臺灣博碩士論文加值系統

視覺式秘密分享是一種圖像秘密的機密保護機制，其最大的特點為解密的過程是利用人眼來辨識機密，完全不需要任何電腦輔助的計算。傳統的視覺式秘密分享方法大都將整張圖像視為一體來處理，因此整張圖像的內容將被限制於一個單一機密等級。Wang等人[27]於2007年提出一個多層次的視覺式秘密分享，將一張圖像的內容分成n個層級，並編碼成n+1張分存圖像。這個方法隨著者參與解密的分存圖像增加，可揭露越多層級的機密資訊。此方法賦予視覺式秘密分享具備分享多個層級機密的功能，但仍有下列兩個弱點: (1)像素擴張量太大，(2)疊合圖像中不同機密層級的區域會出現不同的明亮對比，可能洩漏出尚未解出之機密數量與位置的蹤跡。
在本研究中，我們利用隨機網格的概念設計一個增值式的視覺式秘密分享。在所提出的方法中，機密圖像會被分割為屬於1至n機密層級的多個圖像區域，並且編碼成n+1張分存圖像。這個方法具有下列特性: (1)每一張分存圖像都看似雜訊，無法從中看出任何機密資訊，(2)每一張分存圖像都與原圖的大小一樣，(3)任t (2≦t≦n+1) 張分存圖像能還原t-1個層級的機密，(4)疊合所有的分存圖像可解密出全部的機密，(5)在疊合圖像中，尚未解碼的機密區域與數量是完全不可知的，(6)解碼的過程是由人眼來檢視疊合圖像中所呈現的訊息，不需要任何電腦輔助的計算。

Visual cryptography (VC) is an image-based secret protection mechanism in which the decoding process is conducted by inspecting the stacked shares with the unaided eye without any computation. In traditional VC schemes, the content of an image is usually processed as a single secret, which limits the whole image to a unique secret property. Recently, Wang et. al. [27] proposed a multilevel VC scheme in which the secrets on the original image are concealed in such a way that more levels of secrets are revealed when more shares are engaged in the decoding process. The scheme endows an image with multiple secret levels in VC applications; however, it also suffers from two weaknesses: (1) the pixel expansion is large, and (2) the contrast varies among the regions of different secret levels on the stacked images.
In this study, we propose a non-expansible VC scheme using the concept of random grids with the ability of incremental revealing of the secrets. In the proposed n-level incrementing VC using random grids, the secret image S is designated to multiple regions associated with n secret levels, and encrypted to n+1 shares which exhibits the following properties: (1) each share has noise-like appearance and cannot obtain any secret in S, (2) the size of each share is the same as S, (3) any t (2≦t≦n+1) shares can be used to reveal up to t-1 levels of secrets, (4) all secrets in S can be disclosed when all of the shares are available, (5) all of the regions containing not-revealed-yet secrets have same contrast with the background on the stacked shares, and (6) the decoding process is done by inspecting the well-stacked shares with unaided eye without computer computation.

摘要 i
Abstract ii
Chapter 1 Introduction 1
1.1 Background 1
1.2 Motivation 3
1.3 Thesis organization 4
Chapter 2 Related Work 5
2.1 Naor and Shamir’s visual cryptography scheme 5
2.2 Kafri and Keren’s scheme of Image Encryption by Random Grids 9
2.3 Multilevel visual cryptography 13
Chapter 3 The Proposed Method 16
3.1 The proposed n-level IVCRG 16
3.2 Construction of the 2-level IVCRG 19
3.3 Construction of the 3-level IVCRG 23
Chapter 4 Experimental results 29
Chapter 5 Conclusion and future work 39
Reference 40

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