Say Bob wants to tell Alice a secret message, then they start some message exchanges protocol. At the end, Alice will receive the secret message and can be convinced that the message was not produced earlier (and obviously not later) than the time they communicated.

An attacker (Bob included) will try to win by forging the secret message before they start the message exchanges.

As for a real world scenario, consider the secret message a picture, showing something of Alice's interest. Bob sends this picture to Alice, Alice however, doubt the picture was taken a long time ago, so she challenges Bob to send as well some random angles of the things showing in the picture as she requested until she is convinced that the things showing in the picture are there at the time they communicate. Note that the things in the picture can be there a long time ago (e.g. an old house) , but the secret message (the picture showing the old house) was only produced at that moment.

The security of the above scenario is only based on Bob not knowing the random choice of the angles Alice will pick and the infeasibility of Bob quickly creating a fake picture at the said angle. Can cryptography provide something better than these?

Somehow I believe the "infeasibility to create variants of the secret message" is a must assumption to guarantee the proofs. So may be in real world this cannot be a simple plaintext message, but sound, picture, video, 3D model, etc., which are difficult to edit?

  • $\begingroup$ I have no experience in the issue you described. But I think perhaps the article en.wikipedia.org/wiki/Timestamp could be of some interest to you. $\endgroup$ – Mok-Kong Shen May 2 '17 at 10:07
  • $\begingroup$ @Mok-KongShen Yes sure. A timestamp in a form that is unpredictable yet decipherable to tell the exact time. Problem is how can this timestamp be incorporated into the message so that it cannot be removed or changed. $\endgroup$ – user1589188 May 3 '17 at 0:43

If the the message is generate by a one-way function then incorporating in the message a sample from a randomness beacon (such as https://www.nist.gov/programs-projects/nist-randomness-beacon) will prove that the message was generated at a time later than when the sample was taken.

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  • $\begingroup$ Are you aware of the legal position regarding such a beacon? Could it be used to provide an incontestable alibi when I'm accused of robbing a bank for example? Or for proof of prior art in a patent? Or is it just a geeky novelty toy? $\endgroup$ – Paul Uszak May 2 '17 at 20:59
  • $\begingroup$ @stromboli Thank you for letting us know such devices. Proving the message was generated later than the beacon together with an online interactive protocol surly can prove the message generation time in that exact moment. Do you think there can also be a way to do so with an offline non-interactive protocol? Like the message was prepared first and send later but still the time it was generated can still be proved? $\endgroup$ – user1589188 May 3 '17 at 0:36
  • $\begingroup$ @PaulUszak It is definitely not a toy but a service that the NIST provides based on needs. I don't see how it can serve as alibi since you can obtain samples retroactively. I also don't think it is relevant for proof of prior art because the beacon lets you prove you did something after some time whereas in prior art you want to prove that it predates a given time. The main use of the beacon though, I believe, is when a multiple-party needs a neutral, shared source of randomness for all parties. $\endgroup$ – stromboli May 3 '17 at 3:27
  • $\begingroup$ @user1589188 You can prove the generation time in an offline protocol if you use a trusted third party to timestamp the message, for example a camera that has built-in timestamping and which has been audited to prove it wasn't tampered with. $\endgroup$ – stromboli May 3 '17 at 3:34
  • $\begingroup$ @stromboli Yes, that means the security is based on the tamper resistance property of the device. I am more hoping a cryptographic way. $\endgroup$ – user1589188 May 3 '17 at 3:40

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