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During World War II, this happened constantly. The British had broken the German ciphers, and the US had broken the Japanese codes and ciphers. (That division of labor wasn't planned, it was just how it worked out.) During the North Africa campaign, one of the critical factors controlling Rommel's ability to attack the British was his ability to get supplies across the Mediterranean from Italy by ship. The Italians were responsible for these supply convoys -- and the British were reading the codes which carried those orders. Obviously when such convoys were planned, the Royal Navy wanted to be out there to meet them, but didn't want to give away the game. So whenever a convoy was planned, the British would send a scout plane out to where the convoy would be. They were scouting constantly anyway, so this wasn't particularly a surprise. That plane would then radio the information back and the Italians would see the plane and intercept its message (and though they couldn't read it, it didn't take too much intelligence to know what it said), so when the RN showed up and attacked it wasn't unexpected. The crew of the plane didn't know wny they were sent where they were; they just thought they'd been lucky. And this kept the secret that the British already knew where the convoy would be. In early 1943 as the campaign in the Solomon islands began to go against Japan, Admiral Yamamoto decided to tour certain front-line positions. His itinerary was radioed to all the units in question so that they could be prepared for him, and the code used was one that the Americans had broken and could read. The message was translated and instantly sent to the office of Admiral Nimitz. The importance of Yamamoto to the Japanese cause can't be underestimated; his death would have the same kind of effect that Churchill's death would have had on the Allies. And now they knew where he'd be on a certain date in the future, which means they had the opportunity to kill him. Should they? This was hotly debated, because it included the possibility that the Japanese might figure out where the information had come from. But they decided to take the risk, and included a cover story that his plans had been discovered by the legendary "Coast Watchers" (whose reputation was enormous anyway, so this wasn't implausible). 16 P-38 Lightnings from Guadalcanal flew to Bouganville and intercepted the Betty bomber in which he flew and shot it down. The effect on the Japanese was devastating; they never found another commander for the rest of the war who was his equal. And they did not change their codes. Sometimes if you reveal intelligence, the mere information itself will let your opponent know where it came from. If that information was only in a few places, that's where they'll search. If the Republic of Pongoraku releases a memo they claim was written from the President of the US to the VP, US security people will then investigate everyone who has the ability to see such memos. This is obvious. If one of them is a Pongoraku spy, there's a good chance they'll figure it out -- and then Pongoraku will lose that asset and not be able to get anything else from him in future. (And likely he'll be killed or imprisoned.) If the message was sent using a particular cipher, that cipher will probably be changed. If Pongoraku was reading that cipher, they'll lose the ability to read it. So obviously Pongoraku is going to want to be careful with the kind of information they receive; if they reveal it at all, they'll couch it in different terms. They'll announce that "We have information that" and then talk vaguely about what they've learned, in such a way as to disguise the fact that they've been reading Presidential memos. They may reveal only part of what they know. They may, in fact, deliberately change some of it and announce things they know are wrong, so as to pretend that their source of information is less than reliable. It's all part of the game. As long as they don't get too specific, then US security people won't have as much of a clue where to look, and the Pongoraku spy will be much less likely to be found, or the specific cipher much less likely to be changed, and Pongoraku will keep that intelligence asset for the future. Which brings us the US accusations against bin Laden. The Taliban are demanding "proof" that bin Laden is involved. (So are some Americans.) But what the Taliban are really demanding is that the US reveal enough information so that the Taliban can figure out where their security leaks are and plug them, depriving the US of future inform 
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