I remember there being a mini scandal about this feature being limited to 3 uses or something, but can't seem to find the story right now.
https://support.google.com/photos/thread/405321957?hl=en&msg...
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.
Only in storage quota views do Photos and Drive (and Gmail, etc.) commingle.
Just remember that Google always kills its products, and they are an advertising company.
I'm joking, I hope, but Google ain't what it used to be.