A document authentication method employs Krawtchouk decomposition to analyze and compare document images. When printing an original document, the original document image is segmented into image patches, which preferably correspond to individual symbols of the document. Krawtchouk decomposition is applied to each image patch. The image patches are classified into image patch classes using their Krawtchouk coefficients. The locations of all image patches belonging to each class are obtained and stored along with the Krawtchouk coefficients for each class. When authenticating a target document, the same segmentation, Krawtchouk decomposition and classification steps are applied to the target document image, and the locations of all image patches belonging to each class are obtained. The image patch classes and the locations of image patches belonging to each class for the original and target document image are compared to detect alterations present in the target document.