Non-expansible Flip-flop Visual Cryptography with Perfect Security

More and more researchers focus on the visual cryptography scheme that embeds more than one secret image. Most of them generate size-expanded transparencies for decoding only by human eyes. In this paper, a non-expansible flip-flop visual cryptography (FFVC) scheme is proposed. Firstly, stacking two generated transparencies, one of the secret images can be decoded. Another secret image can be decoded by flipping one of the transparencies and then stacking the transparencies again. Each generated transparency leaks out the secret images about neither pixel values nor the relation of some pixels to obtain perfect security.