A method implemented in a data processing apparatus for authenticating a target document image by comparing the target document image with an original document image, comprising: comparing the target document image with the original document image at a first level, including: (a) segmenting the target document image into a plurality of first-level units; (b) segmenting the original document image into a plurality of first-level units, wherein the first-level units in the target and original document images are paragraph units each containing a paragraph of text, line units each containing a line of text, or word units each containing a word of text; (c) comparing the plurality of first-level units in the target document image with the plurality of first-level units in the original document image to detect alterations in the target document image, including: comparing a count of the first-level units in the target document image with a count of the first-level units in the original document image to detect alterations in the target document image; and if no alteration is detected in step (c), comparing the target document image with the original document image at a second level, including: (d) segmenting the target document image into a plurality of second-level units; (e) segmenting the original document image into a plurality of second-level units, wherein the second-level units in the target and original document images are line units each containing a line of text, or word units each containing a word of text, or character units each containing a character of text, and wherein the second-level units are smaller than the first-level units in the target and original document images; and (f) comparing the plurality of second-level units in the target document image with the plurality of second-level units in the original document image to detect alterations in the target document image, including: comparing a count of the second-level units in the target document image with a count of the second-level units in the original document image to detect alterations in the target document image. (a) segmenting the target document image into a plurality of first-level units; (b) segmenting the original document image into a plurality of first-level units, wherein the first-level units in the target and original document images are paragraph units each containing a paragraph of text, line units each containing a line of text, or word units each containing a word of text; (c) comparing the plurality of first-level units in the target document image with the plurality of first-level units in the original document image to detect alterations in the target document image, including: comparing a count of the first-level units in the target document image with a count of the first-level units in the original document image to detect alterations in the target document image; and comparing a count of the first-level units in the target document image with a count of the first-level units in the original document image to detect alterations in the target document image; and (d) segmenting the target document image into a plurality of second-level units; (e) segmenting the original document image into a plurality of second-level units, wherein the second-level units in the target and original document images are line units each containing a line of text, or word units each containing a word of text, or character units each containing a character of text, and wherein the second-level units are smaller than the first-level units in the target and original document images; and (f) comparing the plurality of second-level units in the target document image with the plurality of second-level units in the original document image to detect alterations in the target document image, including: comparing a count of the second-level units in the target document image with a count of the second-level units in the original document image to detect alterations in the target document image. comparing a count of the second-level units in the target document image with a count of the second-level units in the original document image to detect alterations in the target document image.