The Study of Stegnography and Watermark on Binary Images

在隱像術不易察覺要求的先決條件下，灰階及彩色影像有較大的藏密範圍，如LSB(least significant bit)對像素值改變小，除了藏秘量大外，可用的方法也多，更重要的是藏入秘密亦不易被察覺，然而許多方法適用於灰階及彩色影像卻無法適用僅含兩像素值的二元影像，因此藏密於二元影像較灰階或彩色影像困難許多。然而，在經濟考量上，相對於灰階或彩色影像，二元影像有較小的儲存空間，可提高傳輸效率。在實際考量上，日常文書處理及許多早期的文件皆使用二元影像，如醫學，商業，法律，乃至個人文件應用及水墨繪圖，書法，電子圖書館等，經常需要加入浮水印或附註說明來防止偽造或竄改。生活上，二元影像藏密技術亦是不可或缺的一環。 二元影像從早期的文件影像利用，空白字，或字間空白及近年的語意學的高階藏密法藏密， 但無法同時滿足於二元影像在不易察覺性與藏密量的要求，因此現在大部份的方法仍以區塊藏密法為主，區塊藏密法即一張影像切出固定的區塊，再判定區塊是否為可藏密邊緣區塊，藏密後不會造成視覺品質不佳的結果。 在二元影像中，任意更動一像素於均勻的二元影像區塊，如全黑區塊中的一白點或全白區塊中的一黑點，均極易引起注意，為能滿足隱像術不易察覺的要求，二元影像中的藏密法多利用黑白的邊緣來藏。許多方法在黑白邊緣選出如3x3區塊大小，只中心一點的藏密方式藏量非常有限，有的方法區塊選擇邊緣寬度大於2位元的區塊， 如1x4， 1x6，或mxn區塊(m>=3且n>=3)，變動的部份超過邊緣範圍，所以藏密後出現明顯的模糊邊緣。整張圖容易被發現有人為加工無法滿足隱像術不易察覺的要求。近年來許多學者更將研究延伸到無失真影像還原上，有學者以數學運算來達到無失真，卻無法清楚區分區塊是否藏密，又未能記錄區塊藏密與否，無法有效達成還原性，有的學者用壓縮法來達到無失真的目的，然而壓縮效果不佳，對原本就亟需藏密量的二元影像，無法達到實用的效果，更有學者以極少或不存在於圖中的區塊型態作為識別藏密區塊，僅記錄這些極少區塊出現位置當成額外資訊或秘鑰來達到還原性，然而這些極少或不存在於圖中的區塊型態經常不是邊緣區塊，一旦大量藏入極易被察覺人為加工的圖像，造成顯著視覺品質不佳的結果。除了上列的缺失外，過去的研究大多忽略了單一區塊藏密最大藏量的討論，以致藏量都非常低。 如何在最大藏量達成最小變動量，同時兼顧到藏密後不易察覺性或達成無失真還原的實用性，是本研究最大的貢獻，達成上述條件即在區塊藏密不易察覺的絕對要求下，排除均勻的全黑或全白狀況，其他情況皆可藏達最高藏密量的考量下找出最佳區塊型態。2x2區塊型態在連續性與平滑性不易察覺考量上皆為最佳條件，2x2區塊在扣除不可能發生在邊緣的全黑及全白區塊藏密量可達[log2(24-2)]約3.8位元。不但區塊比一般所採用的小，提出變動長度藏密法來達成藏密量極大化為一般藏量的數倍，在如此優勢的條件下，我們提出配對(maximum pair matching)法達成最小的單位像素變動(per pixel distortion)。我們保證2x2區塊中可藏將近3(2.857)個位元的密秘，而且每一2x2區塊變動位元數保證不超過2位元。換句話說我們的單位像素變動平均只有0.34比一般0.5小得非常多，更重要的是採用文字比對的方式，我們時間消耗卻非常低，因此我們的研究以強過其它研究數倍的成效，達到目前二元影像的新紀元。在應用上，我們也利用我們高藏量低變動性的特性設計無失真的還原性機制，除了我們提出六進位霍夫曼壓縮法外，壓縮率雖提高許多，我們更進一步提出還原插入法(reversible bi-linear interpolation method)，在視覺效果變化不大的情況下，我們不但保存了原有的圖像資料於圖像中，即使用我們擴大的影像當原圖用其它方法來藏，我們的藏密量仍是大幅領先其它方法且單位像素變動率更小，我們還可達到還原性的目標。未來我們的研究將朝向過去別人所沒有作過的浮水印偵測及復原機制，開創二元影像的新標竿。Stegnography on binary images, however, is much more difficult than that on grayscale or color counterparts because only two distinct values are available for each pixel. A variety of applications of applying stegnography to binary images have been identified. Most stegnography schemes on binary images adopt a block pattern based approach. With only two colors for each pixel in binary image, arbitrarily flipping pixels in the uniform region of binary images can cause significant artifacts. For imperceptibility, most schemes embed secret on the edges of binary images. Significant visual artifacts might appear due to edge blurring if pixels outside the edge are altered. To maximize the data hiding capacity, variable length coding techniques can be applied to compress the hidden information. Based on the considerations of smoothness and connectivity of the images, we adopt blocks of size 2x2 as our basic data hiding units. Information embedding capacity is not the ultimate concern and can be traded for better visual quality. The pair matching method proposed in this thesis guarantees at most 2-bit alteration in any embedded 2x2 block. In other words, the average per pixel distortion is only 0.34, which is much less than 0.5 of most other schemes. With regard to issue of confidentiality, a seed value is employed as the secret key. Without such information, the locations of the information embedded cannot be identified, which prevents any attempt of unauthorized information retrieval. The pseudo random number generator used in choosing the information embedding blocks also tends to exhibit an even distribution in locations