My presentation on Image-based Security prepared for the Faculty Development Programme on "Research Issues in Digital Image Processing". This covers the concepts of Visual Cryptography and is based on the book "Handbook of Image-based Security Techniques"

1. Image-based Security
Adri Jovin J J, M.Tech., Ph.D., B.G.L.,
Assistant Professor (Sl. Gr.), Department of Information Technology
Sri Ramakrishna Institute of Technology

2. VC???
VC
Visual Cryptography
24-05-2021 Image-based Security 2

3. Introduction
• Secret sharing – vital cryptographic application
• Secret sharing – introduced in 1979 by Shamir
• Uses Lagrange interpolation, polynomial and intersection of affine hyperplanes
• Based on matrices and prime numbers
• More specifically designed for numeric and text data
• Today, images contain sensitive data
• Illegal modification of images have become very common
24-05-2021 Image-based Security 3

4. Visual Cryptography
• Proposed by Naor and Shamir in 1994
• Exclusively for images
• Divide image into two or more shares
• On appropriate superimposition the original image is restored
24-05-2021 Image-based Security 4
k-out-of-n VC

5. 2-out-of-2 VC
k=2, n=2
• Each pixel p of a secret binary image is encoded into a pair of black and white subpixels for
each of the two shares using the codebook.
• If p is white/black, one of the first/last two columns under the white/black pixel in the
codebook is selected randomly so that the probability of selection will be 0.5.
• Irrespective of whether p is black or white, a single pixel is encoded into two subpixels of
black-white or white-black with equal probabilities.
24-05-2021 Image-based Security 5

6. Applications
• Trojan Free Secure Transaction
24-05-2021 Image-based Security 6

7. Applications (Contd…)
• Authentication
• Access Control (Joint accounts in banks)
• Transaction Tracking (Software licensing)
• Watermarking (Ownership identification)
24-05-2021 Image-based Security 7

8. Fundamental Principles
Visual
Cryptography
Contrast
Basis
Matrix
Pixel
Expansion
24-05-2021 Image-based Security 8

9. Pixel Expansion
• Number of subpixels in which sub pixel p of the secret image Isec is encoded in every share
• Must be kept minimal
• Ideal value is m=1 (1 pixel will be encoded by one pixel in each share)
24-05-2021 Image-based Security 9

10. Contrast
• Black pixels are recovered with 100% accuracy
• White pixels may suffer a black mix
• Important measure to judge the effectiveness of VC
• All approaches suffer some contrast loss
• Increase in pixel expansion reduces contrast loss
• Contrast value must be high
24-05-2021 Image-based Security 10

11. Basic Matrices
• Used to construct shares
• Most VC methods are based on binary secret image, which results in two basis matrices, one
for black and the other for white.
𝑆0 =
0 1
0 1
, 𝑆1 =
0 1
1 0
𝐶0 =
0 1
0 1
,
1 0
1 0
𝐶1 =
0 1
1 0
,
1 0
0 1
24-05-2021 Image-based Security 11

12. Concept of Black and White Pixels
• In general, intensity value for white is 1 and for black it is 0
• In VC, white is denoted as 0 and black is denoted as 1
• Share staking – similar to logical OR
REASON???
24-05-2021 Image-based Security 12
TRUTH TABLE
1 1 1
1 0 1
0 1 1
0 0 0

13. Formation of a basis matrix
Some points for observation:
1. Each row of both S0 and S1 contains 𝑛 1s and 𝑛 0s
2. OR operation on rows of S0 contain single 0 in the resultant vector
3. OR operation on rows of S1 contain all 1s in the resultant vector
4. Pixel expansion is directly dependent on the number of participants as m=2n-1, so if n is large
then pixel expansion will be exponentially large
5. Contrast 𝛼 also depends on the number of participants as 𝛼 =
1
2𝑛−1. If the value of n
increases 𝛼 decreases.
24-05-2021 Image-based Security 13

