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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 decryption becomes a mechanical operation that does not require a computer. One of the best-known techniques has been credited to Moni Naor and Adi Shamir, who developed it in 1994.〔Naor, M. and A. Shamir. Visual cryptography, Advances in cryptology. Eurocrypt ’94 Proceeding LNCS, 950:1–12, 1995.〕 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 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 overlaid, the original image would appear. There are several generalizations of the basic scheme including k-out-of-n visual cryptography.〔Verheul, E.R. and H.C.A.van Tilborg. Constructions and properties of k out of n visual secret sharing schemes. Design Codes and Cryptography, 11(2):179–196, 1997.〕〔Ateniese, G., C. Blundo, A. De Santis, and D. R. Stinson. Extended capabilities for visual cryptography. Theoretical Computer Science, 250:143–161, 2001.〕 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. Normally, there is an expansion of space requirement in visual cryptography. But if one of the two shares is structured recursively, the efficiency of visual cryptography can be increased to 100%.〔Gnanaguruparan, M. and Kak, S. Recursive hiding of secrets in visual cryptography. Cryptologia, vol. 26, pp. 68-76, 2002.〕 Some antecedents of visual cryptography are in patents from the 1960s.〔Cook, Richard C. (1960) (Cryptographic process and enciphered product ), United States patent 4,682,954.〕〔Carlson, Carl O. (1961) (Information encoding and decoding method ), United States patent 3,279,095.〕 Other antecedents are in the work on perception and secure communication.〔Kafri, O. and E. Keren. Encryption of pictures and shapes by random grids. Optics Letters, Vol. 12, Issue 6, pp. 377–379 (1987)〕〔Arazi, B., I. Dinstein, O. Kafri. Intuition, perception, and secure communication. IEEE Transactions on Systems, Man and Cybernetics. Vol. 19, Issue 5, pp. 1016–1020 (1989)〕 Visual cryptography can be used to protect biometric templates in which decryption does not require any complex computations. == Example == In this example, the image has been split into two component images. Each component image has a ''pair'' of pixels for every pixel in the original image. These pixel pairs are shaded black or white according to the following rule: if the original image pixel was black, the pixel pairs in the component images must be complementary; randomly shade one ■□, and the other □■. When these complementary pairs are overlapped, they will appear dark gray. On the other hand, if the original image pixel was white, the pixel pairs in the component images must match: both ■□ or both □■. When these matching pairs are overlapped, they will appear light gray. So, when the two component images are superimposed, the original image appears. However, considered by itself, a component image reveals no information about the original image; it is indistinguishable from a random pattern of ■□ / □■ pairs. Moreover, if you have one component image, you can use the shading rules above to produce a ''counterfeit'' component image that combines with it to produce any image at all. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Visual cryptography」の詳細全文を読む スポンサード リンク
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