Patent ID: 7643181

Claim:
A method for determining if a printed image is unaltered by determining if said image includes coded information and a copy detection feature linked to said coded information, said method comprising the steps of: a) scanning said image to recover said coded information and said copy detection feature; b) testing said coded information and said copy detection feature; where said copy detection feature includes robust elements and is determined to be unaltered if copy detection feature is determined to have been linked to said coded information by the steps of: generating said copy detection feature as a pseudorandom function of said coded information; creating a robust descriptor of said copy detection feature; and incorporating said descriptor into said coded information, and said testing step comprises the substeps of: a) regenerating said robust descriptor from said recovered copy detection feature; b) recovering said robust descriptor from said coded information; c) comparing said recovered and said regenerated robust descriptors; and d) if said descriptors are at least substantially similar; then, e) regenerating said copy detection feature as a pseudorandom function of said coded information; f) computing a distance between said recovered copy detection feature and said regenerated copy detection feature; and g) indicating that said link exists if said distance is less than a predetermined threshold; and h) accepting said printed image as unaltered if said testing step indicates that said coded information and said copy detection feature are linked.