1. A method comprising: acquiring digital image data of an image of at least a selected authentication region of a document, wherein the document has been forged or altered at least in the authentication region from its original state; extracting at least one feature from the acquired image data from the authentication region; combining the extracted features to form a digital fingerprint of the forged document; designating the digital fingerprint as tending to evidence a forgery or alteration; storing the designated digital fingerprint in a database record for use in detecting forged or altered documents; and repeating the foregoing steps over additional documents that have been forged or altered so as to build a forgery database comprising multiple digital fingerprint records for use in detecting forged or altered documents.