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 )

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

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

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

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

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

7 Hidden in The Basis Image: Mickey The Counterfeit Image: Minnie The Secret Image: Airplane

8 The Basis Image: Mickey The Counterfeit Image: Minnie The Expanded Basis Image: Mickey-3X3 The Expanded Counterfeit Image: Minnie-3X3

9 Airplane Mickey-3X3Minnie-3X3

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

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

14 Share 1Share 2

15 shadow 1shadow 2 camouflage cover image stego image1stego image2 secret image Meaningful Visual Cryptography for Grayscale Image

cover image shadow1 140 transparent stego image shadow2 stego image shadow1 shadow2

stego image1 stego image2 stacked result

18 Experimental Results shadow1 cover image1stego image1 shadow2 cover image2stego image2

19 stego image1 stego image2 stacked result

20 shadow1 cover image1stego image1 shadow2 cover image2stego image2

21 stego image1 stego image2 stacked result

22 Meaningful and Progressive Visual Cryptography Meaningful shares: friendly content Progressive visual cryptography Improve the stacked result progressively

23 Progressive Visual Cryptography (t, 6) (2, 6)(3, 6)(4, 6)(5, 6)(6, 6)

24 Meaningful shares: friendly content Progressive & Meaningful Visual Cryptography shadow 1shadow n camouflage cover image stego image1stego image n … … secret image

25 secret image cover image secret imagecover imageshadow image extended secret image shadow image 1shadow image 2shadow image 3shadow image 4

26 secret image cover image t =2t = 3

27 (2, 6)(3, 6)(4, 6)(5, 6)(6, 6) Experimental Results

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

32 Participant 1 Participant 2 Share 1 Share 2 Stack Recovered secret image Hacker Authentication Original secret image

33 Share 1 Share 2 Correct stacking result Share 1 Share 2 Correct stacking result Legal extra confidential image Legal revealed secret image Authentication

34 Tampered share 1 Share 2 Tampered share 1 Share 2 Tampered secret image Nothing Tampered revealed secret image Authentication

35 Grayscale image LenaGrayscale image BaboonBinary main secret Halftone image of LenaHalftone image of BaboonBinary extra confidential image Authenticatable VSS Scheme for Binary Secret

36 Share image AShare image B Stacking resultRevealed extra confidential image Authenticatable VSS Scheme for Binary Secret

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

38 Share image AShare image B Stacking resultRevealed extra confidential image Authenticatable VSS Scheme for Color Secret

39 Visual cryptography (Cont.) Share 1 Share 2 Secret 1 “ VISUAL ” Secret 2 “ SECRET ” Stacking Rotating 40 o

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

41 Conclusions An alternative for hiding visual secret Less secret Low computation cost

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