利用平移隨機網格之多重視覺機密影像配置系統

Visual Multi-Secret Image Sharing Schemes by Shifting Random Grids

張若怡(Jo-Yi Chang)

指導教授 ： 阮夙姿

國立暨南國際大學/科技學院/資訊工程學系/碩士(2012年)

摘要

視覺密碼學(Visual Cryptography, VC)也稱為視覺機密分享(Visual Secret Sharing, VSS)方法，是一種靠人眼解密的影像加解密技術。其有別於傳統密碼學，最初是由學者Naor和Shamir 所提出的一項影像分享加解密技術。此項技術結合傳統密碼學與資訊分享的概念，將機密影像經加密後得到多張分享影像(Share)，並將影像分享給參與者，參與者由所拿到的該張分享影像，無法顯現出任何與原始影像相關的資訊；而將所有分享影像疊合起來，則會顯示機密影像中的內容。視覺機密配置方法的主要精神在於解密時，無須仰賴任何複雜的數學計算以及大量的電腦運算，亦不需任何密碼學知識，僅需將分享影像重疊並利用人類視覺系統即可辨識、解密。
本論文所提出的視覺機密配置系統，是利用視覺密碼學中的隨機網格技術設計而成。近來，在此領域中有越來越多的研究，著眼於如何能以越少張數的分享影像一次加密多張秘密影像。但目前已知的方法僅能加密一至四張機密影像，且其失真率會隨機密影像之數量變多而提升，導致其可加密的機密影像數量有所限制。在本篇論文中，我們提出一種使用隨機網格技術加密多重機密影像的方法，探討如何使用隨機網格(Random Grids)技術，不需要用到傳統VC的編碼簿(Codebook)，使得加密後的影像像素不會擴增，從而將多張機密影像加密於兩張分享影像中，且其失真率可隨使用者需求調整。此方法是利用隨機網格技術去對機密影像的每個像素做計算，使所有的機密影像像素達到最高的使用率，由此可降低解密時的失真率，進而得到更多的資訊量。

關鍵字

視覺密碼學 ； 視覺機密配置 ； 隨機網格 ； 多重機密影像 ； 分享影像 ； 失真率 ； 編碼簿

並列摘要

Visual cryptography (VC), also called visual secret sharing (VSS), which is different from traditional cryptography. Visual Cryptography was proposed by Naor and Shamir in 1994, which combines traditional cryptography with conception of information sharing, encoding a secret image into n pieces of share images. We give the share images to participants who will obtain noise-like, unidentified images which cannot be restored to original secret image when participants only have one share image. The main concept of visual secret sharing is that any complicated mathematical measurement is needless when the sharing scheme decodes the secret images, nor a large amount of computer operation and knowledge of cryptography. The only thing we need to do is collecting k shared images and superimposing them, and then the original secret image can be distinguished, decoded, and reconstructed by human visual system.
The proposed visual secret sharing schemes were designed by using the random grid (RG). Recently, it has drawn more and more attention to encode more than one secret image into two shares by RG-based VSS techniques. However, previous techniques can only encrypt at most four secret images at one time, and the distortion of the reconstructed secret images is obvious when the number of the secret images increases up to now. In this thesis, we propose a RG-based scheme which is able to encrypt multi-secret images into two shares by shifting random grids. As compared with the traditional VC-based VSS, RG-based VSS need not to design the codebook of conventional VC, and the size of share images will not be expanded. Also, users can adjust the distortion in our schemes. Our schemes calculate almost each pixel of the secret images by shifting random grids. These schemes achieve the best utilization of all of the pixels on secret images, and reduce the quantity of distortion when decrypting the secret images. Therefore, users can get more information by using our shifting random grids techniques.

並列關鍵字

Visual Cryptography ； Visual Secret Sharing ； Random Grids ； Multi-Secret Images ； Share ； Distortion ； Codebook

參考文獻

[1] J.-L. Bai, “Random-based secret image sharing scheme,” Master's Thesis, Computer Science and Information Engineering, Ming Chuan University, 2005.

[2] G.R. Blakley, ‘‘Safeguarding cryptography keys,’’ in Proceedings of AFIPS 1979 National Computer Conference, vol. 48, New York, USA, 1979, pp. 313–317.

[3] T.-H. Chen, Y.-S. Lee and C.-L. Li, “High-capacity multi-secret sharing by random grid,” in Proceedings of The First International Workshop on Cloud, Wireless and e-Commerce Security, Fukuoka, Japan, November 4-6, 2010.

[4] T.-H. Chen, and K.-H. Tsao, “Image encryption by (n, n) Random Grids,” in Proceedings of 18th Information Security Conference, Hualien, May 29-30, 2008.

[5] T. H. Chen and K. H. Tsao, “Visual secret sharing by random grids revisited,” Pattern Recognition, vol. 42, no. 9, 2009, pp. 2203-2217.