利用資訊隱藏技術在PDF檔案上做秘密傳輸、驗證及分享之研究

Abstract

隨著網路技術的發展，今人皆利用網路來交換資訊。PDF檔案格式有各種優異特性，如合乎開放標準、跨平台及提供高品質的閱覽與列印，因此已變得非常普及。在本論文中我們利用PDF檔案做為掩護檔案，提出數種資訊隱藏的方法，並將其應用在秘密傳輸、驗證以及分享之上。在秘密傳輸方面，我們提出了兩種基於資訊隱藏的傳輸技術。第一種是將秘密資訊編碼，成為PDF檔案中代表空白的16進位碼，用以取代原本的空白。而第二種則是將文字區塊插入到PDF檔案當中，使其不會顯現在PDF的瀏覽畫面之中。對於秘密驗證，我們修改PDF檔案中的產生數(generation numbers)來藏入驗證訊號，以達到驗證秘密資訊真偽性的目的。在秘密分享方面，我們將秘密PDF文件和數個PDF掩護檔案，利用“互斥或”(exclusive-OR) 和 “重合”(coincidence)兩種運算，產生一份中間的秘密資訊，並將它藏入這些PDF掩護檔案中得到分享檔案。最後我們將這些分享檔案分發給每一位參與者。藉由收集每一份分享檔案，我們可將秘密PDF文件復原回來。實驗結果證明以上所提出各方法的可行性。

With the development of network techniques, people exchange information on the Internet nowadays. The PDF becomes very popular today because of its outstanding capabilities like open-standard, cross-platform, and high-quality viewing and printing. In this study, several data hiding methods for applications of secret transmission, authentication, and sharing are proposed using PDF files as cover media. For secret transmission, we propose a method for secret transmission by two data hiding techniques. The first is a technique based on a space coding scheme and the second is a technique of inserting invisible texts into PDF files. For secret authentication, we propose a method for authentication of secret messages for fidelity by modifying the generation numbers in a PDF file to embed authentication signals of the secret messages. For secret sharing, we use a secret PDF document and cover PDF files to create an intermediate secret message by exclusive-OR and coincidence operations, and hide it into the cover PDF files by the second previously-mentioned data hiding technique to get shares. The shares are then distributed to the participants for them to keep. By collecting all the shares, the secret PDF document can be recovered. Experimental results show the feasibility of the all proposed methods.