When was a message signed.

[mikeywith]

So, how can people who see the message after that 100th block verify it was signed before it?

Oh, past that 100th block in your example this approach is no longer viable, if it's just a random message someone checking there would be no way to know the upper bound of the message, we would need a completely different approach to prove that after block 101 a message was indeed signed in block 50.

There should be some creative ways that won't be straightforward, maybe something like the above suggestion by vjudeu which I would need to evaluate in the time (3 am here)

[1714611958]

1714611958

[vjudeu]

There should be some creative ways that won't be straightforward

The main idea behind it is simple: you post a transaction on-chain, and then, it is possible to refer to that later. Which means, if there is some transaction from 2012, that is confirmed on-chain, which contains R-value, matching your commitment, then it is impossible, to change it now (in 2024), without re-mining all of those blocks, and changing R-value of that signature today.

Note that you don't have to make another payment, just to confirm some message, if you don't want to. It can be outsourced, one transaction is sufficient to confirm all pending commitments, regardless of their size, which means if it would be enforced on the protocol level, then mining pools can just be responsible for that, and provide that kind of service (and it would cost them zero additional on-chain bytes, because commitments could be stored by users, and they could provide just some 256-bit hashes, to be committed).

[NotATether]

There is no parameter in the bitcoin signed message that will tell you exactly when it was signed. ECDSA does not have such a capability, and that's really all a signed message has. Like others said, the best you can do is add a date yourself, or a blockhash, or something else that could only be known at the time it was signed, to prevent backdating.

[mikeywith]

There should be some creative ways that won't be straightforward

The main idea behind it is simple: you post a transaction on-chain, and then, it is possible to refer to that later.

I understand your idea, it's basically a commitment scheme that consists of two phases, commit and reveal, it's like writing a letter and putting it in a locked box, only to come in the future and reveal the key to that box, given that box has been there since 2012 for everyone to see, it's only becomes a matter of giving someone the keys so they can verify what is/was inside the message which you couldn't have changed since the box was put in place you can't access (in your example the transaction on the blockchain)