New Visual Secret Sharing Schemes With Non-Expansible Shadow Size Using Non-binary Sub Pixel

Abstract

Visual secret sharing (VSS) scheme is a perfect secure method that protects a secret image by breaking it into shadow images (called shadows). Unlike other threshold schemes, VSS scheme can be easily decoded by the human visual system without the knowledge of cryptography and cryptographic computations. However, the size of shadow images (i.e., the number of columns of the black and white matrices in VSS scheme [1]), will be larger than the original image. Most recent papers about VSS schemes are dedicated to get a higher contrast or a smaller shadow size. In this paper, the gray (non-binary) sub pixel in the proposed method is completely different from the black and white (binary) sub pixel in the conventional VSS scheme. The new definition of gray sub pixel lets the proposed VSS scheme have non-expansible shadow size. The term “non-expansible” means that the sizes of the shared secret and shadows are same.