臺灣博碩士論文加值系統

視覺密碼機制( Visual Cryptography Scheme; VCS )的重要特色是基於像素層級的運算，因此解密時不需要任何電腦設備 (也就是「相疊即視」特性)。在(k, n)視覺密碼機制下，秘密影像會被加密得到n張子影像。任意的k張子影像疊合，在即可由人的眼睛直接解密，若少於k張子影像則無法解回原本的秘密影像。由於VCS視覺解密的獨特性，因此有很廣泛的應用。例如：視覺認證、數位浮水印、安全顯示器和數位影像智慧財產權保護機制等。
最近，Hou等人推出了(2, n)基於權限的VCS(PVCS)。藉由子影像的不同權限級別，擁有更高權限的子影像可以恢復更多的訊息，而較低的權限的子影像具有較少的訊息恢復能力。 (2, n)-PVCS的門檻值是兩位參與者，當兩位或更多位參與者可解回該秘密的圖像。此外，堆疊出來的秘密影像品質取決於所有參與的子影像其權限總和。但是，Hou等人的(2, n)-PVCS回復後的影像的對比度和權限級別的總和的不一致性。因此，本論文提出改善過的Hou等人(2, n)-PVCS（簡稱Hou等人(2, n)-EPVCS）來解決這種不一致的問題。然而，這樣的EPVCS並不能適用於任意權限值的PVCS，並且它也有子影像其白度不均等的問題。
在本文中，我們重新定義Hou等人的(2, n)-PVCS權限。並解決了上述兩個關鍵問題。我們所提的新機制能符合(2, n)-PVCS的安全、對比度條件，以及權限值條件，並解決以上兩個關鍵的問題: 任意特權值與每個子影像都擁有相同的白度。

Visual Cryptography Scheme (VCS) is based on pixel-wise operation, which can be easily implemented without computer. In a (k, n)-VCS, a secret image is encoded into n shadow images that distributed to n participants. Any k participants can reveal the secret image by stacking their shadow images and decode the secret image via human visual sight, but (k-1) or fewer participants will not gain any secret information. This simple decoding way is the so-called stacking-to-see property, on which VCS can be applied on visual authentication, digital watermarking, security display, and embedding private information.
Recently, Hou et al. introduced a (2, n) privilege-based VCS (PVCS) with various privilege levels of shadow images. In Hou et al.’s (2, n)-PVCS, a shadow with a higher privilege contributes more recovered information, while a lower privileged shadow has the less recovery capability. The threshold of (2, n)-PVCS is two, and thus two or more participants can reconstruct the secret image. Moreover, the visual quality of stacked result depends on the total sum of privilege levels for all involved shadows in reconstruction. Unfortunately, Hou et al.’s (2, n)-PVCS has the inconsistency of the contrast of recovered image and the sum of privilege levels. Accordingly, an enhanced Hou et al.’s (2, n)-PVCS (for short, Hou et al.’s (2, n)-EPVCS) is proposed to solve this inconsistency problem. However, Hou et al.’s EPVCS is not a general solution to implement all PVCSs with arbitrary privilege levels, and it also has the unequal whiteness of shadows. In this thesis, we also propose a new (2, n)-PVCS to solve these two critical problems.

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 Hou et al.’s (2, n)-PVCS 8
Chapter 3 Motivation 11
Chapter 4 Main Results 17
4.1 Proposed scheme 17
4.2 Relation Between The Proposed (2, n)-PVCS and Hou et al.’s (2, n)-EPVCS 22
Chapter 5 Experiment and Comparison 27
5.1 Experimental Results 27
5.2 Comparison 30
Chapter 6 Conclusion and Future Work 35
Appendix 37
References 41

[CIMA05] S. Cimato, A. De Santis, A.L. Ferrara, B. Masucci, Ideal contrast visual cryptography schemes with reversing, Information Processing Letters. 93 (2005) 199–206.
[CIMA06] S. Cimato, R. De Prisco, A. De Santis, Probabilistic visual cryptography schemes, The Computer Journal. 49 (2006) 97–107.
[EISE02] P.A. Eisen and D.R. Stinson, Threshold visual cyptography schemes with specified whiteness, Designs, Codes and Cryptography. 25 (2002) 15–61.
[HOU13] Y.C. Hou, Z.Y. Quan, C.F. Tsai,A.Y. Tseng, Block-based progressive visual secret sharing, Information Sciences 233 (2013) 290–304.
[HOU15] Y.C. Hou, Z.Y. Quan, C.F. Tsai, A privilege-based visual secret sharing model, J. Vis. Communication Image R. 33 (2015) 358–367.
[ITO99] R. Ito, H. Kuwakado, H. Tanaka, Image size invariant visual cryptography, IEICE Trans. on Fund. of Elect. Comm. and Comp. Sci. E82-A (1999) 2172–2177.
[LIU11] F. Liu, C. Wu, X.J. Lin, Cheating immune visual cryptography scheme, IET Information Security5 (2011) 51–59.
[NAOR94] M. Naor and A. Shamir, “Visual Cryptography,” Advances in Cryptology-EUROCRYPT’ 94, LNCS 950, pp. 1-12, 1994.
[SHIM79]　A. Shamir, How to share a secret, Communication ACM. 22 (1979) 612–613.
[SHYU12] S.J. Shyu, H.W. Jiang, Efficient construction for region incrementing visual cryptography, IEEE Transactions on Circuits and Systems for Video Technology. 22 (2012) 769-777.
[SHYU13] S.J. Shyu, H.W. Jiang, General constructions for threshold multiple-secret visual cryptography schemes, IEEE Transactions on Information Forensics Security 8 (2013) 733–743.
[VERH97] E.R Verheul, H.C.A Van Tilborg, Constructions and properties of k out of n visual secret sharing scheme, Designs, Codes and Cryptography. 1 (1997) 179–196.
[YAN14] X. Yan, S. Wang, X. Niu, C.N. Yang, Essential visual cryptographic scheme with different importance of shares, ICONIP 2014. LNCS 8836, pp. 636–643.
[YANG04]　C.N. Yang, New visual secret sharing schemes using probabilistic method, Pattern Recognition Letters 25 (2004) 481–494.
[YANG12] C.N. Yang, H.W. Shih, C.C. Wu, L. Harn, k out of n region incrementing scheme in visual cryptography, IEEE Transactions on Circuits and Systems for Video Technology. 22 (2012) 779–810.
[YANG14] C.N. Yang, D. Wang, Property analysis of XOR based visual cryptography, IEEE Transactions on Circuits and Systems for Video Technology. 24 (2014) 189–197.
[YANG15A] C.N. Yang and C.Y. Lin, Almost-aspect-ratio-invariant visual cryptography without adding extra subpixels, Information Sciences. 312, (2015) 131–151.
[YANG15B] C.N. Yang, C.C. Wu, Y.C. Lin, C. Kim, Constructions and properties of general (k, n) block-based progressive visual cryptography, ETRI Journal. 37 (2015) 979-989.