to the experimental results, the proposed scheme can obtain better performance than the RIVC.

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