It's intended that server-to-server messages be signed with the private key that matches the public key baked into the user's profile data. I'm not sure whether

It's intended that server-to-server messages be signed with the private key that matches the public key baked into the user's profile data. I'm not sure whether the final specification requires this, or merely recommends it; it may have gotten pushed off to a future update in order to get the spec out on time.