A rare four-rotor M4 Enigma cipher machine,

German, circa 1944,

Footnotes

Anyone who has seen the film the Imitation Game will know the story of how the staff and Bletchley Park cracked the German Army three rotor Enigma however this more complicated M4 four rotor Enigma presented the team with a more difficult problem. As the operators had a choice of eight scramblers rather than five. Also with the reflector – the standard reflector was fixed while in this example the reflector could be fixed in any one of 26 orientations hence the number of possible keys was increased by a factor of 26.

The German Navy were gaining an upper hand in the Battle of the Atlantic and Admiral Karl Donitz has developed a highly effective two-stage strategy for naval warfare which begun with his U-boats spreading out and scouring the Atlantic for Allied convoys. Once found, it would then call other U-boats to the scene and the attack would only commence once a large pack of U-boats were present. For this strategy it was imperative for the German Navy to have secure communications which was provided by this Enigma.

Between June 1940 and June 1941 The Allies lost on average 50 ships per month and unless these losses could be drastically reduced Britain was in danger of losing the Battle of the Atlantic and subsequently, the war. Churchill later wrote:
"Amid the torrent of violent events one anxiety reigned supreme. Battles might be won or lost, enterprises might succeed or miscarry, territories might be gained or quitted, but dominating all, our power to carry on war, or even keep ourselves alive, lay in our mastery of the ocean routes and the free approach and entry to our ports."
So the pressure was on for the team at Bletchley to be able to crack the M4 Enigma.

The simplest strategy for cracking the naval enigma was to capture the German code books. Several plans were made including one devised by Ian Fleming, creator of James Bond and then working for Naval Intelligence involving crashing a captured bomber in the English Channel near a German ship. The ship would then approach the plane to rescue their colleagues and the British crew would then board the ship, capture the crew and steal the code books. This plan eventually came to nothing and the books were eventually captured during daring raids on U=boats and German weather ships.

These documents gave Bletchley the means to bring an end to German naval intelligence blackout and they could then pinpoint the location of U-boats and convoys could be steered pass the enemy. British destroyers could even go on the offensive, seeking out and sinking U-boats.

It was imperative that the German High Command never suspected that the system had been cracked as if they had, they would upgrade the machines and once again Bletchley would be back to square one. So the information had to be used very carefully.

The Naval M4 Enigma machines were made in much smaller quantities than the 3 rotor Army examples and in addition, the majority of these were lost when either when the ships and boats were sunk in combat or scuttled by their crews at the end of the war.