Ultra

One of the world’s best-kept secret is hiding the fact that the Allies broke the Enigma cipher, later, the Japanese “Purple” cipher, the German Lorenz cipher, and the Italian C-38 cipher. Allies had blown every code the Axis used out of the water, thanks to the work of the Polish Cipher Bureau, and the Bletchley Park mathematicians including Alan Turing, and the American Signal Intelligence Service. The collective intelligence from all these broken codes was called Ultra. But they couldn't let the axis know that their codes were broken. What do you do when your code gets broken? You make a new, harder one. The allies couldn't let that happen.

So they basically lied to their own men. If they wanted to take out an Axis supply ship after finding it through Ultra, they had a spy plane fly over where they knew the ship would be, then they sunk it. They also had to hide the broken codes from their own soldiers, lest they revealed it under careless talk. One of the few times they were forced to sink ships immediately, they covered it by sending a message in a code they knew the Germans had broken, to a spy in Naples, congratulating him of his success. The spy didn't exist, but the Germans intercepted the message and assumed everything was still good with Enigma.

They didn’t even reveal Ultra after the war. They saw to it that the Enigma machines were sold to potential enemies in the Third World, who continued to use the broken codes for years. Ultra wasn't revealed in its full extent until 1974, 29 years after the war. Never has a secret of such massive importance been so well kept for so long.