1. A method of encoding an unprotected document to ensure that unauthorized changes to the unprotected document can be detected, the method comprising the steps of: defining an unmodulated array of protection marks associated with the unprotected document, and, for each protection mark in the array, modulating an attribute of the protection mark according to a property of information printed on the unprotected document in an area associated with the protection mark to thereby form a modulated protection mark, wherein: the area associated with the protection mark partially overlaps an area associated with another protection mark in the array, so that unauthorized changes to the document in the partially overlapping area associated with the protection mark and the other protection mark can be detected in relation to both the protection mark and the other protection mark; and incorporating the modulated protection marks into the unprotected document to thereby form a protected document.