Trivia

The caption before the end credits, detailing the fact that the Royal Navy captured the first Enigma machine, was only added after an outcry in Britain, where it was believed that Hollywood was trying to claim the credit for the Americans (whose forces captured no German Naval Enigma material until 1944).

The Enigma machine used in the movie was genuine and not a prop. It was obtained from a collector for use in the movie.

A History Channel review of this movie, which aired soon after its release, included a German World War II U-Boat commander. At the end of the show, he was asked for his opinion of the authenticity of the movie. His response was; "They got one thing right in the movie. There were U-Boats in the North Atlantic during the Second World War."

In reality, the submarine U-571 was never captured. The submarines U-559 and U-110 were the ones captured with the codebooks, but by the British Navy in August 1941, four months before the United States entered the war. U-570 however was a U-boat captured in late 1941 (by Britain) near Iceland, and subsequently deployed as H.M.S. Graph until early 1944. Its appearance was used by the Royal Navy to fool German ships and submarines, and destroy them by surprise.

The Germans introduced a new, four-rotor version of their Enigma machine in February 1942, and the code was practically unbreakable until the Allied capture of associated code books from a submarine in October 1942. "Shark" (as the Naval Four Rotor Enigma cipher was know by Bletchley Park) was broken regularly from December of that year. In real life, it was a British operation, and did not involve a deception like that depicted in the film. The sub itself, the U-559, sank shortly after the code books were removed.