1. A document authentication method implemented in a data processing system, comprising: (a) obtaining an original document image representing an original document; (b) segmenting the original document image into a plurality of original image patches; (c) performing Krawtchouk decomposition for each original image patch to generate a set of Krawtchouk coefficients for each original image patch; (d) classifying the plurality of original image patches into a plurality of original image patch classes using the Krawtchouk coefficients generated in step (c), each original image patch class being defined by a set of Krawtchouk coefficients, each original image patch being classified into one and only one original image patch class, to generate list of locations of original image patches belonging to each original image patch class; wherein the Krawtchouk coefficients for each original image patch class and the list of locations of original image patches belonging to each original image patch class constitute original image patch classification data; and (e) storing the original image patch classification data.