臺灣博碩士論文加值系統

在本論文中，我們提出一個像素不擴展之灰階視覺密碼的方案，以解決像素擴展所造成的數倍儲藏容量。我們的方法可以呈現三種不同深淺的黑色子區塊並同時加密多個點，這將會使得疊合影像有更好的品質。
我們所提出的方案是利用區塊加密的方法來達到像素不擴展的目的。首先，我們將一張x×y 的機密影像切割成2×2 大小的子區塊，並將這些子區塊藉由我們的方法歸納為三種類別。它們分別是0%、50%、100%的黑色子區塊。然後，它們將透過區塊加密的規則分別對應到50%、75%、100%的黑色疊合子區塊。此外，我們利用調整75%黑色疊合子區塊的位置來達到與機密影像更相似的疊合影像。

這篇論文主要共現有三：
(1) 機密影像與疊合影像相同大小，可避免浪費儲存空間。
(2) 三種不同深淺的黑色子區塊可更細膩的呈現機密影像。
(3) 透過調整75%黑色疊合子區塊而得到更相似機密影像的疊合影像。

我們相信本論文的研究結果對未來視覺密碼學的相關研究上能有更進一步的改良發展且在應用上能有相當的助益。

In this thesis, we propose a scheme for gray-level images without pixel expansion. It solves the difficulty in carrying shares by using pixel expansion scheme and the requirement of more storage space. Our method can present three different levels of black sub-block and encrypt multi pixels at the same time. This makes the stacked image has better visual quality.
In the proposed scheme, we utilize the method, block coding, to encrypt the secret image and reach the purpose of pixel unexpansion. At first, we divide the secret image (x × y pixels) into n sub-blocks which are 2×2 blocks. The sub-blocks can be grouped into three types by our manner. They are 0%, 50% and 100% black sub-blocks. Then, they will individually correspond to three different shades of the black blocks (50%, 75% and 100% black blocks) that are stacked sub-blocks through our coding rules. In addition, we offer an improvement that revises the position of 75% black stacked sub-blocks and it will get more similar with the secret image than the unimproved.

The contributions in this thesis are as follows:
(1) The size of the stacked image is the same as the secret image. This makes us to avoid wasting a lot of storage space and carry the shares easier.
(2) The black blocks of three different degrees can be more delicate to show the secret information. It has better visual effect for the stacked image.
(3) After revising the position of 75% black stacked sub-blocks, the stacked images will be more similar with the secret image.
As expected, visual cryptography will be employed in future. We trust that results of our research in this thesis will be probably practical and efficient to establish a useful visual cryptography scheme with above advantages in applications.

Contents

Chinese Abstract……………………………………………………………………..I
English Abstract……………………………………….…………………………….II
Acknowledge………………………………………………………………………..IV
List of Figures…………………………………………………………………….VI
List of Tables………………………………………………………........................VII

Chapter 1: Introduction……………………………………………………………..1
1.1 Background…………………………………………………………………..1
1.2 Research Motivation and Purpose…………………………………………...4
1.3 Organization of This Thesis………………………………………………….5

Chapter 2: Previous Works………………………………………………………….6
2.1 The Principle of Visual Cryptography………………………………………..6
2.2 Halftone……………………………………………………………………..10
2.2.1 Ordered Dither………………………………………………………..11
2.2.2 Error Diffusion………………………………………………………..13

Chapter 3: The Proposed Scheme…………………………………………………17
3.1 An Unexpanded VC Scheme by Encrypting a Single Pixel….......................17
3.2 An Unexpanded VC Scheme by Block Coding……………………………..20
3.3 Reduction of Error Information……………………………………………..25
3.4 The Improvement of Performance…………………………………………..30

Chapter 4: Conclusions and Future Research……………………………………33

Reference………………………………………………………………………...….34


List of Figures

Figure 2.1 An example of the 2-out-of-2 visual cryptography scheme with two sub-pixels………………………………………………………………….8
Figure 2.2 An example of the 2-out-of-2 visual cryptography scheme with four sub-pixels………………………………………………………………….9
Figure 2.3 Grayscale image and half tone…………………………………………...10
Figure 2.4 Grayscale image and half tone…………………………………………...11
Figure 2.5 Ordered dithering method………………………………………………..11
Figure 2.6 Ordered dithering………………………………………………………...13
Figure 2.7 Error diffusion method…………………………………………………...14
Figure 2.8 Error diffusion……………………………………………………………15
Figure 3.1 An invalid VC scheme…………………………………………………...18
Figure 3.2 A VC scheme by encrypting a single pixel……………………….……...20
Figure 3.3 Grouping the sub-blocks…………………………………………………24
Figure 3.4 The results of block coding………………………………………………24
Figure 3.5 The results of the improved block coding………………………………..29
Figure 3.6 Encrypting white field……………………………………………………30
Figure 3.7 A problem of block coding ………………………………………………31
Figure 3.8 The comparison of stacked images………………………………………32
Figure 3.9 The comparison of stacked images………………………………………32


List of Tables

Table 2.1 The 2-out-of-2 visual cryptography scheme with two sub-pixel…………...8
Table 2.2 The 2-out-of-2 visual cryptography scheme with four sub-pixels………….9
Table 3.1 Coding rules for recovering the secret completely………………………..18
Table 3.2 Coding rules of VC scheme by encrypting a single pixel…..…………….19
Table 3.3 The new categories after grouping………………………………………...21
Table 3.4 The corresponding stacked sub-blocks……………………………………22
Table 3.5 Block coding rules…………………………………………………...........23
Table 3.6 The added block coding rules for the 50% and 100% black………...........26
Table 3.7 The added block coding rules for 75% black stacked sub-block………….27
Table 3.8 Improved block coding rules for 50% black stacked sub-block…………..28
Table 3.9 Improved block coding rules for 100% black stacked sub-block…………29
Table 3.10 The improvement for the 75% stacked black sub-block…………............31

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