[jeroenhd]

You don't need crypto for that, though, client side certificates have existed for such use cases for decades now. Companies aren't using them so nobody knows about them and therefore browser vendors don't care about the UX and therefore nobody wants to use them, but that cycle will happen for as long as people don't implement such login methods.

If there was a way for end users to sync webauthn logins, we'd solve this problem without ever resorting to any kind of blockchain whatsoever.