在1994年Noar 與Shamir 所提出。視覺密碼(Visual Cryptography)是對影像加密的一種方法，它在加密時不須要複雜的計算過程，解密時，只須將雜亂的分享影像(Share Images)進行重疊即可利用人類視覺系統取得隱藏的機密訊息(Secret Image)。可惜的是，由此取得的秘密訊息是由黑與白所組成，因而導致影像品質並不理想，再加上傳統的視覺密碼只能使用兩張以上的分享影像隱藏一張秘密影像。因此，近年來，陸續有不少學者提出相關的研究，改善影像品質且隱藏較多的機密影像。在本研究中，提出新的彩色視覺密碼，不僅改善了分享影像和解密後機密影像在視覺上的品質，且可在有限的分享影像中藏入更多的訊息。在安全性方面，可確保無法從單一的分享影像來察覺出機密影像的資訊。

The visual cryptography, proposed by Noar and Shamir in 1994, is a kind of image encrypting. It is different from traditional cryptography, because the share images can be generated without any computation and the secret image can be reconstructed with stacking share images via human visual system. Unfortunately, the reconstructed secret information is almost composed of black and white pixels, so inducing the quality of image is not desirable. In addition, the traditional visual cryptography is merely used more than two share images to encrypt a secret image. Therefore, many savants proposed relevant researches that improve image quality and hide more information continually. In this thesis, we propose a new color visual cryptography scheme based on modified visual cryptography. This scheme is not only bettering quality of share and secret images, but also embedding more information in the limited shares. In security, it can not perceive the secret information from the single share image.

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