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 a list of locations of original image patches belonging to each original image patch class, wherein the set of Krawtchouk coefficients that define each original image patch class are updated each time an original image patch is classified into that class, so that the Krawtchouk coefficients that define each class are statistical values calculated from Krawtchouk coefficients of all original image patches that have been classified into that 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.