The Security Defect Of A Multi-Pixel Encoding Method

A visual cryptography scheme (VCS) encodes a secret image into several share images, such that stacking sufficient number of shares will reveal the secret while insufficient number of shares provide no information about the secret. The beauty of VCS is that the secret can be decoded by human eyes without needing any cryptography knowledge nor any computation. Variance is first introduced by Hou et al. in 2005 to evaluate the visual quality of size invariant VCS. Liu et al. in 2012 thoroughly verified this idea and significantly improved the visual quality of previous size invariant VCSs. In this paper, we first point out the security defect of Hou et al.'s multi-pixel encoding method (MPEM) that if the secret image has simple contours, each single share will reveal the content of that secret image. Then we use variance to explain the above security defect.