I'll play along with the hypotheticals and assume that the NSA don't lie when questioned by Congress. In that scenario, the NSA can give reasonably straight ans

I'll play along with the hypotheticals and assume that the NSA don't lie when questioned by Congress. In that scenario, the NSA can give reasonably straight answers that don't compromise national security. "Are you spying on suspected terrorists?" - "Of course we are. That's our job." "Who are you spying on, and how?" - "We can't reveal that. It's National Security." "Are you spying on people that you have no reasonable suspicion of being terrorists?" - "No." "Fine". The problem comes when you have lie about that last one when asked by Congress.