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