> If NSA and only NSA can crack a particular system, they probably wouldn't mind using it for their own secrets.
How do you think they could assess that they, and only they will ever be able to exploit a particular cryptographic vulnerability at any time over the next few decades?
They can’t, they would be well aware of that, and they are extremely risk averse.
> And anyway why is there any reason to believe they really do use the system they say they use?
Because these systems exist widely throughout government today.