Block-based progressive visual secret sharing

Block-based progressive visual secret sharing (BPVSS) is an encryption technique that utilizes the human visual system to recover a secret image. The recovery method is based on building up different image blocks step by step. In the last few years, several research projects have been devoted to the study of BPVSS. However, they have some common drawbacks. Their dispatching matrices are chaotically designed by adding some redundant 0/1 s in order to maintain the size as a fixed value, which makes the methods hard to be implemented. In addition, the pixel expansion ratio grows faster when the number of participants increases and the visual quality of the restored image is quite poor. In this paper, we propose a non-expanded block-based progressive visual secret sharing scheme with noise-like and meaningful shares, respectively. Experimental results demonstrate the feasibility of this scheme. Our design has several advantages over other related methods: (1) the model is concise, easy to implement, and the number of participants need not be restricted; (2) with noise-like shares, the contrast of the restored image is 50%, which means that the hidden message can be clearly recognized by the naked eye; (3) when transparencies are transferred from noise-like patterns into a meaningful cover image, the contrast in the shares and the restored image is 25%, which is still superior to that obtained in other studies; (4) our scheme is more suitable for grayscale and color secret images; and (5) the size of the transparencies is the same as that of the secret image.