視覺密碼是一種秘密分享方案，用於分享可視秘密訊息。本篇論文“結合資料隱藏與視覺密碼以安全擷取隱藏之資訊”中，我們用它來保護一個「視覺金鑰」來作隱藏資訊的擷取。一般而言，視覺密碼方案可生成 n 個雜亂的子影像分給 n 個參與者。當 k 個或更多參與者直接疊合他們的子影像時，能通過人眼直接看秘密影像。但是，少於k個子影像則無法解回秘密影像。大部分現有的視覺密碼方案專注於提高解密影像的視覺品質也就是清晰度、或減少子影像的擴張。
本論文以植基於漢明碼的資料隱藏技術在(k, k)視覺密碼疊合的子像素列向量來做資料隱藏。漢明碼資料隱藏與癥狀解碼技術可以將 t 個秘密位元，使用癥狀嵌入到 (2t − 1) 個位元中。以最常使用的 (7, 4)漢明碼為例，可以將 3 個秘密位元嵌入到 7 個位元。在實現(k, k)視覺密碼時，我們需任意排列視覺密碼的基本矩陣，以符合安全性。在本文所提的視覺密碼與資料隱藏相結合的模型，我們排列視覺基本矩陣以得到疊合的漢明碼徵狀是我們嵌入的加密資料，而加密這些嵌入資料的金鑰則顯現於視覺密碼疊合的「視覺金鑰」，最後解密得回原隱藏資料。更理論地說，可視的「視覺金鑰」被分享到 k 個子影像當中，於此同時密文也將被嵌入到這 k 個子影像當中。當同時擁有 k 個子影像時，我們可以通過將 k 個子影像疊合在一起來直接得到視覺金鑰 (註: 我們以十六進制格式表示金鑰以更容易得到金鑰)，並且在此同時也可以從疊合後的結果使用癥狀解碼擷取加密隱藏資料，最終安全地擷取資料。反之，若是只有(k − 1)子影像，則無法解回「視覺金鑰」、也無法擷取加密之隱藏資料，這能夠保證資料的安全性。
總結來說，本論文方案提供安全擷取資料隱藏有其優勢、能有更多應用場景。除此之外， 理論分析和實驗結果也證明了其安全性和可行性。

Visual cryptography scheme (VCS) is a special type of secret sharing to share secret visual data. In the thesis “A Combination of Data Hiding with Visual Cryptography for Secure Extraction of Data Hiding”, we use it to safeguarding a “visual key” for extracting hidden data. In general, a secret in VCS is shared to n shadow images (referred to as shadows) and delivered to n participants. The secret image can be visually decrypted via human visual system only when k or more participants directly stack their shadows. However, with less than k shadows, the secret image cannot be recovered. Most VCSs are dedicated on improving the visual quality of recovered image or reducing the pixel expansion.
In the thesis, the Hamming code based data hiding (DH) is used to embed the secret data in stacked row vectors of (k, k)-VCS. The Hamming code data hiding and syndrome decoding technique could embed t secret bits into (2t −1) bits via syndrome. Consider (7, 4) Hamming code as an example. We may embed 3 secret bits into 7 bits. When realizing (k, k)-VCS, we have to randomly permute the columns in VCS basis matrix to achieve security. In the combination of VCS and DH, we use the stacked result of VCS row vectors as a Hamming code with the syndrome, which is our encrypted data. Meantime, the stacked result of VCS is to reveal the “visual key”. To formally describe the proposed approach, our “visual key” (note: we represent the key in hexadecimal format to make it easier visually reveal the key) is shared into k shadows, and meanwhile the data is embedded into the stacked row vectors of k shadows. Thus, when stacking k shadows, the key is revealed by human visual system directly, and syndrome decoding is used to extract encrypted hidden data from the stacked row vectors. Finally, we could decry the encrypted data to the original hidden data. On the other hand, (k −1) or fewer shadows, both “visual key” and encrypted data cannot be obtained for correctly decode data.
In conclusion, the proposed scheme with secure extraction of DH has its advantage, and can have more intended applications. In addition, theoretical analyses and experimental results demonstrate its security and feasibility.

Chapter 1 Introduction 1
Chapter 2 Preliminaries 5
2.1 VCS 5
2.2 Hamming Code 6
Chapter 3 The Proposed Approach 9
3.1 Application Scenario and Motivation 9
3.2 The Model 10
3.3 Initial Phase 13
3.4 Sharing and Hiding Phase 16
3.5 Reconstruction and Extraction Phase 20
3.6 Property Analysis 22
Chapter 4 Experimental Results and Discussions 29
4.1 Visual Illustration 29
4.2 Pixel Expansion, Contrast and Embedding Rate 30
4.3 Secure Extraction 36
4.4 On Constructing The Base Matrix 40
4.5 Comparison 45
Chapter 5 Conclusion and Future Work 49
References 51

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