Visual cryptography for grey level images

Visual cryptography is a cryptographic paradigm introduced by Naor and Sham ir [Lecture Notes in Comput. Sci., Vol. 950, Springer, Berlin, 1995, p. 1]. Some predefined set of participants can decode a secret message (a black a nd white image) without any knowledge of cryptography and without performin g any cryptographic computation: Their visual system will decode the messag e. In this paper we define and analyze visual cryptography schemes for grey le vel images whose pixels have g grey levels ranging from 0 (representing a w hite pixel) to g - 1 (representing a black pixel). Moreover, we give a nece ssary and sufficient condition for such schemes to exist. (C) 2000 Elsevier Science B.V. All rights reserved.