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?