Re: That broke rule 1 of creating secure services
You seem to think rule 1 is "let cops with a warrant snoop". If you set up a system where you cannot snoop because the communication is from one phone to another, you can tell the cops with the warrant "sorry, we can't help you".
Other than them trying to do like the FBI tried to do to Apple and make you push an update that changes that behavior, it is safe for criminals.
The reason most things aren't safe is that making a 100% secure communications channel is REALLY hard. Even with the unlimited resources of Apple, Google, and Microsoft they don't always get security right, so what chance does some guy selling a fly by night app have?