臺灣博碩士論文加值系統

影像偽裝術(image steganography)，有別於密碼學直接將欲傳送的資料做加密(encryption)，是將重要的訊息嵌入在另一張看似無異樣的數位影像之中再傳送，以提升通訊的隱蔽性。它的安全性取決於不可覺察以及不可偵測機密訊息之存在的程度。偽裝分析(steganalysis)則是一門偵測此類秘密通訊行為的科學。
JPEG影像是現今網際網路最廣為使用的儲存格式；而JPEG影像偽裝技術則是以JPEG格式作為偽裝影像，因此研發一套安全又實用有效的JPEG影像偽裝術是個相當有意義且迫切解決的議題。前此已提出的JPEG影像偽裝技術並不多，而且大多使用LSB翻轉的觀念，因而無法抵擋基於區塊效應改變量的偽裝偵測攻擊。
有鑒於上述之缺點，我們提出一個可以克服之方法，我們的想法為藉由修改儘可能少的像素便可嵌入儘可能多的訊息位元，作法是參考Chen、Pan與Tseng在2002年所提出的資訊隱藏方法，平均在一區塊中只需修改一個係數即可。如此一來，修改後的訊息會稀疏地分布在區塊中，可使得發生在區塊邊緣的統計失真大大地降低以及重複取代的特性明顯地減少。
根據實驗結果證明我們的想法及作法是對的而且具有下列的特性：
1. 使用目前最常見的JPEG影像儲存格式。
2. 具有高不可覺察性。
3. 可抵檔Chi-square統計偵測攻擊。
4. 可抵檔Calibration統計偵測攻擊。
5. 訊息嵌入容量較J-Steg、F5及Outguess等著名的JPEG偽裝軟體為高。

Image steganography, which differs from cryptography that directly encrypts the data transmitted, embeds important messages into a digital image that appears usual before transmission to improve the concealment of communication. Its security depends on the ability to bypass the awareness of others and to resist the detection of secret messages. Steganalysis is the art of detecting acts of secret communication.
The JPEG image format is currently the most widely used storage format in the Internet. JPEG image steganography applies the JPEG format as the cover image, thus to develop an efficient, safety and useful JPEG image steganography technology is a meaningful and urgent problem. There are only a few JPEG image steganography techniques formerly proposed, yet most utilize the LSB flipping concept, which cannot resist detection attacks based on the variations of block effects.
Observing the flaws mentioned above, we propose a method to remedy the problems. Our main concept is to embed as much message bits as possible while minimizing the number of modified bits. Our method is based on the data hiding method proposed by Chen, Pan and Tseng in 2002, and it only needs to adjust one coefficient of average in a block. In this way, the modified message will be sparsely distributed in the block, which significantly reduces statistical distortion that may occur at the block edges and also drastically diminishes the characteristics of repeating replacement.
Experiment results show that our concept is correct and has the following properties:
1. Applying the most commonly used image storage format, the JPEG format.
2. Possesses high undetectability.
3. Is capable of resisting the Chi-square statistical detection attack.
4. Is capable of resisting the Calibration statistical detection attack.
5. Higher embedding capacity when compared with several renowned JPEG steganographic software such as J-Steg, F5 and Outguess.

誌謝 I
中文摘要 II
ABSTRACT III
目錄 V
圖目錄 VII
表目錄 IX
第一章 緒論 1
1.1 研究動機與目的 1
1.2 論文架構 5
第二章 基本數位影像理論 6
2.1 數位影像格式 6
2.2 影像偽裝技術探討 7
2.2.1 空間域數位影像資料格式 8
2.2.2 頻率域數位影像資料格式 9
2.3 JPEG靜態影像壓縮 10
2.3.1 離散餘弦轉換 11
2.3.2 量化 12
2.3.3 可變長度編碼 14
2.4 影像品質的鑑定 18
第三章 相關文獻探討 20
3.1 植基於二元影像偽裝技術 20
3.1.1 矩陣編碼 20
3.1.2 CPT演算法 22
3.1.2.1 二元影像的資料隱藏法介紹 22
3.1.2.2 二元影像的資料隱藏步驟 24
3.2 植基於JPEG壓縮影像的相關偽裝技術 28
3.2.1 Kobayashi所提出的方法 28
3.2.2 Chang所提出的方法 29
3.2.3 JPEG-JSteg 29
3.2.4 F5 31
3.2.5 Outguess 32
3.3 資訊偽裝偵測與分析 33
3.3.1 Chi-square偵測攻擊 34
3.3.2 Calibration偵測攻擊 35
第四章 以JPEG為偽裝影像格式的安全偽裝技術 41
4.1 研究目的與分析 41
4.2 嵌入流程 42
4.3 萃取流程 48
第五章 實驗結果 50
5.1 實驗方法 50
5.2 安全性 51
5.2.1 不可覺察性 51
5.2.2 不可偵測性 55
5.2.2.1 Chi-square偵測結果 55
5.2.2.2 Calibration偵測結果 57
5.3 嵌入容量 60
5.4 實驗討論 63
第六章 結論 64
6.1 結論 64
6.2 未來研究方向 65
參考文獻 66

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