Meta-information, such as a digital watermark, that is associated with a document is encoded by using characters, or sequences of characters, that are "invisible" when rendered. Characters such as spaces, tabs, carriage returns, backspaces, line feeds, and so on are used to encode the watermark. The...http://www.google.de/patents/US6769061?utm_source=gb-gplus-sharePatent US6769061 - Invisible encoding of meta-information
