“How to Time-Stamp a Digital Document”, 1991 (; backlinks; similar):
The prospect of a world in which all text, audio, picture, and video documents are in digital form on easily modifiable media raises the issue of how to certify when a document was created or last changed. The problem is to time-stamp the data, not the medium.
We propose computationally practical procedures for digital time-stamping of such documents so that it is infeasible for a user either to back-date or to forward-date his document, even with the collusion of a time-stamping service.
Our procedures maintain complete privacy of the documents themselves, and require no record-keeping by the time-stamping service.
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