Cheating The (2,N) Visual Secret Sharing Scheme
Horng et al. proposed a method that allows colluding parties to cheat an honest party in visual cryptography. They take advantage of knowing the underlying distribution of the pixels in the shares to create new shares that combine with existing shares to form a new secret message of the cheaters choosing.
We know that 2 shares are enough to decode the secret image using the human visual system. But examining two shares also gives some information about the 3rd share. For instance colluding participants may examine their shares to determine when they both have black pixels and use that information to determine that another participant will also have a black pixel in that location. Knowing where black pixels exist in another party's share allows them to create a new share that will combine with the predicted share to form a new secret message.
In this way a set of colluding parties that have enough shares to access the secret code can cheat other honest parties.
Read more about this topic: Visual Cryptography
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—Ovid (Publius Ovidius Naso)
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—Henry David Thoreau (1817–1862)