What are Visual Secret Sharing Schemes?

Naor and Shamir [NS94] developed what they called visual secret sharing schemes, which are an interesting visual variant of the ordinary secret sharing schemes.

Roughly speaking, the problem can be formulated as follows: There is a secret picture to be shared among n participants. The picture is divided into n transparencies (shares) such that if any m transparencies are placed together, the picture becomes visible, but if fewer than m transparencies are placed together, nothing can be seen. Such a scheme is constructed by viewing the secret picture as a set of black and white pixels and handling each pixel separately. See [NS94] for more details. The schemes are perfectly secure and easily implemented without any cryptographic computation. A further improvement allows each transparency (share) to be an innocent picture (e.g. a picture of a landscape or a picture of a building), thus concealing the fact that secret sharing is taking place.

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