How to time-stamp a digital document By Stuart HaberW. Scott Storn
Journal of Cryptology
January 1991, Volume 3, Issue 2, pp 99–111
https://page-one.live.cf.public.springer.com/pdf/preview/10.1007/BF00196791
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.