A novel visual secret sharing scheme for multiple secrets via error diffusion in halftone visual cryptography

Visual Cryptography is an encryption technique where a secret image is cryptographically encoded into n meaningless share images. In visual secret sharing scheme (k, n) the secret images can be visually revealed by stacking together any k or more transparencies of the shares and by inspecting less than k shares, one cannot retrieve the secret image. In Visual Secret Sharing based on halftoning, the continuous-tone image is first transformed into a halftone image, and then encrypted. The visual secret sharing scheme for multiple secrets is intended to encrypt more than one secret image into the same quantity of share images to increase the encryption capacity compared with the original VSS scheme. Two binary secret images are encrypted as in without pixel expansion and the shares are embedded into the halftoned image to avoid the suspicion of the intruders. These halftone shares are concurrently error diffused to give visually pleasing effect. During decryption process, the halftone shares are directly stacked to reveal the first secret and the second secret could be revealed by stacking one share image and the other with a rotation angle of 180. The simulation result shows that when stacking the halftone shares, the first secret is revealed and when the second share is rotated to 180° and stacked to the first the second secret is retrieved which is of better quality.