Visual Cryptography — How to Use Images to Share a Secret

Abstract

In this paper, we give a survey of visual cryptography schemes, a new type of cryptographic schemes which was first introduced by Naor and Shamir [9] in 1994. Visual cryptography schemes can be considered as encryption schemes based on graphical data. In visual cryptography schemes as special instances of secret sharing schemes, the secret information is encoded by the construction of several fragments, called shares which are distributed secretly to different, not necessarily trustworthy parties. In order to reconstruct the secret image, a qualified subset of these parties has to combine their shares. For example, in (K, n)—threshold schemes there are n different parties and each subset of at least k parties is qualified. Visual cryptography schemes are perfect, i.e., forbidden (i.e., not qualified) subsets of parties learn no information at all about the encrypted image (in the information-theoretic sense). In contrast to conventional encryption (resp. secret sharing) schemes, visual cryptography schemes allow the decryption to be done directly by the human visual system, i.e., without performing any sophisticated cryptographic computations.