Image based security
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The document discusses image-based security, focusing on visual cryptography, a method developed by Naor and Shamir to securely share images by dividing them into multiple shares. It explains key concepts, including pixel encoding, applications, and various types of visual cryptography approaches such as traditional, threshold, and dynamic visual cryptography. The document emphasizes the importance of contrast, pixel expansion, and the potential applications in secure transactions, authentication, and watermarking.
Image based security
- 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