Overcoming the suspicion in transmission scheme based on CDES

[[abstract]]CDES (Confused Document Encrypting Scheme) 是一項提供資訊隱藏(Information Hiding或是稱之為Steganography)的文件保護機制。與傳統加密機制不同的是，它並不直接傳送加密過的訊息，而是將秘密訊息隱藏在正常並具有意義的文件當中。傳送端分別傳送多份欺敵文件(Cheating text)以及一份經過加密的索引檔案(Plaintext index file)給接收端，每份欺敵文件的內容皆為有意義的文字所組成，就算被人截取，也很難察覺出，哪些文件只是一般訊息，哪些才隱藏著大量的秘密訊息。但是，本研究發現在這樣的傳送機制下，反而會讓正在竊聽網路封包的竊聽者(Eavesdropper)起疑心；尤其是對那份唯一加密過的索引檔案，不免懷疑這些檔案之間的關聯性。為了改善CDES在實際應用上的這個問題，本論文中，除了基於CDES原有文件保護機制，並透過圖像隱藏技術的加入，將索引檔案隱藏在圖像中，藉以達到保護秘密訊息的目標。研究中並實際將此一方式運用在電子郵件服務上，透過改善CDES，讓訊息傳送過程中，其傳遞內容更具意義以及合理化，讓竊聽者不會對內容產生懷疑。[[abstract]]Confused Document Encrypting Scheme (CDES) is a document protection technique for data hiding (Steganography), which uses meaningful cheating texts to confuse eavesdroppers. It is a simple and effective file protection scheme. However, CDES has a problem that its plaintext index file (PIF) was transmitted in encrypted form, which looks suspicious. In this thesis, we proposed an improved messaging protection framework based on CDES. As many users include emoticons like smiling faces in their instant messages, it is also natural for many users to include pictures and photographs in their emails. By applying the image-hiding technique to hide the PIF in an emoticon or a photo, emails protected by CDES looks more natural and less suspicious, which could provide a better protection to the secret texts.[[note]]碩