Flip visual cryptography (FVC) with perfect security,conditionally-optimal contrast,and no expansion

This paper proposes a flip visual cryptography (FVC) scheme with perfect security, conditionally optimal contrast, and no expansion of size. The proposed FVC scheme encodes two secret images into two dual-purpose transparencies. Stacking the two transparencies can reveal one secret image. Flipping one of the two transparencies and then stacking with the other transparency can reveal the second secret image. The proposed scheme is proved to have conditionally optimal contrast: its contrast is optimal if the double-secrets non-expanded FVC scheme is required to have perfect security. The perfect security is also proved.     

Optimal (2, n) and (2, infinity) visual secret sharing by generalized random grids

Based on generalized random grids, this paper proposes two visual cryptography methods denoted as (2, n) GRG and (2, infinity) GRG. The (2, n) GRG is suitable for the pre-decided number of shares, and the (2, infinity) method is suitable for the adjustable number of shares. The proposed (2, n) GRG achieves better contrast on the stacking result, and the proposed (2, infinity) GRG enables extending the number of shares anytime. Based on the definition of contrast in Shyu’s work in 2007, we also demonstrate that the stacking result of (2, n) GRG is close to the theoretical bound of the contrast, and the stacking result of (2, infinity) GRG achieves the theoretical bound of the contrast.     

VCPSS: A two-in-one two-decoding-options image sharing method combining visual cryptography (VC) and polynomial-style sharing (PSS) approaches

This paper presents a novel method to combine two major branches of image sharing: VC and PSS. n transparencies are created for a given gray-valued secret image. If the decoding computer is temporarily not available at (or, not connected to) the decoding scene, we can still physically stack any t received transparencies (tn is a threshold value) to get a vague black-and-white view of the secret image immediately. On the other hand, when the decoding computer is finally available, then we can get a much finer gray-valued view of the secret image using the information hidden in the transparencies. In summary, each transparency is a two-in-one carrier of the information, and the decoding has two options.