[ycmbntrthrwaway]

> 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.

[t0b]

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.