臺灣博碩士論文加值系統

數位影像偽裝術(簡稱影像偽裝術；Image Steganography)是一種有別於密碼學之加/解密作法的秘密通信技術，主要是利用有意義且非機密性的數位影像來掩護及傳送可以用位元串(bit stream)表示的秘密訊息，使得發送與接收雙方之外的第三者或攻擊者難以用肉眼察覺出或用統計方法偵測出秘密訊息的存在以及通訊行為之進行的一種技術。
2003年，Wu與Tsai提出一個相當簡單而且有效的影像偽裝術，稱為像素值差異技術(pixel-value differencing technique; PVD)。它的觀念與作法主要是根據相鄰的兩個像素值的差異級別來決定嵌入此兩像素的秘密訊息之位元數。如此得以在比較平滑的區域嵌入較少量的訊息而在像素值差異較大的邊緣區域則嵌入較多的訊息來降低偽裝影像的失真(distortion)度，達成人類肉眼的不可覺察性(imperceptibility)。此外，根據實驗結果指出他們的方法也可以抵擋由Fridrich等人在2001年所發表的RS統計偵測的攻擊。
鑑於像素值差異技術的簡單性與有效性，Wang、Wu、Tsai與Hwang等人在2008年利用模數運算，提出一個降低像素值修改量的改良作法，進一步提高了像素值差異法所產生的偽裝影像之品質。
本論文參考Wang等人的作法並利用權值的觀念提出一個新的像素值差異嵌入技術，同時進行一系列的實驗。實驗結果證明，當嵌入的秘密訊息數量相同時，利用本論文的作法所產生的偽裝影像具有比Wang等人的結果更好的影像品質，也即具有更高的PSNR值(Peak Signal to Noise Ratio)。尤其在高嵌入容量的要求下，當Wang等人的結果之PSNR值降低到無法滿足不可覺察性的要求時，我們的作法所產生的偽裝影像仍然具有相當安全的PSNR值。因此，我們的方法比前此已發表的所有植基於像素值差異的嵌入技術更適於偽裝術的實際應用。

Image steganography is a secret communication technique that is different from encryption/decryption methods from cryptology, where image steganography utilizes non-confidential and meaningful digital images to cover and transmit secret messages that can be expressed with bit streams. With such a technique, attackers or any third party aside from the sender and the recipient are unable to discover the existence of the hidden message through the human eye or statistical detection.
In 2003, Wu and Tsai proposed a simple and effective image steganography technique, calling it the pixel-value differencing technique. Its main concept and method is to decide the amount of information embedded in two neighboring pixels based on the difference of their pixel values. In this way, regions with higher smoothness will embed less information and those with lower smoothness will have the opposite, therefore reducing the degree of distortion of the image and achieving imperceptibility. In addition, experimental results show that their method is capable of resisting Fridrich et al.’s RS steganalysis.
Because of the simplicity and efficiency of the pixel-value differencing technique, Wang, Wu, Tsai, and Hwang proposed an improved method with lower pixel value modification using modulus computation in 2008, which further increased the image quality of the steganography utilizing the pixel-value differencing technique.
In this paper, we refer to Wang, Wu, Tsai, and Hwang’s method and propose a new pixel-value differencing technique implementing the concept of using “weights”, followed by carrying out a series of experiments. The results prove that our proposed method has higher stego-image quality than Wang, Wu, Tsai, and Hwang’s method when embedding the same secret message, implying that we have higher PSNR value. Especially under the requirement of high embedding capacity, our method still maintains a safe PSNR (Peak Signal to Noise Ratio) value where the PSNR value of Wang, Wu, Tsai, and Hwang’s method drops too low and fails to satisfy imperceptibility. Therefore, compared with previously proposed embedding techniques that base on pixel-value differencing, our method provides greater suitability for practical applications.

誌謝 i
摘要 ii
Abstract iv
目錄 vi
圖目錄 viii
表目錄 ix
第一章 緒論 1
1.1 前言 1
1.2 研究動機 6
1.3 研究目的 7
1.4 論文架構 7
第二章 數位影像基本理論 8
2.1 數位影像的格式 8
2.2 影像偽裝技術探討 10
2.2.1 空間域影像偽裝技術 10
2.2.2 頻率域影像偽裝技術 11
2.3 影像品質的鑑定 12
第三章 文獻探討 13
3.1 Wu與Tsai的影像偽裝技術[12] 13
3.1.1 Wu與Tsai[12]的嵌入演算法 16
3.1.2 Wu與Tsai[12]嵌入法範例 17
3.1.3 Wu與Tsai[12]的萃取演算法 19
3.1.4 Wu與Tsai[12]萃取法範例 20
3.2 Wang等人的影像偽裝技術[17] 21
3.2.1 Wang等人[17]的嵌入演算法 22
3.2.2 Wang等人[17]嵌入法範例 24
3.2.3 Wang等人[17]的萃取演算法 26
3.2.4 Wang等人[17]萃取法範例 26
3.3 RS統計偵測法之回顧[10][11] 28
3.3.1 鑑別函數： 28
3.3.2 翻轉函數： 28
3.3.3 RS偵測法：偵測嵌入訊息的存在 30
第四章 我們的改良作法 32
4.1 我們的想法 32
4.2 嵌入演算法 41
4.3 萃取演算法 43
4.4 簡例 44
第五章 實驗結果與討論 48
5.1 實驗環境 48
5.2 實驗方法 48
5.3 實驗結果 50
5.3.1 不可察覺性實驗 50
5.3.2 最大嵌入容量實驗 53
5.3.3 不可偵測性實驗 54
第六章 結論 56
6.1 結論 56
6.2 未來研究方向 57
參考文獻 58

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