Decoy (U)

Using the Hydra cipher sent through the secret Enigma code machine, Admiral Donitz talked freely with his U-boats as they ranged the Atlantic, seeking and sinking Allied convoys. At Bletchley Park, British cipher experts eavesdropped on Enigma and read almost every Hydra signal to and from the U-boats. Thanks to Bletchley, the Royal Navy had a secret eye in Donitz's headquarters. But suddenly the 'eye' was blinded: in February 1942, the U-boat command changed the Hydra cipher and altered the Enigma machine. The 'Great Blackout' had started: the Admiralty no longer knew where the U-boats were or what they were doing.