[rocqua]

The nice solution here that would please security-conscious people with an opt-in would be for that opt-in to prevent automatic re-encryption and re-transmission under the new key.

To expand on the example given above, if the police get your phone, turn it off and wait for a while. You might have quite a few incoming unreceived messages. They can then simply take the sim, put it in a new phone, and register that with whatsapp. They can then read all messages sent to you since they turned of your phone.