Sure, this could certainly leak some information, but it's hard to argue that this is a "backdoor".

A way of exploiting it is talked about in the article which would effectively allow Facebook/Whatsapp, or whoever is coercing them, to read all messages sent by a particular device.

It makes WhatsApp effectively not E2E encrypted. All messages can be recovered by Facebook. How is that NOT a backdoor?

No, all messages cannot be recovered by Facebook. Read the article - messages that are not yet delivered can potentially be read; if it has been delivered it cannot be retrieved.

You go read the article. The deciding factor is not whether the message has been delivered, but whether WhatsApp servers report to the device that the message has been delivered. There's nothing stopping them from claiming that no messages have been delivered and thus recovering all messages (as long as they had been preselected for false delivery reports) despite true delivery status.

> but whether WhatsApp servers report to the device that the message has been delivered

It is hard to check what WhatsApp does, but in Signal it is not the server, but a recipient who sends delivery receipt. WhatsApp then has to either recognize encrypted receipts or allow only one-way conversation during attack. Carrying out the whole attack just to decrypt "hi, are you here?" is not really interesting.

The delivery receipt is the message that is directly sent after the message has been delivered. Not too hard to distinguish those from other text messages.

So they can recover the messages, right? However, wouldn't these messages still be encrypted? Sure, they force a key change, and the messages are encrypted using the new key and sent. Theoretically, an attacker could have multiple copies of the same message, but these messages would still be encrypted under a variety of different keys right? Wouldn't the content of the messages still be secure?

Unless the key-change forces the user to be using an insecure key-pair, but is that actually happening?

New encryption (public) key is selected by the attacker, so he knows the decryption (private) key. Basically attacker just puts real device offline and registers his own device.

Wouldn't the attacker need to be authenticated as the user of the real device for this to work?

All messages, sent while a person is offline. It is bad, but not nearly as bad as "all messages"

It is in fact all messages. They can simply not deliver the first message and force a resend record that mesaage. Afterwards force again a resend with the old encryption key and deliver that mesaage. No one would get a notification.

I can see how you would leave the receiver in the dark by sending them the original, deferred message, but how would asking the sender's device to resend with a different key not result in a notification?

Furthermore, as soon as the sender attempts to deliver another message to the recipient, they would get another notification (because the encryption key changed back to the real key); alternatively the attacker could continue blocking (and reading) messages to the recipient, but the lack of delivery would be noticeable.

You could escalate it into a MITM rather easily, though, by attacking both ends; but again, a key change notification should be displayed to both parties.

Assuming the closed sourced app works as advertised, obviously.

Yes, you are right. But I think most people did not enable the security option so they wouldn't detect any interception of messages.

Well, what you are describing is a regular MITM attack. Unless you validate fingerprints, this is a risk with _all_ public key-based protocols.

Can't they (WhatsApp) simulate a user being offline?