Extended visual cryptography scheme with an artificial cocktail party effect

Visual cryptography schemes have been introduced in 1994 by Naor and Shamir [9]. These kind of schemes have been also well described by C. Blundo, A. De Santis and D.R. Stinson in [3]. In this case, a secret image I may be encoded into n shadow images called the shares, and to give exactly one such shadow image to each member of a group P of n persons. Certain qualified subsets of participants can visually recover I, but other, forbidden sets of participants have no information on I. A visual recovery for a set X consists of photocopying the shares given to the participants and then stacking them. Shortly afterwards the discovery of visual cryptography schemes Droste gave a generalization of such schemes, and Ateniese et al, formalized the idea of Naor and Shamir of an extension of the model which conceals the very existence of the secret image. Ateniese et al have called this formalization, Extended Visual Cryptography [5, 7,10]. In order to encode and hide a given set I₁, I₂, . . . , I_k of gray-level images, in this paper, we propose an Extended Visual Cryptography Scheme for which the decoding process simulates a cocktail party effect.