「可視秘密碎片」馬賽克畫—一種新的藝術與其在資 訊隱藏上的應用

Abstract

在本論文中，我們提出了一個新的藝術畫 - 「可視秘密碎片馬賽克畫」，並 應用在這種藝術畫，發展出三種不同的資訊隱藏技術 - 秘密傳輸、影像隱匿術 及秘密分享。這種藝術畫是將一張「秘密影像」切割成許多正方形碎片，並用這 些碎片當成元件，組成起來的新型式馬賽克畫。首先，我們針對人類視覺對顏色 的敏感度，提出了一表示影像色彩分佈的公式，並用它從資料庫中篩選出與秘密 影像最相像的候選圖片，來當作製作馬賽克畫的「目標影像」 。有了秘密影像以及 目標影像之後，我們利用本研究所提出的一個貪婪演算法，將秘密影像的碎片逐一嵌合 到目標影像上。另外，當資料庫中的圖片數量不足時，選出的目標影像的色彩分佈會與 秘密影像不同，導致製成的馬賽克畫和目標影像相差過遠。為此我們也提出了一個彌補 這種情況的方法。 在秘密傳輸方面，我們是在製作可視秘密碎片馬賽克畫的時候，根據「在同一長方 圖容器中(histogram bin)的秘密影像碎片會擁有相似的顏色」的這項特性，交換秘密影 像碎片所對應到的目標影像區塊的標籤，來達到藏入秘密訊息的效果。在影像隱匿術方 面，我們是將秘密文件轉換成灰階影像，並用它來製成一新的馬賽克畫，藉此達到隱藏 秘密文件的效果，並根據灰階值提出另一個特徵值的計算方法，用以提高馬賽克畫製作 的速度。最後，在秘密分享方面，我們是將一張秘密影像分成多張可視秘密碎片馬賽克 畫，並將秘密影像的碎片平均分散在各個目標影像裡。 除了上述的方法外，我們還提出了幾個增加安全性的方法，確保藏入的秘密資訊不 被駭客發現並攻擊。以上的方法皆有實驗結果證明它們在視覺方面的良好成效，以及在 資訊隱藏技術上的可行性。

iii

created from a grayscale image of a secret document, yielding a steganographic effect of hiding the secret document into the mosaic image, though visibly. The selection of the most similar target image from a database is based on a newly-proposed k-feature of the grayscale value, which speeds up the image creation process. Finally, for secret sharing, a secret image is shared to yield multiple secret-fragment-visible mosaic images. In order to disperse tile images evenly in selected target images, the target images take turns randomly to pick appropriate tile images. In addition, various security enhancement measures were proposed to make the embedded data more random to prevent hackers’ attacks. Experimental results show the feasibility of the proposed methods for image creation and data hiding applications. In this study, a new type of art image is created, which is called secret-fragment-visible mosaic image. And this kind of new mosaic image is used for three applications of data hiding, namely, covert communication, steganography, and secret sharing. First, the newly-proposed secret-fragment-visible mosaic image is created to be composed of rectangular-shaped fragments which come from division of a secret image. A new 1-D h-colorscale is proposed to represent the color distribution of an image based on the color feeling of human vision. To create a secret-fragment-visible mosaic image, a new image similarity measure based on the h-colorscale is proposed, and the most similar candidate image from an image database is selected accordingly as a target image. Then, a greedy algorithm is adopted to fit every tile image in the secret image into an appropriate block in the target image. Furthermore, to solve the problem of using an insufficiently-large database, a remedy method by enlarging the size of a target image is also proposed. Secondly, for covert communication, based on the fact that tile images which are in an identical histogram bin have similar colors, the tile images in an identical histogram bin are reordered, or equivalently, the relative positions of the tile images are switched, to embed secret message bits imperceptibly. Third, for image steganography, a grayscale secret-fragment-visible mosaic image is