Method and apparatus for protecting visual information with printed cryptographic watermarks

Abstract

A method and device for protecting visual information against unauthorized access and modification using a printed cryptographic watermark includes printing a first array of shapes on a first sheet of material to be protected and printing a second array of shapes on a second sheet of material, which is transparent so as to form a developer for developing a watermark encoded in a combination of the first and second arrays of shapes. The watermark is encoded by preparing each array using black and white pixels. Each pixel, which may be a square, rectangle, circle, hexagon or other shape, is split into first and second collections of subpixels, the first collection of subpixels appearing in the first array of shapes and the second collection of subpixels appearing in the second array of shapes. When the transparent second sheet of material is positioned directly on top of the first sheet of material with the second array of shapes aligned with the first array of shapes, the first sheet of material may be viewed through the transparent second sheet of material. In this manner, the encoded watermark, which was not visible in either one the two individually, becomes visible. Additionally, a first image may be encoded in the first sheet, a second image may be encoded in the second sheet, and the watermark may be viewed as a third image that is visible in the combination of the first and second sheets.

Dependencies are inferred from the claim text.