@KawaSeadrake Like, there are ways to do it, but Mastodon wasn't designed that way. I *think* you could have the poster's instance sign the post/files with the remote user's public key, but the user's private key would have to be stored in a client application or something. No good if the remote instance has access to the key, because a leak is likely to contain both (to say nothing of a malicious admin).
@KawaSeadrake (And obviously we have this problem with twitter, but they have a lot of cash, folks, and infrastructure dedicated to keeping stuff pretty secure.)