Research On The Extended Visual Secret Sharing Methods

Secret sharing is an important research area of Cryptography,the visual secret sharing(VCS)has been proposed as a new research direction in recent years.It is an encryption technique that utilizes human visual system in recovering secret image and it does not require any complex calculation.A(k,n)-VCS encoded a secret image into n random-looking shares that is different from an "innocent-looking" meaningful image.The secret can be visually recovered by stacking k or more shares.The VCS can be divided into two categories:the sharing image with random-looking and meaningful content.In existing schemes,most have pixel expansion,the distortion of recovered image,the low quality and so on.This paper researches the below question,and mainly obtain some results as follows:(1)Most existing schemes have given the construction of basis matrices and realized the meaningful sharing images.However,they did not give a general method of mathematical.In this paper,we analyze the property of the extended matrix and give a new method to construct the extended matrix.This approach is simpler.(2)Conventional VCS generated shares by expanding a secret pixel to m sub pixels.m is not a square integer,and resulting in the distortion of recovered image.Most existing VCSs generated share images are meaningless.In this paper,we present a construction which implements the meaningful shares with aspect ratio invariant based on the above property.And our scheme like the original extended visual cryptography scheme(EVCS)does that every secret pixel is exactly reconstructed.The pixel expansion and the contrast of recovered image approximately equals convention VCS.(3)The pixel expansion is an important problem in the EVCS.In this paper,we present a general construction method of probabilistic EVCS for binary secret image.In our scheme,the pixel expansion can be set to a user-defined value.The qualities of recovered image and share images,measured by average contrasts,are adjusted to be friendly for the dealer by different basis matrices.When s= m+1,the average contrast of recovered image is optimal with m·?/(m?1).The deterministic EVCS can be viewed as a special case in the schemes proposed here.(4)Many VCSs without pixel expansion have been proposed.But they recover secret image with low contrast due to the stacking operation.To improve the quality of recovered image,XOR-based VCS has been proposed.However,shares constructed from XOR-based VCS are random-looking.In this paper,we propose an(k,k)extended visual cryptography scheme(EVCS)by "XOR"operation to solve above mentioned problems.This is implemented by diving the share image into two parts using a parameter and operating the secret pixels and cover pixels respectively.Comparing with previous schemes,the qualities of share images and recovered image are improved,and our scheme also suits the "OR"operation.