Comment by Fry-kun

So many people confuse encryption with authentication... Suppose you're a whistleblower trying to contact a reporter using WhatsApp. You've never met in person, so you send a message over WhatsApp, "Hello!" The reporter replies, "Hi!" You now have a big problem -- you don't KNOW that it was the reporter who replied to you, it might've been a nefarious 3rd party who already intercepted your original message and is now replying. WhatsApp's only mechanism for checking if that's the case is comparing some numbers out-of-band (e.g. in person, with QR codes). What's even more tragic is that WhatsApp doesn't track/show which contacts you may have already authenticated.. good luck remembering which of your 100 contacts are verified