Secret Image Sharing Using Quadratic Residues

Secret image sharing is a technique for protecting image by splitting secret image into many shadow images that are transmitted and stored separately. In the scheme proposed in this paper, each pixel of secret image is encrypted using quadratic residues, and the cipher text is split into two bytes which are shared into two shadow images. According to the restriction of grey level images, we design two methods, namely lossy method and lossless method. The lossy method maintains the size of shadow images and the experimental results show that this method achieves high PSNR values. The lossless method enlarges shadow images to hold the overflow information. Furthermore, with quadratic residues, it is difficult to find square roots modulo a composite number, so the proposed scheme achieves image transmission secrecy.