As a follow up to Whatsapp encryption keys discussion and from WhatsApp security whitepaper, it seems that most of the components to a fully secure end-to-end encryption are already there but what seems to be lacking is a fully trusted open-source client that does not secretly transmit private keys to the server.
Not claiming WhatsApp client does it, but we would never know unless the client is open source.
I'm wondering if it's possible, with the current information that's available from WhatsApp or are there still any proprietary missing piece that is stopping us from building that?