Multi-pixel Encryption Visual Cryptography

A visual cryptography scheme (VCS) is a secret sharing method, for which the secret can be decoded by human eyes without needing any cryptography knowledge nor any computation. In their pioneer work, Naor and Shamir mentioned that encrypting a block of pixels simultaneously may result in better result. Inspired by that idea, we first define multi-pixel encryption visual cryptography scheme (ME-VCS), which encrypts a block of t (1 ≤ t) pixels at a time. Then we give an upper bound of the overall contrast of ME-VCS. We also give a lower bound of the pixel expansion of (n,n,t)-ME-VCS. At last, we built a contrast-optimal ME-VCS from a contrast-optimal VCS and built an optimal (n,n,t)-ME-VCS from an optimal (n,n)-VCS.