so as to lower the perceptibility of the artifacts. Due to algorithmic simplicity, the execution times of the proposed schemes are, on average, much faster than those of the rival schemes. For reversibility, a senary Huffman compression method is proposed. Combining the senary Huffman compression method and the pair matching technique leads a very efficient reversible data hiding scheme outperforming prior arts by a large margin in both capacity and imperceptibility aspects. Finally, a reversible data hiding scheme working on an extended images via interpolation is proposed. Due to image expansion, most pixel alternations occur on interpolated pixels and only those pixels belonging to the original image need to be recovered. Moreover, under the devised block selection criteria, an altered pixel value can be recovered from its adjacent pixel values. The visual quality of the stego image is improved as well because no two data embedded blocks abut against each other. Extensively simulation results conducted in this thesis have shown that the proposed data hiding schemes have the advantages of high information capacity, better visual quality, full reversibility, and high confidentiality for data hiding in binary images.Table of Contents Chapter 1 Introduction 1 1.1Research motivations 2 1.2 Objectives and research scope 4 1.3 Organization 7 Chapter 2 Background 9 2.1 Imperceptibility 9 2.2 Capacity 10 2.3 Reversibility 11 Chapter 3 Related Works 13 3.1 Non-reversible schemes 13 3.1.1 Tseng et al.'s method 13 3.1.2 Chen et al.'s method 14 3.1.3 Wu et al.'s method 14 3.1.4 Yang et al.'s method 15 3.2 Reversible schemes 16 3.2.1 Tsai et al.'s method 16 3.2.2 Pamboukian et al.'s method 17 3.2.3 Ho et al.'s method 18 Chapter 4 Senary Huffman Compression 20 4.1 Preliminary 20 4.2 Notations and definitions 21 4.3 The Proposed scheme 21 4.3.1 embedding phase 22 4.3.2 extracting phase 27 4.4 Experimental Results 28 Chapter 5 Watermarking Authentication Schemes 31 5.1 Proposed authentication watermarking schemes 32 5.1.1 2x2 Dual-Pair Block (DPB) 32 5.1.2 Matching-Pair (MP) method 36 5.1.3 Dual-Pair Block authentication (DPBA) scheme 40 5.1.4 senary Huffman compression authentication (SHCA) watermarking scheme 44 5.1.5 security analyses 49 5.2 Experimental results 50 5.2.1 test bench images 51 5.2.2 evaluations of DPBA scheme 53 5.2.3 evaluations of SHCA scheme 58 Chapter 6 A High Capacity Data Hiding Scheme Based on Block Patterns 63 6.1 Background 63 6.1.1 NSB and DPB 64 6.1.2 transformation of secrets from binary into septenary values 66 6.1.3 matching pair (MP) method 67 6.2 Proposed data embedding and extracting scheme 73 6.2.1 data embedding phase 73 6.2.2 data extracting phase 74 6.3 Experimental results 75 Chapter 7 Hiding Information in Binary Images with Complete Reversibility and High Embedding Capacity 83 7.1 Background 84 7.1.1 image pre-processing by interpolation 84 7.1.2 complementary form of basic data embedding unit 85 7.1.3 conversion of binary secret to septenary values 87 7.1.4 selection of embeddable blocks 88 7.1.5 symbol remapping by maximum pair matching (MPM) 90 7.2 Proposed reversible data hiding scheme 94 7.2.1 data hiding phase 94 7.2.2 data extraction phase 96 7.3 Experimental results 97 Chapter 8 Conclusions and Future Work 108 8.1 Conclusions 109 8.2 Future work 113 References 11