We invade the privacy of people from a few hundred years ago all the time and it's considered fine. Do you think there will be a breakthrough in encryprion breaking soon enough for it to matter?
Browsers have to frequently deprecate cryptosystems that have become insecure. That's not possible with data frozen inside the blockchain.
Also, we're at a point where quantom computers are just starting to become practically usable. So yes, I think the point of a "cryptographic breakthrough" that will crack some configurations is quite likely.
And if you're not encrypting data with AES (or one of a handful of other algorithms), then you should be worried _now_.
> If AES is broken in your lifetime, you're going to have _way_ bigger problems than somebody decrypting your blockchain ciphertext.
I'm not so sure about that. Not a lot of encrypted data is simply lying around at rest, available for everyone to run attacks against. Most encrypted data is either ephemeral (encrypted data connections) or secured by additional measures (e.g. to even get the raw bytes of an encrypted partition, you need access to the machine, appropriate permissions, etc) That gives the data owners various opportunities to react and mitigate the risks: Stop processes that send sensitive data, unmount sensitive partitions, detete data, etc.
You can't do a lot to protect data on the blockchain - it's literally out there for everyone to access.
My point was that data owners have options to limit damage - e.g. immediately stopping any data transmission and not producing any future ephemeral data.
And just to nitpick about blockchains, ledgers, etc.: they don’t need to be world-readable. You can protect them the same as you would a regular database.
Then you'll need some central entity to manage access to the chain. If you already have a central entity, you can just use a regular database instead of a blockchain and save yourself all the energy waste.
I think the key aspect is that it is a database that no single person or organisation can delete or alter - not even the developers or operators of the database themselves. The only operation possible is append.
But this property requires that the majority of nodes participating in the chain are not under your control. When the nodes are under your control, you could just order them to swap out the current chain with one you just made up. (Which is effectively how git's "history rewriting" features work) This doesn't provide any more integrity than an ordinary database.
On the other hand, if you want an append-only database and you already have a central gatekeeper that you trust (as required for access enforcement), you also can use an ordinary database and have the gatekeeper enforce the append-only property. No blockchain required.
Elliptic curve signatures of the blocks are _significantly_ more fragile to quantum attacks than AES.