> but you can likely at least retrieve that record and be pretty sure the data is accurate and wasn't tampered with.
You will have no way to tell if this data came from the legitimate Ethereum blockchain that was in use in 2021, a forked chain, or even a completely fake one which has zero blocks in common with the real one.
The authenticity guarantee in blockchains doesn't come from cryptographic schemes, it comes from the network agreeing on some shared truth. If the network is no more, you have no way to tell the truth.
If the network is totally gone, it's indeed more tricky. But if it still exists, and if there's no indication that the main network and chain was ever disrupted between then and now, you can be pretty confident the data is accurate if you join the network.
If no one uses it anymore or if it's so little-used that it's schismed into tons of other chains over the years, I think you can still probably obtain the original data you're looking for, but it'll take more effort to verify its authenticity.
Especially if the data you're looking for occurred at a time when the network was healthy and intact (like 2020), all you need to do in 2040 is find a block number and corresponding root hash that existed in 2020. I think these'll likely be possible to find even if the network's dead, and you can compare them against several different sources to increase confidence that they're not fake. Then as you scour the internet and download different published blockchain copies, you can truncate it back to that block number and compare the hash.
It's possible your search will be futile and you'll be unable to find a trustworthy record of block numbers and root hashes or that you'll be unable to find a verified 2020 blockchain, but I think your odds will be pretty good if the internet hasn't collapsed. Either way, as Vitalik pointed out, the odds are way higher you'll be able to find that compared to some data you entered into some SaaS in 2020.
> you can be pretty confident the data is accurate if you join the network.
And how do you know the network you're joining is actually the original ethereum network and a completely different blockchain?
Like every peer to peer network in existence, an Ethereum node needs to connect to a reliable first node (a “tracker” in the bittorrent protocol, idk what's the name in the ethereum world). If they are down, you're on your own to join the network, and you have little guarantee that the network you're joining is working on the original ethereum blockchain.
In fact, after the ethereum foundation is gone, what guarantee do you have that you are running an actual ethereum node and not something running a modified protocol?
>And how do you know the network you're joining is actually the original ethereum network and a completely different blockchain?
The "if you join the network" is contingent on the network still being active and widely used; presumably with the Ethereum Foundation or some successor also still being active. I may not've made that fully clear in the first paragraph.
If that were the case, you could ask the same question right now. And the answer is that they publish an official client you can download and should be able to trust. This would be the case in 2040 if the network is still active and the foundation still exists.
If the foundation is gone and the network is mostly dead, then it'd indeed be much harder or perhaps impossible. The second part of my answer covers that scenario:
>If no one uses it anymore or if it's so little-used that it's schismed into tons of other chains over the years, I think you can still probably obtain the original data you're looking for, but it'll take more effort to verify its authenticity.
>Especially if the data you're looking for occurred at a time when the network was healthy and intact (like 2020), all you need to do in 2040 is find a block number and corresponding root hash that existed in 2020. I think these'll likely be possible to find even if the network's dead, and you can compare them against several different sources to increase confidence that they're not fake. Then as you scour the internet and download different published blockchain copies, you can truncate it back to that block number and compare the hash.
>It's possible your search will be futile and you'll be unable to find a trustworthy record of block numbers and root hashes or that you'll be unable to find a verified 2020 blockchain, but I think your odds will be pretty good if the internet hasn't collapsed. Either way, as Vitalik pointed out, the odds are way higher you'll be able to find that compared to some data you entered into some SaaS in 2020.
You will have no way to tell if this data came from the legitimate Ethereum blockchain that was in use in 2021, a forked chain, or even a completely fake one which has zero blocks in common with the real one.
The authenticity guarantee in blockchains doesn't come from cryptographic schemes, it comes from the network agreeing on some shared truth. If the network is no more, you have no way to tell the truth.