It sounds like what you're looking for is what's come to be known as The GLOMAR response.
The 'official' original GLOMAR response read:
"We can neither confirm nor deny the existence of the information requested but, hypothetically, if such data were to exist, the subject matter would be classified, and could not be disclosed."
It was designed as, essentially, a means for the CIA to cope with Freedom of Information requests in which merely revealing the existence of documents which comply with a request, whether or not the documents themselves were disclosed, would itself be material and damning information. It is the utter and complete lack of an answer. "I will not tell you whether I know the answer to your question, but if I did know it, I wouldn't tell you."
Since it's original use, the 'GLOMAR response' has taken on something of a life of it's own, as documented by Radiolab recently. It has become rather standard government boilerplate for all sorts of non-responses, to say nothing of it's use by corporations, celebrities, and even just in personal conversations.