14. Various Dimensions of Visual Cryptography
Visual Cryptography
Traditional
Visual
Cryptography
(TVC)
Threshold
Visual
Cryptography
Extended Visual Cryptography (EVC)
Halftone
Visual
Cryptography
Tagged Visual
Cryptography
Friendly Visual
Cryptography
Size Invariant
VC
Progressive
Visual
Cryptography
Dynamic Visual Cryptography (DVC)
XOR – based
Visual
Cryptography
Visual
Cryptography
with Multiple
Secrets
Verifiable
Visual
Cryptography
Multitone
Visual
Cryptography
Random Grid
based Visual
Cryptography
24-05-2021 Image-based Security 14

15. Traditional Visual Cryptography
• All approaches based on Naor and Shamir’s approach
• Possess the following properties:
1. The dimensions of all shares will be larger than the original secret image.
2. Each share will be visually random in nature.
3. The secret can be recovered just by stacking the shares.
24-05-2021 Image-based Security 15

16. Threshold Visual Cryptography
• Generalization of Traditional Visual Cryptography
• Denoted as (k,n) – VCS, where k is the threshold
value and n is the number of participants
• The secret shall be revealed only if k number of
participants meet together
24-05-2021 Image-based Security 16

17. Extended Visual Cryptography
• Many features are added to traditional visual cryptography
24-05-2021 Image-based Security 17
Extended Visual
Cryptography
(EVC)
Halftone Visual
Cryptography
Tagged Visual
Cryptography
Friendly Visual
Cryptography
Size Invariant
VC
Progressive
Visual
Cryptography

18. Halftone Visual Cryptography
• Halftone images are generated by the reprographic technique that
simulates continuous images through the use of dots, which creates an
almost grayscale image.
• More appropriate for visual cryptography compared to threshold binary
images because of the randomness.
24-05-2021 Image-based Security 18

19. Tagged Visual Cryptography
• Method to address the problem of share handling in case of
large number of shares
• Shares contain information about both the image’s tag as
well as the secret
24-05-2021 Image-based Security 19

20. Friendly visual cryptography
• Visual cryptography with meaningful shares
24-05-2021 Image-based Security 20

21. Size Invariant visual cryptography
• The dimensions of the secret and shares are identical.
24-05-2021 Image-based Security 21

22. Progressive Visual Cryptography
• Clarity and contrast of the secret image is improved by stacking more and more shares.
24-05-2021 Image-based Security 22

23. Dynamic Visual Cryptography
• Extended visual cryptography with additional features
24-05-2021 Image-based Security 23
Dynamic Visual
Cryptography
(DVC)
XOR – based
Visual
Cryptography
Visual
Cryptography
with Multiple
Secrets
Verifiable Visual
Cryptography
Multitone Visual
Cryptography
Random Grid
based Visual
Cryptography

24. Multitone Visual Cryptography
• All visual cryptography approaches which are made up for gray scale/color images
24-05-2021 Image-based Security 24

25. Visual cryptography with multiple secrets or multi secret sharing
• A greater number of secrets can be shared with the same set of shares
24-05-2021 Image-based Security 25

26. XOR-based visual Cryptography
• Computation-based visual cryptography approach which is used for perfect recovery of the
secret.
24-05-2021 Image-based Security 26

27. Verifiable Visual Cryptography
• Used to enhance the power of any existing VC approach by providing tamper detection
capability to the shares.
• Can only be implemented with computation at the recovery end, therefore it may increase the
complexity of the VC approach.
24-05-2021 Image-based Security 27

28. Random grid-based visual cryptography
• Transparent with randomly distributed intensities of either white (transparent) or black
(opaque).
• Generate unexpanded shares by default
24-05-2021 Image-based Security 28

29. References
Shivendra Shivani, Suneeta Agarwal, Jasji S. Suri. (2018). Handbook of Image-based Security
Techniques.
Image Courtesy: Handbook of Image-based Security Techniques
24-05-2021 Image-based Security 29

30. Thank you!
24-05-2021 Image-based Security 30