Visual Cryptography

Definition: Visual cryptography or visual secret sharing represents a group of effective schemes for image and video hading and watermarking.

Visual cryptography (VC) or visual secret sharing (VSS) schemes constitute probably the most cost-effective solution within a (k,n) -threshold framework. The VSS schemes use the frosted/transparent representation of the shares and the properties of the human visual system to force the recognition of a secret message from overlapping shares without additional computations or any knowledge of cryptographic keys.

As it is shown in Figure 1, the conventional VSS schemes operate on a binary input. Following the encryption procedure in a (k,n) -threshold framework, the secret binary pixel is encrypted into n blocks of m 1 × m 2 binary pixels. The actual share blocks are randomly generated through the column permutation of the n × m 1 m 2 basis matrices. By repeating the process for all pixels of a K 1 × K 2 secret (input) image, the VSS scheme produce n binary shares with dimensions of m 1 K 1 × m 2 K 2 pixels. Each noise-like binary share is distributed to one of n participants. The secret image is visually revealed (Figure 2) only if any k (or more) recipients stack their shares printed as transparencies together on an overhead projector or specialized display.