Study On The Extension Of Block-based Visual Cryptography

Visual cryptography is a secret image sharing method.A(k,n)-threshold visual cryptography scheme encodes the secret image into n shares(encoded images),at least k shares can reconstruct the secret image.Block-based visual cryptography divides the secret image into non-overlapping blocks,different shares can reconstruct different blocks.Block-based visual cryptography can keep different secrets in the same secret image,different participants can reconstruct different secrets.This thesis is about the extension and optimization of block-based visual cryptography,to embed extra infor-mation through block-based visual cryptography and solve the pixel expansion problem.First,a block-based visual cryptography scheme with polynomial-based secret sharing is proposed.It is found that the generated pixels is regular after analyzing the encoding of block-based visual cryptography,which makes it possible to keep extra information in the shares.There is a mapping between different patterns and corre-sponding numerical values after numbering pixel patterns,so that the pattern sequence can be used to represent the corresponding numerical sequence.In order to embed ex-tra secret image,it is converted to a numerical sequence at first.Then encoding it by polynomial-based secret sharing to generate an encoded sequence.At the end,the en-coded sequence is mapped to the corresponding pattern sequence and saved in different shares.The experimental results show that no information about the secret can be ob-tained from single share.The proposed scheme offers two decode methods:decode the carrier image by superimposing the shares,or decode the embedded images by extract-ing the corresponding blocks in shares and using an interpolation method.Second,we propose a two-dimensional lattice-based halftone processing technol-ogy.It is used to construct a size invariant block-based visual cryptography scheme by combining with two-in-one scheme.Based on a two-dimensional lattice,the secret im-age is divided into pixel blocks of the same size and non-overlapping.Those pixel blocks is converted into all white or all black pixel blocks by block-based halftone technology.After that,all pixel blocks are encoded to generate shares by block-based visual cryp-tography.The experimental results show that shares generated by the proposed scheme have the same size as the secret image,which solves the pixel expansion problem.