Visual cryptography
- 1. WELCOME TO THE SEMINAR TITLED VISUAL CRYPTOGRAPHY Presented by: SHAHID MANZOOR ZARGAR
- 2. SEMINAR OVERVIEW INTRODUCTION VISUAL CRYPTOGRAPHY SECRET SHARING SCHEME VISUAL CRYPTOGRAPHY FOR BINARY IMAGES VISUAL CRYPTOGRAPHY FOR HALFTONE IMAGES VISUAL CYPTOGRAPHY FOR COLOR IMAGES IMPLEMENTATION ADVANTAGES DISADVANTAGES APPLICATION
- 3. INTRODUCTION Cryptography is the science and art of transforming messages to make them secure and immune to attack. Process of Cryptography Plain Text/image Encryption Cipher Plain Text /image Decryption Channel It is the practice and study of techniques for secure communication in the presence of third parties (called adversaries).
- 4. Visual cryptography Visual cryptography (VC) was introduced by Moni Naor and Adi Shamir at EUROCRYPT (1994). Used to encrypt written text/ pictures etc in a perfectly secure way. Decoding is done by human visual system, without any computation. Encryption involves creating “shares” of the image which in a sense will be a piece of the image. Give of the shares to the respective holders. Decryption – involving bringing together an appropriate combination and the human visual system.
- 5. Secret sharing scheme in Visual Cryptography Suppose the data D is divided into n shares D can be constructed from any k shares out of n Complete knowledge of k-1 shares reveals no information about D k of n shares is necessary to reveal secret data.
- 6. OVERVIEW OF Visual Cryptography Share1 Stacking the share reveals the secret Share2 Encryption Decryption
- 8. 2 out of 2 scheme(2 subpixels) Black and white image: each pixel divided in 2 sub-pixels Randomly choose between black and white. If white, then randomly choose one of the two rows for white.
- 9. 2 out of 2 Scheme (2 subpixels) If black, then randomly choose between one of the two rows for black.
- 10. 2 out of 2 Scheme (4 subpixels) Each pixel encoded as a 2x2 cell in two shares (key and cipher) Each share has 2 black, 2 transparent subpixels When stacked, shares combine to Solid black Half black (seen as gray)
- 11. Horizontal shares Vertical shares Diagonal shares 2 out of 2 Scheme (4 subpixels)
- 12. 2 out of 2 Scheme (4 subpixels)
- 13. Visual cryptography for Binary Images Naor and Shamir proposed a scheme take a pixel from secret image extend it onto 2x2 subpixel Randomly choose 1 out of 2 share.
- 14. Visual Cryptography for gray-level images (The halftone technology ) Transform the gray-level image into black-and white halftone image Halftone technology uses 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
- 15. visual cryptography for color images Rijmen and Preneel’s method Each pixel is divided into 4 subpixels, with the color red, green, blue and white. In any order, we can get 24 different combination of colors. We average the combination to present the color. To encode, choose the closest combination, select a random order on the first share. According to the combination, we can get the second share.
- 16. visual cryptography for color images Rijmen and Preneel’s method Pattern1 Pattern1Pattern2 Pattern2 Combined Result Combined Result
- 17. visual cryptography for color images
- 18. Advantages Simple to implement, Such a technique thus would be lucrative for defense and security. Decryption algorithm not required (Use a human Visual System). So a person unknown to cryptography can decrypt the message. We can send cipher text through FAX or E-MAIL. Lower computational cost since the secret message is recognized only by human eyes and not cryptographically computed.
- 19. Advantages Send innocent looking transparencies, e.g. Send images a dog, a house, and get a spy message with no trace.
- 20. Disadvantages The contrast of the reconstructed image is not maintained. Perfect alignment of the transparencies is troublesome. Due to pixel expansion the width of the decoded image is twice as that of the original image. Leads to loss of information due to change in aspect ratio. Its original formulation is restricted only to binary images. Additional processing is required for colored images.
- 21. Implementation Platform: .Net Framework 3.5 Language: C# Reason for selecting .Net: Previous experience working on .Net. .NET also has lot of support for image acquisition and manipulation. Other Platforms: Matlab, Python etc
- 22. Application Watermarking Steganography Remote electronic voting Bank customer identification
- 23. Example of watermarking Fig: A twenty euro bank note held against the light to show watermark.
- 25. Example of internet voting system Fig 1. The screen image and transparency are random dots before aligning. Fig 2. After aligning the transparency and screen image,the voter password (EH) is revealed.