用於多媒體安全的影像驗證與秘密傳輸的新技術之研究

Abstract

影像驗證是保護數位影像內容一種有效率的方法，此技術的目的在檢驗數位影像的完整性與真確性。當一張數位影像遭受竄改或破壞時，影像驗證技術可以偵測竄改並將影像遭竄改的部分定位出來。本論文中，我們針對灰階公文影像、黑白影像與灰階影像分別提出了可修復式之影像驗證方法。所提出的方法不只可以偵測並定位竄改，還能進一步地修復影像遭到竄改的部分。對於灰階公文影像與黑白影像，我們首先將輸入的影像轉換成含有透明層的PNG格式影像，接著利用Shamir的秘密分享技術將驗證訊號與修復訊號轉換成許多的秘密碎片，並以特定的方式將秘密碎片散佈在透明層中，以產生受保護的影像。如此一來，在一張遭受竄改的影像，只要能從透明層中蒐集回足夠數量並存活的秘密碎片，再經由Shamir秘密分享的反程序，便可進行影像修復。此外，本論文亦提出一個彩色影像驗證方法，該法是立基於我們所提出的一個利用影像透明層來隱蔽資料的新型資訊隱藏方法。而在灰階影像驗證方面，我們設計了一種盒子碼，它的概念是將像素中較重要的幾個位元壓縮成較短的位元。此盒子碼兼具驗證訊號與修復訊號的功能，因此我們利用此盒子碼來發展可修復式灰階影像驗證。 在多媒體安全方面，我們提出一個具自我驗證能力，並以常見的試算表作為機密訊息載體的新型機密傳輸方法。我們將機密訊息透過Shamir秘密分享技術轉換成特定數量的數字碎片，將這些數字碎片依據所設計的原則去替換掉試算表中某些數字; 在擷取機密訊息時，只要藉由檢查多份數字碎片運算後之結果是否一致，便可達到自我驗證的功能。如此一來，使機密訊息接收者在不需任何輔助資訊的情況下，便可確認機密訊息的真確性與完整性。在可逆式資訊隱藏領域，我們提出一個兩階段式直方圖位移之可逆式資訊隱藏技術。藉由兩種直方圖位移方式的搭配，所提出的方法可以重複性地操作以達到更大的資訊隱藏量，並且在擷取所藏入的資訊時，本法不需任何額外的輔助資訊即可進行。 以上所提出的方法之效能皆已經由理論分析與實驗做過評估，結果顯示了它們在實際應用上的可行性。

Image authentication is an effective technique to protect digital image content and the goal of such techniques is to check the integrity and fidelity of a digital image. It means that image authentication techniques are capable of providing the function of tempering detection and tampering localization when an image has been tampered with. In this dissertation, repairable image authentication methods for grayscale document images, binary images and grayscale images are proposed, respectively. The proposed methods can not only detect and localize the tampered region but further repair the tampered region for an attacked image. For authentication of grayscale document images and binary images, an input cover image is first transformed into a stego-image in the PNG format with an alpha channel plane. Subsequently, the information used for authentication and image repairing are transformed into partial shares by the Shamir’s secret sharing method, which are then distributed in a well-designed manner into an alpha channel plane to create a stego-image in the PNG format. In this way, data repairing can be applied to tampered region by a reverse Shamir scheme after collecting enough shares from the alpha channel plane. In addition, a color image authentication method which is based on a new data hiding method proposed is developed in this dissertation. The data hiding method conceals data by utilizing the character of the transparency of the alpha channel plane. For authentication of grayscale images, a method which is based on the concept of compressing a number of the most significant bits of a pixel’s gray value into a shorter “bin code” is proposed. The bin code is for use both as an authentication signal for the pixel and as an index for generating the data for repairing the pixel when it is authenticated to have been tampered with. In the aspect of multimedia security, a new covert communication method which takes Spreadsheets as the cover medium and applies Secret Sharing method to yield a self-authentication capability is proposed. Through checking the consistency of the copies in value computed from secret shares, the proposed covert communication has the capability of self-authentication. In the field of reversible data hiding, a blind two-phase reversible data hiding method based on histogram shifting is proposed. Two types of histogram-shifting techniques are executed in two phases such that the proposed method can be performed iteratively during the data embedding process and blindly during the data extraction process. The feasibility and effectiveness of all the above proposed method have been demonstrated by theoretical analyses and experiments.