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Cheating Prevention in Visual Cryptography

Cheating Prevention in Visual Cryptography. IEEE TRANSACTIONS ON IMAGE PROCESSING,2007 指導老師:李南逸 報告者:黃資真. Outline. Introduction Cheating in VC Three cheating methods Generic transformation for cheating prevention. Conclusion. Introduction.

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Cheating Prevention in Visual Cryptography

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  1. Cheating Prevention in Visual Cryptography IEEE TRANSACTIONS ON IMAGE PROCESSING,2007 指導老師:李南逸 報告者:黃資真

  2. Outline • Introduction • Cheating in VC • Three cheating methods • Generic transformation for cheating prevention. • Conclusion

  3. Introduction • Propose three cheating methods against VC or EVC schemes. • Propose some necessary criteria for a VCS to be secure against cheat-preventing. • Propose a generic method that converts any VCS to another VCS with the property cheating prevention.

  4. Cheating in VC • Two types of cheaters: • Malicious participant(MP) • Malicious outsider(MO) • A cheating process against a VCS: • Fake share construction. • Image reconstruction phase.

  5. Cheating in VC

  6. Three cheating methods- Cheating a VCS by an MP • Assume that is the cheater. • Uses his genuine share as a template to construct a set of fake share. • Fake share are indistinguishable from its genuine share.

  7. Three cheating methods- Cheating a VCS by an MP Input: .(Assume that the cheater is ) Fake Share construction phase: Assume: (1) Each pixel of has x black and y white subpixels. (2) chooses a fake image and prepares r= (m/x)-1 fake shares. • For each white pixel of the fake image, copy the corresponding subpixels of the pixel in to each fake share. • For each black pixel of the fake image, randomly assign x black and y white subpixels to each fake share. Image reconstruction phase (the fake image): If ,the stacking of genuine shares and fake shares reveal the fake image.

  8. Three cheating methods- Cheating a VCS by an MP

  9. Three cheating methods- Cheating a VCS by an MO • MO can cheat even without any genuine share at hand. • Use the optimal (2,2)-VCS to construct the fake shares for the fake image.

  10. Three cheating methods- Cheating a VCS by an MO Input : none. Fake share construction phase: The MO chooses a fake image. • Encode the fake image into two fake shares with the optimal (2,2)-VCS. • Generate enough pairs of fake shares with various sizes and subpixel distributions, 1≦i ≦r for some r. Image reconstruction phase (the fake image): Let . The stacking of and two fake shares shows the fake image for some c, 1≦c≦r.

  11. Three cheating methods- Cheating a VCS by an MO

  12. Three cheating methods- Cheating a EVCS by an MP • Use the fake shares to reduce the contrast between the share images and the background. • The fake image in the stacking of fake shares has enough contrast against the background since the fake image is recovered in perfect blackness.

  13. Three cheating methods- Cheating a EVCS by an MP • be the threshold for contrast that human eyes distinguish the image from the background.

  14. Three cheating methods- Cheating a EVCS by an MP Input: .(Assume that the cheater is ) Fake Share construction phase: Assume: chooses a fake image • Create . The share image of is removed by changing d black subpixels into white subpixels in each black pixel. d is the difference between the numbers of black subpixels of a black and a white pixel. • Create temporary fake shares ,1≦i ≦r, by using according to CA-1. • Randomly change d white subpixels into black subpixels of each pixel of the share image in , 1≦i ≦r. • Construct by randomly adding black subpixels to each pixel in ,1≦i ≦r. Image reconstruction phase (the fake image): Same as in CA-1

  15. Three cheating methods- Cheating a EVCS by an MP

  16. Generic transformation for cheating prevention • An cheat-preventing method should have the following properties: • It does not rely on the help of an on-line TA. • The increase to pixel expansion should be as small as possible. • Each participant verifies the shares of other participants. • The verification image of each participant is different and confidential. • The contrast of the secret image in the stacking of shares is not reduced significantly. • A cheat-preventing method should be applicable to any VCS.

  17. Generic transformation for cheating prevention Input: Shares construction phase: • Let • Use as basis matrices for generationshare ,1≦i ≦n, of • ,1≦i ≦n, choose a verification image and generate a verification share as follows: • For each white pixel in the verification image, put the pixel of (m+2)-dimensional[100…0] (subpixels) to . • For each black pixel in the verification image, put the pixel of (m+2)-dimensional[0100…0] (subpixels) to .

  18. Generic transformation for cheating prevention Share verification phase: Before stacking their share, checks whether shows his verification image, where is another participant.

  19. Generic transformation for cheating prevention

  20. Conclusion • Proposed three cheating methods against VCS and EVCS. • Proposed an efficient transformation of VCS for cheating prevention.

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