US Patent 7028188 Document authentication using the physical characteristics of underlying physical media

A method for authenticating a document in which a document key for the document is generated by examining one or more attributes of a physical media that underlies the document. An original image is then imparted onto the physical media so that the original image is associated with the document key in a way that enables a subsequent recovery of the document key from the original image. This tying together of the underlying physical media, through the document key, with an original image enables detection of a forgery which was performed either through an alteration of the original image, or ink stripping and re-printing, or a printing of the original image on another physical media.