A system for time-stamping a digital document, including for example text, video, audio, or pictorial data, protects the secrecy of the document text and provides a tamper-proof time seal establishing an author's claim to the temporal existence of the document. Initially, the author reduces the document...http://www.google.de/patents/US5136647?utm_source=gb-gplus-sharePatent US5136647 - Method for secure time-stamping of digital documents
