Improving recovered image quality in secret image sharing by simple modular arithmetic

Shamir's polynomial based secret image sharing has been widely adopted to protect an image by splitting it into multiple shadows with GF(251) calculation. However, mod 251 operation requires pixel values in the secret image to be modified into the range of [0,250], and leads to image distortion. In this paper, a method for improving recovered image quality in secret image sharing is proposed. To obtain computational efficiency, simple modular arithmetic on a prime number is still adopted. Pixels in the secret image are divided into non-overlapping blocks, and each block contains several N-bit subblocks. Modification strategies for different values of N are given to ensure that each subblock is fit for sharing, as well as theoretical analysis for choosing optimal N. Experimental results by the proposed method are demonstrated, illustrating improved recovered image quality and almost distortion free reconstructed image are achieved.