視覺密碼學

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

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. secret message Traditional (t, n)-secret sharing (Shamir’s Method 1979) (2, 3)-secret sharing Alice Bob Ken shadows secret retrieval Shamir, A. (1979): How to Share a Secret, CACM, Vol. 22, Nov. 1979, pp. 612-613.

4. How to Share a Secret (t, n)-threshold Scheme • Ex: (2, 3)-threshold Scheme (The secret k=7) 3 shadows: (1, 9), (2, 0), (3, 2) y (3, 2) (2, 0) (1, 9) x

5. How to Share a Secret (t, n)-threshold Scheme Ex: (3, 5)-threshold Scheme(t=3, n=5, k=13, p=17) Bob Ken Jesse Eva Alice ID1=1 ID2=2 ID3=3 ID4=4 ID5=5 (1, 8) (3, 10) (5, 11) Lagrange Interpolation k

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

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

9. Mickey-3X3 Minnie-3X3 Airplane

10. Visual Cryptography for Binary Image • 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

11. Secret pixel Share 1 Secret image stacked result Share 2

13. Visual Cryptography for Grayscale Image • 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

14. Share 1 Share 2

15. cover image Meaningful Visual Cryptography for Grayscale Image secret image shadow 1 shadow 2 camouflage stego image1 stego image2

16. transparent shadow1 shadow1 167 97 140 150 shadow2 shadow2 stego image1 167 140 stego image2 cover image

17. 167 97 150 140 167 170 140 83 stego image1 stacked result stego image2

18. Experimental Results cover image1 stego image1 shadow1 cover image2 stego image2 shadow2

19. stego image1 stego image2 stacked result

20. cover image1 stego image1 shadow1 cover image2 stego image2 shadow2

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. cover image Progressive & Meaningful Visual Cryptography • Meaningful shares: friendly content … shadow 1 shadow n secret image camouflage … stego image1 stego image n

25. extended secret image secret image cover image shadow image secret image cover image shadow image 1 shadow image 2 shadow image 3 shadow image 4

26. secret image cover image t =2 t = 3

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

28. Visual Cryptography for Color Image RGB: TV’s and Monitors CMY: Color Printing Press

31. Experimental Results shadow image 1 shadow image 2 stacked image 2

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

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

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

35. Authenticatable VSS Scheme for Binary Secret Grayscale image Lena Grayscale image Baboon Binary main secret Halftone image of Lena Halftone image of Baboon Binary extra confidential image

36. Authenticatable VSS Scheme for Binary Secret Share image A Share image B Stacking result Revealed extra confidential image

37. Authenticatable VSS Scheme for Color Secret Color image Lena Color image Baboon Color main secret Color halftone extra confidential image Color halftone image of Lena Color halftone image of Baboon

38. Authenticatable VSS Scheme for Color Secret Share image A Share image B Stacking result Revealed extra confidential image

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

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

42. Thanks for your listening