Systems and methods are disclosed which enable the establishment of file dates and the absence of tampering, even for documents held in secrecy and those stored in uncontrolled environments, but which does not require trusting a timestamping authority or document archival service. A trusted timestamping authority (TTSA) may be used, but even if the TTSA loses credibility or a challenger refuses to acknowledge the validity of a timestamp, a date for an electronic document may still be established. Systems and methods are disclosed which enable detection of file duplication in large collections of documents, which can improve searching for documents within the large collection.