An anti-tampering method for processing documents is disclosed. The method comprises, in regard to an encoding step, the steps of resolving (in a step 2303) in regard to an N-level image to be recorded, a pixel of the image into a major component having N possible values, selecting (in the step 2303) a pattern element depending upon the major component and the position of the pixel in the image, and recording the selected pattern element (in a step 2308) onto a transfer medium. In regard to a corresponding decoding step the method comprises extracting (in a step 2405) from the recorded document, a retrieved pattern element for said pixel, determining a pattern element (in a step 2407) depending upon a major component extracted from the retrieved pattern element and the position of the pixel on the recorded document, and comparing (in a step 2409) the retrieved pattern element and the said determined pattern element.