1. Introduction to Visual Cryptography

2. Overview Grayscale VC Color VC What is Visual Cryptography
How to share a secret
Concept of Digital Halftoning
Grayscale VC
Color VC
Bit-level based secret sharing
Conclusion

3. Visual Cryptography Traditional cryptography: Visual Cryptography:
Both Encrypt & Decrypt
Needs the knowledge of cryptography, keys and possess high computational complexity
Visual Cryptography:
Use the characteristic of human vision to decrypt encrypted images
Needs neither cryptography knowledge nor complex computation

4. Visual Cryptography

5. How to Share a secret (k, n) Threshold Scheme
k points in 2-D plane (x1 , y1) , … , (xk , yk) with distinct xi ‘s, there’s one and only one polynomial q(x) of k -1 degree such that
q(xi ) = yi for all i
Assume q(xi ) = a0+a1x+…+ak-1xk-1
Where D = a0 = q(0)
evaluate D1 = q(1) ,..., Di = q(i) ,…. ,Dn = q(n)
cannot solve with the knowledge of just k-1
values

6. Continuous-tone VS Halftone
Render the illusion of a continuous-tone image based on two-tone (half-tone) display
Black/white gray-level image
Darker area denser black points per area
Whiter area sparser black points per area

7. Continuous-tone VS Halftone

8. Basic Theorem of VC

9. VC For Binary Image
Enlarged

11. Grayscale VC

13. Color Model RGB: TV’s and Monitors CMY: Color Printing Press
Use Additive Color Use Subtractive Color

14. Color Image Printing

15. Color VC - Method(1)

16. Color VC - Method(1)

17. Color VC - Method(2)

18. Color VC - Method(2)

19. Color VC - Method(3)

20. Color VC - Method(3)

21. Bit-level based secret sharing
The drawback of visual sharing schemes
Cannot restore its original quality
Convert continuous-tone images into halftone images
Bit plane 1
Bit plane b

22. Bit-level based secret sharing
Every pixel can be expressed as
We can obtain 8 shares(by size invariant VC)
If white: If black:
Every share can decomposed into 2 sub-share
Then combine the sub-share(with order of MSB~LSB ) → 2 grayscale share images 27 26 25 24 23 22 21 20
(1 , 1)
(0 , 1)
(0 , 0)
(1 , 0)

23. Conclusion Traditional VC only works on binary images
Grayscale VC and Color VC→provide secure ways to transfer images on the internet
Advantages of VC:
1.exploit human eyes to decrypt secret
images with no computation
2.easy to implement→no complex computation
Disadvantages of VC:
1. the image will become 4 times bigger
2. the quality of the restored image become
worse

24. Conclusion
Utilizes bit-level decomposition and stacking operations to both encrypt and decrypt B-bit image by computer
Bit-level based secret sharing allows for perfect reconstruction of the input B-bit image
Can encrypt binary, gray-scale and color images
Retain the size of the reconstructed image

25. References
[1] M. Noar and A. Shamir, “Visual Cryptography,” Advances in Cryptography
Eurocrypt’94, Lecture Notes in Computer Science, Springer-Verlag, Vol. 950, pp. 1-12, 1995.
[2] AShamir, “ How To Share a Secret, ” Communications of the ACM, vol.22,no. 11,
1979,pp
[3] C.Y. Chang, Visual cryptography for color images, MS thesis, National Central University,
Taiwan, 2000.
[4] Y.C. Hou, Visual cryptography for color images, Pattern Recognition 36 (2003) 1619–1629.
[5]Chao-Yong Hsu, Chun-Shien Lu, and Soo-Chang Pei, "Joint Screening Halftoning and Visual
Cryptography for Image Protection," Int. Workshop on Digital Watermarking (IWDW),
Lecture Notes in Computer Science, 2006
[6]  陳尚寬,” A rapid-decoding secret sharing scheme based on visual cryptography”