傳統視覺密碼是一種利用秘密分享機制將機密影像加密成n張分享影像，只要疊合其中的k張以上的分享影像，就能夠利用人類視覺系統直接解密，無需透過電腦運算；反之，小於k張則無法從中獲得任何與機密影像有關的資訊。然而傳統視覺密碼學有像素擴展與分享影像不具任何意義的缺點。
相關學者提出新的視覺密碼，將機密影像利用簡單的數學運算嵌入至偽裝影像當中，解密時，透過數學運算就能夠還原機密影像，這是不同於傳統視覺密碼的加密與解密方法，雖然不能直接利用人類視覺系統解密，但是也改善了傳統視覺密碼的像素擴展與分享影像不具任何意義等缺點。
本研究提出一種以秘密分享機制為基礎的灰階影像隱藏技術，其方法是將一張灰階機密影像分成個位數及百位數、十位數二部份，並分別嵌入至1張灰階輸入影像與n-1張灰階輸入影像當中。透過將機密影像像素值分解，再分別嵌入至灰階輸入影像中的方式，最少只要使用4張灰階輸入影像就能夠達到影像隱藏的目的，且所有的分享影像仍然能夠維持不錯的PSNR值，並且使攻擊者無法利用肉眼從分享影像中窺視出有關機密影像的任何資訊。解密時，只要取得所有的分享影像，再利用簡單的數學算式，就能夠還原機密影像。

Original visual cryptography is a secret sharing method that encrypts a secret image into n shares, and decrypts the image by overlaying k or more shares over each other without any computation. However, by inspecting less than k shares, one cannot gain any information on the secret image. The original visual cryptography have two weaknesses which are pixel expansion and noise-like shares.
A lecture review has showed that encrypts a secret image into input images and decrypts the image by applying simple arithmetic operations, which is difference from original visual cryptography. Although, it cannot decrypt by human visual system, it has improved pixel expansion and noise-like shares.
In this study. An image hiding technique based on secret sharing scheme has been proposed. The proposed method divides a gray value of the secret image into two separated parts: unit, hundreds and tens. After the gray value separated, one of the n input images was selected for unit encryption and the other input images for hundreds and tens. The experimental result has showed that all shared images similar to the origin input images with high PSNR. The gray value change of the share image if hardly identify by human visual system. Additionally, the decryption of secret image require only simply arithmetic operations to each share.

摘 要.....................................................i
Abstract...................................................ii
誌謝.......................................................iii
目錄.......................................................iv
表目錄.....................................................v
圖目錄.....................................................vi
第一章 緒論................................................1
1.1 研究背景與動機....................................1
1.2 研究目的..........................................4
1.3 論文架構..........................................5
第二章 文獻探討............................................6
2.1 Naor與Shamir之視覺密碼機制........................6
2.2 Kim與Choi等學者之視覺密碼機制.....................8
第三章 研究方法........................................... 13
3.1 秘密分享機制之灰階影像隱藏技術運作原理............13
3.2 秘密分享機制之灰階影像隱藏技術安全性分析..........23
第四章 實驗結果與討論......................................25
4.1 秘密分享機制之灰階影像隱藏技術實作................25
4.2 秘密分享機制之灰階影像隱藏技術安全性實作..........57
4.3 Kim等學者之視覺密碼機制實作.......................60
4.4 本機制與Kim等學者之視覺密碼機制之比較分析.........68
第五章 結論與未來研究方向..................................69
參考文獻...................................................70

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