That is all good and clear, but its not open source and the government would not need have a back door, they can just request the received messages, or am I wrong here?
They can certainly get a warrant to seize and search a phone, yes. They can also get a warrant to seize the contents of the WhatsApp server, for what little good it will do them (assuming we believe they are in fact end-to-end encrypting the messages effectively).