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 the human visual system, without the aid of computers. Visual cryptography was pioneered by Moni Naor and Adi Shamir in 1994. They demonstrated a visual secret sharing scheme, where an image was broken up into n shares so that only someone with all n shares could decrypt the image, while any n-1 shares revealed no information about the original image. Each share was printed on a separate transparency, and decryption was performed by overlaying the shares. When all n shares were overlayed, the original image would appear. Using a similar idea, transparencies can be used to implement a one-time pad encryption, where one transparency is a shared random pad, and another transparency acts as the ciphertext.
Example
In this example, the Wikipedia logo has been split into two shares. Each white pixel in the original logo is split into two of the same small blocks that have half black and white pixels. When these two blocks are overlayed, they line up exactly, and the result is a light-colored block (with half black and half white pixels). Each black pixel in the original logo is split into two complementary small blocks. When these two blocks are overlayed, the result is a completely black box. If each pixel in the original image is split randomly as described above, then each individual share is a totally random collection of blocks. Only when the shares are combined is any information revealed about the original image.
References
- Moni Naor and Adi Shamir, Visual Cryptography, EUROCRYPT 1994, pp1–12 [1].
- Doug Stinson's visual cryptography page
