視覺密碼學分享模型之新設計法

New Designs for Visual Secret Sharing

有別於傳統密碼學（cryptography），視覺密碼學（visual cryptography）是由Naor和Shamir（1995）所提出的一項與影像分享相關的加解密技術。此項技術結合密碼學與資訊分享的概念，將機密影像經加密為n張雜亂無章（noise-like）的分享影像（share），並且將分享影像分配給n個參與者，每人一張。視覺密碼學主要的精神在於解密時，毋需倚賴任何複雜的數學計算以及大量的電腦運算能力，亦不需任何密碼學知識，僅需將k張或k張以上的分享影像重疊，並利用人類視覺系統即可辨識、解碼，重建出原始影像，進而取得所需的機密訊息。本研究是透過藏密學（steganography）的技術，將一張機密影像嵌入到兩張分享影像中。在分享影像上所看到的內容是偽裝影像的資訊，無法直接辨識出機密影像。當分享影像疊合時，將會凸顯出機密影像的內容，而偽裝影像反而會隨著疊合而消失。在實做上，本研究針對2X2的擴展模型提出四個類型、16種不同的分享新模型。並且無論在分享影像或疊合影像上，都能夠產生出25％或50％的黑白色差，因此可以輕易且清楚地辨別出偽裝影像或機密影像的內容。最後，本研究的分享模型的概念簡明且容易實作，使用者可以根據需求來調整影像的黑白色差，以及藏入偽裝影像與機密影像的位置。而在（4,2）／（4,2）分享模型中，分享影像和疊合影像的黑白色差皆為50％，這個結果優於其他學者的研究成果。

Visual secret sharing (VSS) is a kind of image cryptography, proposed by Naor and Shamir (1995), which integrating the concepts of the cryptography and the information sharing. It encodes a secret image into n pieces of noise-like shares and dispatches these shares to the corresponding participants. The main advantages of VC is that it no longer needs complicated computation and specialized knowledge to decrypt the secret, the hidden message will be revealed by naked eyes after superimposing k or more than k pieces of shares. In this paper, we embedded two camouflage images into shares according to the theory of steganography, each share's profile is meaningful and it has no way to reveal secret. Besides, the outline of stacked image will be the secret content and the camouflage images were disappeared along with it. In practice, we proposed four approaches (16 models) to implement it. On the other hand, the black-white contrast of shares and stacked image is either 25% or 50%, that's means that the camouflage images and secret image is easy to recognition. Lastly, our proposed models are easy to implement, and the contrast and embedded location can be adjust according to users need; moreover, both the contrasts of shares and stacked image are better than other researches in the (4, 2)/(4, 2) model.