Visual cryptography is a cryptographic technique that allows visual information (images, text, etc.) to be encrypted in such a way that the decryption becomes the work of the person to be deciphered through visual reading.
One of the best known techniques has been credited to Moni Naor and Adi Shamir, who developed it in 1994. They demonstrated a secret visual exchange scheme, where an image was divided into n actions so that only someone with all n actions could Decrypt the image while any n - 1 actions revealed in the information about the original image. Each action was printed on a separate transparency, and the decryption was performed by superimposing the actions. When all n actions were overlapped, the original image would appear. There are several generalizations of the basic schema that includes k-out-of-n visual cryptography.
Using a similar idea, transparencies can be used to implement single-pad encryption, where a transparency is a shared random pad, and another transparency acts as the ciphertext. Typically, there is an expansion of the space required in visual cryptography. But if one of the two parts is structured recursively, the efficiency of visual cryptography can be increased up to 100%.
Some background of visual cryptography is in patents of the sixties. Other background is in the work on perception and secure communication. Visual cryptography can be used to protect biometric templates in which decryption does not require any complex computation.