Visual cryptography
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Visual cryptography is a cryptographic technique which allows visual information (pictures, text, etc.) to be encrypted in such a way that the decryption can be performed by humans (without computers).
The first visual cryptographic technique was pioneered by Moni Naor and Adi Shamir in 1994. It involved breaking up the image into n shares so that only someone with all n shares could decrypt the image by overlaying each of the shares over each other. Practically this can be done by printing each share on a separate transparency and then placing all of the transparencies on top of each other. In their technique n-1 shares revealed no information about the original image.
References
- Moni Naor and Adi Shamir, Visual Cryptography, EUROCRYPT 1994, pp1–12 [1] (http://www.wisdom.weizmann.ac.il/~naor/PUZZLES/visual_sol.html).