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1 視覺密碼學 Chair Professor Chin-Chen Chang ( 張真誠 ) National Tsing Hua University ( 清華大學 ) National Chung Cheng University ( 中正大學 ) Feng Chia University ( 逢甲大學 ) http://msn.iecs.fcu.edu.tw/~ccc
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2 Outlines Concept of Secret Sharing Concept of visual cryptography Visual cryptography for binary images Visual cryptography for grayscale images Meaningful visual cryptography for grayscale images Meaningful and progressive visual cryptography Visual cryptography for color images Authenticatable VSS Scheme
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3 shadows secret message Traditional (t, n)-secret sharing (Shamir ’ s Method 1979) secret retrieval (2, 3)- secret sharing AliceKenBob Shamir, A. (1979): How to Share a Secret, CACM, Vol. 22, Nov. 1979, pp. 612-613.
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4 Ex: (2, 3)-threshold Scheme (The secret k=7) 3 shadows: (1, 9), (2, 0), (3, 2) x y (1, 9) (2, 0) (3, 2) How to Share a Secret (t, n)-threshold Scheme
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5 Ex: (3, 5)-threshold Scheme (t=3, n=5, k=13, p=17) Alice KenBobJesseEva ID 1 =1ID 2 =2ID 3 =3ID 4 =4ID 5 =5 (1, 8)(3, 10)(5, 11) Lagrange Interpolation k How to Share a Secret (t, n)-threshold Scheme
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6 Concept of Visual Cryptography Cryptography Traditional Cryptography Symmetric: DES, AES Asymmetric: RSA The encryption and decryption processes need high computation costs Need secret key for decoding Visual Cryptography low computation Fast decoding Without the secret key
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7 Hidden in The Basis Image: Mickey The Counterfeit Image: Minnie The Secret Image: Airplane
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8 The Basis Image: Mickey The Counterfeit Image: Minnie The Expanded Basis Image: Mickey-3X3 The Expanded Counterfeit Image: Minnie-3X3
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9 Airplane Mickey-3X3Minnie-3X3
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10 Naor and Shamir (1994) proposed a (k, k) – VSS scheme Extend a secret pixel into a block of 2 2 sub-pixels Contain two white pixels and two black pixels for each block White pixel: transparent Black pixel: black Visual Cryptography for Binary Image
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11 Secret pixel stacked result Secret image Share 1 Share 2
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13 Transfer the grayscale image to the halftone image Utilize the density of black dots to simulate the content of grayscale image White area: low density of black dots Black area: high density of black dots Visual Cryptography for Grayscale Image
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14 Share 1Share 2
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15 shadow 1shadow 2 camouflage cover image stego image1stego image2 secret image Meaningful Visual Cryptography for Grayscale Image
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16 01 10 152167 140137 cover image shadow1 140 transparent stego image1 01 10 17097 15083 01 10 10 01 140 167 170 83 shadow2 stego image2 167 97 150 shadow1 shadow2
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17 140 167 83 170 140 150 167 97 stego image1 stego image2 stacked result
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18 Experimental Results shadow1 cover image1stego image1 shadow2 cover image2stego image2
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19 stego image1 stego image2 stacked result
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20 shadow1 cover image1stego image1 shadow2 cover image2stego image2
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21 stego image1 stego image2 stacked result
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22 Meaningful and Progressive Visual Cryptography Meaningful shares: friendly content Progressive visual cryptography Improve the stacked result progressively
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23 Progressive Visual Cryptography (t, 6) (2, 6)(3, 6)(4, 6)(5, 6)(6, 6)
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24 Meaningful shares: friendly content Progressive & Meaningful Visual Cryptography shadow 1shadow n camouflage cover image stego image1stego image n … … secret image
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25 secret image cover image secret imagecover imageshadow image extended secret image shadow image 1shadow image 2shadow image 3shadow image 4
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26 secret image cover image t =2t = 3
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27 (2, 6)(3, 6)(4, 6)(5, 6)(6, 6) Experimental Results
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28 RGB: TV ’ s and Monitors CMY: Color Printing Press Visual Cryptography for Color Image
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31 shadow image 1shadow image 2stacked image 2 Experimental Results
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32 Participant 1 Participant 2 Share 1 Share 2 Stack Recovered secret image Hacker Authentication Original secret image
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33 Share 1 Share 2 Correct stacking result Share 1 Share 2 Correct stacking result Legal extra confidential image Legal revealed secret image Authentication
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34 Tampered share 1 Share 2 Tampered share 1 Share 2 Tampered secret image Nothing Tampered revealed secret image Authentication
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35 Grayscale image LenaGrayscale image BaboonBinary main secret Halftone image of LenaHalftone image of BaboonBinary extra confidential image Authenticatable VSS Scheme for Binary Secret
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36 Share image AShare image B Stacking resultRevealed extra confidential image Authenticatable VSS Scheme for Binary Secret
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37 Color image Lena Color image Baboon Color main secret Color halftone image of Lena Color halftone image of Baboon Color halftone extra confidential image Authenticatable VSS Scheme for Color Secret
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38 Share image AShare image B Stacking resultRevealed extra confidential image Authenticatable VSS Scheme for Color Secret
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39 Visual cryptography (Cont.) Share 1 Share 2 Secret 1 “ VISUAL ” Secret 2 “ SECRET ” Stacking Rotating 40 o
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40 Steganography (Cont.) Text 壞的毛病 壞毛病 山東的蘋果 山東蘋果 我的媽媽 我媽媽 主要的問題 主要問題 另外的一件事 另外一件事 其他的問題 其他問題 北京的大學 北京大學 Video Sound
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41 Conclusions An alternative for hiding visual secret Less secret Low computation cost
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