MITM SSH to get the public key and factor it? My understanding from other discussions in this thread is that the publickey authentication protocol is encrypted

MITM SSH to get the public key and factor it? My understanding from other discussions in this thread is that the publickey authentication protocol is encrypted with the server's host key. Are you relying on the user ignoring key mismatch errors, or targeting them on the first connection and relying on them not checking the key fingerprint? Or something else entirely? I'd like to learn if there's something I haven't thought about here.