在XPS文件上做資訊隱藏之新研究

Abstract

隨著電腦與網路技術發展的進步，電子文件比以前越來越普遍。XPS文件是一個版面固定的文字檔案格式，並提供了良好的列印與瀏覽品質。本論文利用XPS格式的特性提出三個資訊隱藏技術做秘密傳輸、文字驗證及隱匿學之研究。 在秘密傳輸的部分，我們提出了一個方法利用XSP文件中的影像當作嵌入資訊的通道。藏入的秘密訊息被編碼成數種切割圖型，而這些用XPS標準所描述的圖型則被用來階層地切割XPS文件中的影像。這些被切割的局部影像再重新組合成為和原來相同的影像。如此可使秘密訊息不會被察覺地隱藏在文件中。 另一方面，因為XPS文件在網路上被傳遞時，很可能有惡意的使用者會嘗試攔截並竄改XPS文件的內容。因此，我們在此論文中提出了一個驗證XPS文件的方法。我們將XPS文件中欲保護的文字編碼成特殊的漸層圖型作為驗證訊號藏入文件本身。之後只要將藏入的驗證訊號抽取出來與目前文件中的文字做比對就可以判斷XPS文件的完整性及真實性。 最後，在許多資訊安全的領域中，保存秘密資訊在文件中且不輕易地被可能的攻擊者發現是需要的。針對此種隱匿學的應用，我們提出將特殊不可視且可變寬度的ASCII碼插入XPS文件的文字間，當作秘密訊息的編碼。用此方法產生的文件外觀與原本的文件相同且不會改變原本文字間的寬度。 本論文也針對所提出的資訊隱藏方法提供了加強其安全性的措施。最後我們提出了相關的實驗結果證明了所提方法之可行性。

With the progress of development in computer and networking technologies, digital text documents nowadays have become much more popular than in the past. The XPS (XML Paper Specification) document is a new text file format which is portable to any computer to produce identical page layouts with high printing and browsing qualities. In this study, three data hiding methods utilizing certain properties of the XPS are proposed for covert communication, text authentication, and steganography. For covert communication, the proposed data hiding method uses a single cover image in an XPS document as a data embedding channel. A secret message to hide is encoded as certain rectangular-shaped patterns according to a table designed in this study, and the patterns are described by Path elements in XPS files for use in dividing an image hierarchically in the XPS document. The divided image pieces then are composed together seamlessly to form the original image, thus accomplishing imperceptible data embedding in the document. Furthermore, since XPS documents are widely used on the Internet, malicious users may try to intercept and tamper with their contents. This requires XPS document authentication for which a novel method is proposed in this study. Specifically, the digests of the texts in an XPS document to be protected, after being encoded by specific gradient patterns, are used as authentication signals. To verify the integrity and fidelity of the text content of a suspicious XPS document, the authentication signals extracted from it are compared with those computed from its current text segments for decision making. Finally, keeping secrets in XPS documents without arousing notice from possible attackers is necessary in many information security fields. For this kind of steganographic application, a method inserting specific invisible and width-adjustable ASCII codes between words in the texts of XPS files for encoding secret messages is proposed. The resulting stego-document appears to be identical to the cover document with no width increasing between the text words. Measures to enhance the security of each proposed method are also suggested. Good experimental results show the feasibility of the proposed methods.