It is possible to back up the WhatsApp folder if using Android, with the likes of total commander or any other file browser.

It can then be decrypted with a simple access to the _sim card_, i.e. a way to receive a message, meaning that anything backed up to Google drive should not considered safe from a dedicated state actor (even if the other party did it). It is quite easy to demonstrate with a new phone or an emulator.

I have doubts on how much the US gov needs to break the Whisper protocol when they have such easy to get side attacks.