A Study of Secret Image Sharing Schemes

偽裝學相似於密碼學，它包括了許多隱蔽通信的方法及技術，用來掩藏及偽裝訊息並使其不易被查覺出來，這兩種技術已經被廣泛地用來保護機密訊息及資料。隨著各種多媒體應用及網路通訊的發展，機密影像分享機制已成為一個重要的安全儲存及秘密通訊的數位影像技術。 Lin跟Tsai學者於2003年提出了一個新型的機密影像分享的技術，他們的方法除了具有影像機密分享的目的之外，同時也能達到偽裝學及影像認證的功能。接著，Yang及Chang等學者分別於2007年及2008年，前後提出了改進影像品質及影像認證的方法。但是這些方法都是使用傳統LSB資訊隱藏的技術，將分享的機密訊息及驗證碼藏入偽裝影像之中，因此會造成偽裝影像失真的情況。 因此，我們結合最佳化像素調整的資料隱藏技術，提出了適用於各種隱藏量及不同認證碼下，提高及改進偽裝影像影像品質的方法。相較於Yang等人、Chang等人及Lin跟Tsai學者的方法，從實驗結果清楚顯示我們的方法針對偽裝影像可以改進及提高4.71%、9.29%及11.10%的影像品質。同時我們也藉由許多實驗去驗證及證明我們的方法一樣具有影像認證的能力。換言之，我們提出的方法除了具有機密影像分享及影像認證的功能之外，同時也改善了偽裝影像的影像品質。 但是這些方法及技術都無法將藏入訊息後的偽裝影像以無失真的方式還原回原本的影像。最近，Lin跟Chan這兩位學者於2010年雖然提出了一種可逆的機密影像分享的技術來改進此缺點，然而，其方法除了參與者最大的個數受到限制的缺點之外，也必需考慮及避免像素值在進行機密分享的過程中發生溢位的情況。 為了可以達到無失真的目的，我們一開始先基於Shamir機密分享的機制，使用額外的記錄檔，提出了一種可逆的機密影像分享的技術。為了解決需要額外儲存記錄檔的缺點，最後，我們使用伽羅瓦域GF（28）的定理，提出了另一種可逆的機密影像分享的技術。此方法不僅可以達到無失真還原機密影像及掩護影像的目的之外，同時也具有良好的影像品質。從實驗結果及討論分析中，可以證明我們提出來的這兩種方法皆優於其它現有的方法。Steganography can be viewed as cryptography. It includes a variety of secret communication methods that embed the existence of the message and makes it appear invisible. Both of them have been used to protect secret information. With the rapid growth of numerous multimedia applications and communications through Internet, secret image sharing has been becoming a key technology for digital images in secured storage and confidential transmission. In 2003, Lin and Tsai proposed a novel secret image sharing scheme. They claimed that their scheme can achieve the goal of secret image sharing with additional capabilities of steganography and authentication. Yang et al. further proposed an improved scheme in 2007. Afterward, Chang et al. also proposed another scheme to improve the authentication ability and visual image quality in 2008. However, the stego-images are obtained by directly replacing the least-significant-bit planes (LSB) of cover-images with secret data and authentication code in most schemes, which will result in the distortion of the stego-images. Therefore, we proposed a novel secret image sharing scheme by applying optimal pixel adjustment process to enhance the image quality under different payload capacity and various authentication bits conditions. The experimental results showed that the proposed scheme has improved the image quality of stego images by 4.71%, 9.29%, and 11.10%, as compared with the schemes recently proposed by Yang et al., Chang et al., and Lin and Tsai. We also provide several experiments to demonstrate the efficacy of authentication capability of the proposed scheme. In other words, the proposed scheme maintains the secret image sharing and authentication ability while enhances the image quality. In addition, there is a common problem in both schemes, that is, the stego-images cannot recover to their original states. Recently, Lin and Chan proposed an invertible secret image sharing scheme in 2010. Nevertheless, the maximum number of participant is limited and the overflow situation must be considered in the secret sharing phase. Then a reversible secret image sharing based on Shamir's scheme is developed in this dissertation. The proposed scheme can achieve lossless reversible for cover image and secret image rely on location map. Finally, a reversible secret image sharing based on Galois Field GF (2^8) scheme without location map is also designed. The proposed scheme not only retrieves the original cover image and secret image, but also achieves the better image quality. According to the experimental simulations and discussions, we show that the proposed schemes have significantly better performance than the others.ABSTRACT i Table of Contents iii List of Tables v List of Figures vi Chapter 1 Introduction 1 1.1 Preliminary 1 1.2 Research Motivation and Objectives 2 1.3 Organization 4 Chapter 2 Related Works 5 2.1 Preliminary 5 2.2 Lin and Tsai's Secret Image Sharing 6 2.3 Review of Yang et al.'s Scheme 6 2.4 Review of Chang et al.'s Scheme 8 2.5 Lin and Chan's Invertible Sharing Scheme 9 Chapter 3 A High Quality Image Sharing and Adaptive Authentication Scheme 10 3.1 Optional LSB Method 10 3.2 The Proposed Scheme 10 3.2.1 High Quality Secret Image Sharing Scheme 10 3.2.2 Secret Image Sharing and Retrieval Algorithms 14 3.2.3 An Example of (2, 3)-Threshold Procedure 16 3.3 Experimental Results 17 3.3.1 Evaluate the Visual Quality 18 3.3.2 Estimate the Authentication Capability 19 3.4 Analysis and Discussion 24 Chapter 4 Reversible Secret Image Sharing Based on Shamir's Scheme 29 4.1 Reversible Secret Image Sharing Procedure 29 4.2 Authentication and Reversible Image Recovery Procedure 30 4.3 Experimental Results 32 4.4 Analysis and Discussion 36 Chapter 5 Reversible Secret Image Sharing Based on Galois Field GF (28) 37 5.1 Galois Field (GF) 37 5.2 Reversible Secret Image Sharing Procedure 39 5.3 Reversible Secret Image Recovery Procedure 40 5.4 Experimental Results 42 5.5 Analysis and Discussion 44 Chapter 6 Conclusions and Future Researches 46 6.1 Conclusions 46 6.2 Future Researches 46 References 4