An Improved Halftone Visual Secret Sharing Scheme for Gray-Level Images Based on Error Diffusion in Forward and Backward Direction

Visual cryptography is data security technique which allows visual information to be encrypted in such away that their decryption can be performed by human visual system. Visual secret sharing scheme encode a secret image into some shares which separately reveal no knowledge about the secret information. The share are obtained using basic visual (2, 2) scheme. The visible image is first converted into halftones shares using halftone error diffusion method. Then secret image is encoded into halftone shares carrying significant visual information. Shares are distributed to participants and then they are super imposed to reveal the secret. In this paper work, the visual quality of recovered secret images obtained after different error diffusion techniques is analysed with respect to PSNR, NCC, MSE and UQI. On analysis it is found that results of recovered secret image are better when errors are distributed uniformly to each of neighboring pixels in both forward and backward direction in the image.