Patent US 8,032,744

A method for proving the validity of a digital document digitally signed using a digital key that corresponds to a digital certificate in a chain of digital certificates issued by certification authorities within a hierarchy of certification authorities. At least one secure digital time stamp is applied to at least one record comprising the digital document, the digital signature, certificate chain data, and information relating to the revocation of certificates by certification authorities within the certificate chain. If, at some later time, one or more digital certificates either expire or are revoked, the timestamp serves as evidence of the integrity of the signed digital document.