Two questions.
My family insist on using Whatsapp for the family chats. I have to keep a copy on a device just so I can communicate with them. I do so under protest, as I was always told it isn't secure. My brother has just said
"oh Whatsapp is encrypted, it's perfectly secure".
First, is it actually as encrypted and safe as my brother claims? That would solve everything.
Second, if it isn't, where can I get some proof that we should switch to Telegram or whatever? Proof which doesn't make me look like a raving loony?
When you type a message a message and send it to your counter part, WhatsApp says it encrypts it and the recipient will decrypt it on their side with WhatsApp. However, WhatsApp is closed source. That means you trust WhatsApp to do what it says.
It's like going to a contractor and telling them your message and handing them a key. The contractor says they'll deliver it to the other party in a manner that nobody else will be able to read that message. You can ask them provide the tools they do it, explain how they do it, and show you how it's done, but they say "no can do, trade secret". Do you trust them?
Alright, let's say you do trust them, they really do make the message unreadable to anybody but the other party. But every time you want to send a message, you have to go to their building, write down the message on a notepad, and then hand it + the key to the messenger. If you told them "Just to be sure, I'd like to verify that nobody else is here possibly looking at the message while I write, nor reading it when you go into the backroom to render it unreadable" and asked "Can I check for other people here?" to which they respond "no can do, trade secret". Do you trust them?
Alright alright, so you still trust them. They won't let you check anything, but you still trust them. The messenger is employed by the one and Sauron Inc. The owner has been caught lying about stuff before, but you trust them. No problem.
Let's says the messenger says "hey, you know, all the communications you have when you go into the small room there, we can make copies for you! if the messages were ever misplaced, this building burned down or anything, you could always have the communication history". You find it a great idea! Wow, it's so convenient. They even suggest to put copies in a building in another city and the building is owned by Darth Vader Inc. You're ecstatic! To get the process started, WhatsApp walks into your room with a bunch of blank papers and chest, then asks you to hand over your key and closes the door behind them. You are escorted out of the building and wait for the process to be over.
A few months later, the city is bombarded by Megatron. The WhatsApp building is destroyed and your communications are gone! The key you had for the messenger to render your communications unreadable? Gone too! Well, luckily you can just go to another WhatsApp building. You enter, say your name, fill in your details and you are escorted to a room that looks just like the one in the building the Megatron destroyed!
The elation is great! ... until you notice that all your messages are readable. Not only that, but the key that's used to make then unreadable by WhatsApp is sitting there on the desk - pristine and undamaged as it ever was.
Wait a moment... how did the unreadable messages and the key get restored? What exactly did Darth Vader Inc. get from WhatsApp?
Must just be a coincidence, right? You probably had the key in your pocked the whole time and gave it to WhatsApp while you were at the reception filling in your contact details. Your trust is unwavering, the security unrattled, and your communication unscathed.
You are right, we don't and can't know if any of what Meta says is true, but at least on the surface it seems to check out. If they are stealing your private key and unlocking all your chats in secret, then they are doing a bloody good job, since no one has leaked anything yet.
Just to clear things a bit, in your analogy you don't hand the courier both the chest and the key. The chest has a special keypad that accepts two keys, one is your key, the other is the recipient's key. What you do is you lock the chest with your key and then give it to the courier, which will deliver the chest to the other party, which will then open the chest with his key. In theory the courier never had access to the key.
Now the issues are that you are indeed writing your message from within the Whatsapp building and you can never know if there cameras watching you or not. You also cannot know if Whatsapp has made a copy of your key, or the recipient's key without your knowledge.
As for how can you recover all your chat history even after you destroy your phone, it's quite easy and Whatsapp doesn't need to know anything in particular. The functionality allows you to make a backup and store it on Google Drive. That backup gets encrypted with your password and it's probably the most secure thing of all, if nothing else because Meta would gain nothing from the backup having poor security (as it would already have all the data if they wanted it) while it would only make them loose face, plus would allow anyone else to gain access to all ~~your ~~their data. After you restore the backup on a new device a new key+padlock pair gets created and the lock gets shared to all your contacts (which will see the yellow box telling them your padlock has changed).
I'm not claiming it doesn't have privacy issues mind you, I'm just saying that you can't be sure either way, unfortunately. Still, better than Telegram that doesn't even encrypt most of your chats.
Maybe that's a new feature? Does WhatsApp require a password when backing up now? Haven't used it in a few years, but back when I had it, the backup to Google didn't require anything besides your phone number and access the google drive on your account - it was only retrievable from WhatsApp and not visible on a Google Drive interface nor API.
They added the password some time ago. I would say maybe a couple years