There are changes all around while I was away. WhatsApp has end-to-end encryption which, I think, is the best feature introduced by them ever and always will be. You might be thinking why is it important at all? If yes, then let me tell you about it.
End-to-end encryption means messages are encrypted on your device and transmitted to the other user where they are decrypted for receiver to read. Every message sent through WhatsApp has lock on them and only the person’s phone you sent the message to has the special key to unlock it. And for added security every message sent through WhatsApp has a unique lock and key for unlocking.
WhatsApp uses AES256 in CBC block cipher mode for encrypting message, which is military grade encryption and cannot be broken (Not atleast during your lifetime). Messages are encrypted then transmitted and for reading those messages a key is needed for decryption. What this means is, your messages are secure and no one in the middle will be able to see them. No hackers, not your ISP and not even WhatsApp team itself.
All the messages (including group messages), calls and media you send to anyone on WhatsApp are encrypted and are safe from prying eyes.
How to enable end-to-end encryption?
It is enabled by default in latest versions of WhatsApp and there is no way to disable it.
How to see if my conversation is end-to-end encrypted or not?
Open "contact info" or "group info" and click on encryption if recipient does not have latest version of WhatsApp which has end-to-end encryption then WhatsApp will show you a message that conversation between you and the recipient is not encrypted. If recipient has latest version, you'll be able to see a screen with a 60 digit number and a QR code which can be used to confirm that your conversation is end-to-end encrypted.
You can scan QR code or compare 60 digit number from other users phone to make sure your conversation is end-to-end encrypted. (This is totally optional)
Full details on WhatsApp’s end-to-end encryption page