World War 2
British Intelligence
One of the greatest technical triumphs of the war was won, not on the battlefield, but in the English countryside at Bletchley Park, the home of the British Government Code and Cypher School.
It was here the British deciphered the top-secret German signals encoded on their 'top secret' Enigma machines. The Allies captured a German U-boat in the North Atlantic on the 9th of May 1941, which had onboard a German Enigma machine, along with its cipher keys and code books. British radio interception networks listened to the apparently meaningless groups of letters encoded by Enigma and transmitted in Morse code. They recorded them, and took them down to Bletchley, where the secret of Enigma was unlocked by matching electro-mechanical computers to the electric wiring of the Enigma machine. In this way British decoders discovered the Enigma keys, the settings that were changed three times a day. Eventually many Enigma signals were being read at the same speed by the British intelligence as the Germans.
Information from the deciphered Enigma signals was codenamed Ultra. It ranged from routine orders to detailed battle plans and invasion tactics. Ultra was surrounded by the greatest secrecy to prevent the Germans discovering that the code had been broken. The British shared Ultra with the Americans but only provided their Soviet allies with summary's of Ultra-derived information. But there was at least one Soviet spy at Bletchley. This was John Cairncross, who provided Soviet intelligence with a detailed picture of Ultra. Even to the end of the war, the Germans never realized that the supposedly unbreakable Enigma code had been cracked.