However, the sender has signed with their private key in kind13. Therefore, the recipient can step by step prove to the public that the message was written by the sender. "Not your sig, not your event. This is how Nostr works." I don't understand what you mean by this. https://image.nostr.build/ef4f50793e286a501390fcdb7f8c7f9ffa4d6fbb34ac7c4d3f25de0b7a370248.jpg