ABSTRACT
A visual cryptography scheme for a set of n participants is a method to encode a secret image, consisting of black and white pixels, into n shadow images called shares, one for each participant. Each share is a collection of black and white subpixels, which are printed in close proximity to each other, so
and Secret
that the human visual system averages their individual black/white contributions. The encoding is done in such a way that certain subsets of participants, called qualified sets, can ”visually” recover the secret image, but other subsets of participants, called forbidden sets, cannot gain any information (in an information-theoretic sense) about the secret image by inspecting their shares. A ”visual” recover for a set of qualified participants consists of xeroxing each share onto a separate transparency, of stacking together the transparencies and projecting the result with an overhead projector. If the transparencies are aligned carefully, then the participants will be able to see the secret image (without any knowledge of cryptography and without performing any cryptographic computation).
