This allows WhatsApp to MITM. Whatapps can rekey both Alice and Bob, decrypt both their messages from that point onwards (incl unsent messages) and forward them re-encrypted with their real keys. The only notification might be that rekeying warning, if the users have turned it on. In this scenario even the double-checkmarks are present.
This is contrary to WhatsApp's claim that even they cannot snoop.
PS: I just check on my phone if those notifications were turned on. There were not. And I'd never turn those off myself, which leads me to conclude that the rekeying notifications are off by default (in their android app)
PS: I just check on my phone if those notifications were turned on. There were not. And I'd never turn those off myself, which leads me to conclude that the rekeying notifications are off by default (in their android app)