A method and apparatus for authenticating currency and other important documents. A preferred embodiment may contain both a substrate, such as paper, and an embedded RFID transponder. An embedded RFID transponder or electronic watermark may contain multiple hierarchical layers of electronic passwords...http://www.google.se/patents/US7221258?utm_source=gb-gplus-sharePatent US7221258 - Hierarchical electronic watermarks and method of use
