"That's on a need to know basis."

If the "That" referred to is information that someone else thinks you could use, they figure out if your knowing it will do them more harm or more good.

"You have no need to know" is a phrase used by "certain people" when they know something you probably do need to know, but the chances of their actually telling you are somewhere between "slim" and "none".

See also: CIA, KGB, MI6, NSA, DIA, NRO, Intellegence, Spys, Espionage, Alphabet soup, and They threw me out! They called me MAD! Said I was a bleedin' security risk, the bastards...