At that point you can do key derivation to get an ed25519 private key from the password and a server-supplied salt (specific to that user), after which the clie

At that point you can do key derivation to get an ed25519 private key from the password and a server-supplied salt (specific to that user), after which the client signs a challenge the server supplied with that private key. When the password was set/changed, the corresponding public key was stored on the server.