If you have broken the enemies crypto AND he doesn't know you have - then you can also send fake messages which they are going to believe. So the Brits could h

If you have broken the enemies crypto AND he doesn't know you have - then you can also send fake messages which they are going to believe. So the Brits could have sent a message ordering a few panzer armies from Normandy to the Pas de Calais. It doesn't seem that this was ever done - presumably because it gave the game away. Even more interestingly - the Brits kept secret that they had cracked enigma into the 1980s because after the war they distributed the 'uncrackable' German code machine to lots of rather gullible allies