Like... Facebook can read all your messages? The encryption only protects you from entities that are not Facebook. With that out of the door, it's not really worth it to even consider other angles of attack, since you now depend on the goodwill of a billion dollar company whose sole purpose and reason for existence is to extract and monetize your data. WhatsApp is a nice chat tool, but it's nothing you should ever use for secure communication. Sending a letter via mail is more safe.