[TacticalCoder]

> 2) Yes. Anyone with the private key could sign a message. And for clarity, such things are not date-stamped in a secure way, so there's no way to know when it was signed.

Certainly if the message included a hash of the last Bitcoin block that'd be a very strong date-stamp.

[retrac]

I would consider that a strong date-stamp too, but it doesn't do what we would need here. It can only set an earliest possible date for the message. It does not bound it on the other end.

It's like using today's newspaper to prove when a photo was taken. If the front page of Dec 13 2022's New York Times is in the photo (and the photo is authentic) then the photograph must have been taken on or after Dec 13 2022. But I could use that same newspaper in 2025 to prove the same thing. The photo was taken on, or after, Dec 13 2022.

Bounding the other end -- to prove that this key was not used after a certain point in time -- is really quite a puzzle with no particularly elegant solution, as far as I know. Trusted central authority that date-stamps and signs messages when it receives them. Or a blockchain, ironically.

[ashpool37]

Is there a way to provably destroy a piece of information? The closest thing I heard of was the Z-Cash "ceremony" [1]. If there was, we could create messages that are provably and absolutely timestamped.

Another possible way would be crafting the message in a way that requires a low-entropy system, like a living human person, a sophisticated computer, or an immense power source. But, this would only be useful on extreme time scales, or within limited space.

[1]: https://z.cash/technology/paramgen/

[ashpool37]

Such a timestamp could only be interpreted as a "no-earlier-than" proof. You couldn't use it to prove that the message was created before a certain date, as all existing bitcoin blocks can easily be referenced in any future messages.