A method for hiding at least one pattern within a visible representation of the other different pattern is presented. The method is carried out by an encoding device and consists of selecting a predetermined key function, and utilizing this function to cerate a binary halftone representation of the visible pattern, in which said at least one pattern is hidden. This halftone representation is such that each cell therein has halftone dots' position distribution corresponding to the gray-level distribution of the at least one hidden pattern. By processing the binary halftone representation by a two-dimensional spatial correlator based on the key function, data representative of the at least one hidden pattern can be extracted.