臺灣博碩士論文加值系統

視覺密碼(Visual Cryptographic Scheme; VCS) 覺密碼是一種秘密影像分享機制。它的解密方式，能以疊合子影像的方式經由人眼直接解密，且不需任何計算。最常使用的視覺密碼是(k, n)門檻式機制，是指將一個秘密影像分成n 張子影像。此處的k 值為解密的門檻值。若少於k，則得不到任何關於秘密影像的資訊。視覺密碼這種獨特的「相疊即視」特性讓他有多樣性的可能應用。
最近，Lou等人介紹了一種彩色轉換之視覺密碼（Color Transfer VCS; CTVCS）係將彩色通道R、G、B的資訊嵌入至(k, n)VCS視覺密碼。Lou等人的CTVCS視覺密碼方案可以藉由堆疊合子影像子影像解回秘密，如同視覺密碼VCS。並且們擬採取有可解回原有之彩色訊息。此種圖像畫質已夠應用於一些商業產品，如電子書和廣告板的ChLCD 顯示裝置。然而Lou等人的CTVCS，其彩色通道的資訊是直接使用子影像的像素來表示示，所以需要金鑰排列這些子像素以達到安全性。因此嚴格來說，Lou等人的CTVCS並不是一個安全的秘密分享機制。
因為半色調圖像是從灰階之圖像藉由數位半色調之技術轉換而得，而彩色通道的彩度與灰度有關係。所以彩色通道的彩度和半色調圖像的像素值之間，存在有密切之關係。植基於影像的這種自然特性，本篇論文設計一個新型的CTVCS，以解決Lou等人的CTVCS需要金鑰排列子影像的像素之缺點。我們正式定義了(k, n)- CTVCS的對比度條件與安全性條件，並提出兩種不需排列影像金鑰之彩色轉換視覺密碼機制。我們也從理論上證明所提的(k, n)-CTVCS滿足所定義的條件。實驗結果驗證了所提方法之可行性。

Visual cryptography scheme (VCS) is a kind of cryptography that can be directly decoded by human visual system when transparent films are stacked. It requires no computation for decryption. VCS is often implemented as a threshold (k, n) scheme, which a secret image is subdivided into n shadow images (called shadows). Any k shadows can be simply superimposed together to recover the secret image. However, (k1) or fewer shadows cannot obtain any secret information. This novel stacking-to-see property of VCS can be applied on various applications.
Recently, Luo et al. introduced a notion of color transfer visual cryptographic scheme (CTVCS) by embedding the information of color channels R, G and B into the conventional (k, n)-VCS. Luo et al.’s (k, n)-CTVCS can visually decode the halftoned secret image by stacking any k shadow images like the conventional (k, n)-VCS, and meanwhile it can obtain a high-quality color image. These visual qualities are good enough to be used in commercial products, e.g., the ChLCD display of electronic paper book and advertising board. However, the information of color channels are represented explicitly on shadows, and thus a key is used to permute color pixels to achieve the security. Therefore, Luo et al.’s (k, n)-CTVCS is, strictly speaking, not a threshold scheme because the recovery of the information of color channels is based on a permutation key.
Because a halftoned image is transformed from a gray-level image by digital halftoning technique, and the intensities of color channels can be converted to a grayscale. Therefore, there definitely exists a tight relation between the intensities of color channels and the pixel value of a halftoned image. By using an inherent property of digital image, we propose a (k, n)-CTVCS with perfect security, and do not need permuting pixels. We formally define contrast and security conditions of a (k, n) - CTVCS. Also, we theoretically prove that our CTVCSs satisfy these conditions. Experiment results demonstrate the feasibility of our constructions.
Keywords: Visual cryptography, Secret Sharing, Threshold scheme, Color transfer VCS, halftoned image.

Chapter 1 Introduction 1
1.1 Background 1
1.2 Contribution of the Thesis 3
1.3 Organization of the Thesis 3
Chapter 2 Preliminaries 5
2.1 Naor and Shamir’s (k, n)-VCS 5
2.2 Luo et al.’s (k, n)-CTVCS 10
Chapter 3 Motivation and Design Concept 15
3.1 Motivation 15
3.2 Design Concept 18
Chapter 4 Construction of (k, n)-CTVCS 23
4.1 Contrast and Security Conditions 23
4.2 The Proposed (k, n)-CTVCS 24
4.3 The Enhanced (k, n)-CTVCS 29
Chapter 5 Experiment and Comparison 37
5.1 Experimental Results 37
5.2 Comparison and Discussion 46
5.2.1 Comparison 46
5.2.2 Discussion 49
Chapter 6 Conclusion and Future Work 53
References 55
Appendix 59

論文投稿中，不方便公布