Bombe
The bombe was an electromechanical machine designed to crack German Enigma cipher machine, which was considered by the Germans to be unbreakable. It was invented in October 1938 by Alan Turing, with an important refinement suggested by Gordon Welchman. The machine was named after an earlier codebreaking device, known as the Bomba, which had been designed by Polish cryptanalysts.
The British bombe
At Bletchley Park, British mathematician Alan Turing designed the bombe on the assumption of the presence of text that analysts believed would be somewhere in the message, a cryptanalytical technique known as cribbing. Using the Turing-Welchman bombe, the Allies were able to read a high proportion of the German Army and Air Force traffic which used the Enigma system.
The German Navy was less certain of the inherent security of Enigma and used a set of eight rotors, three of which were in use at one time, as well as strict communication security measures. The three extra rotors available in the Navy system largely prevented reading of their messages because their wiring wasn't known.
A German U-Boat was captured in May 1941 with all of its cryptological equipment intact, including a complete Enigma machine with all rotors, settings, and code books. The complete key sets meant that for two months the Bombe wouldn't be needed to read the Navy messages but urgent work was carried out to modify the bombes so that they would be able to read later traffic for which the settings wouldn't be known. This led to an immediate change in favor of the Allies in the submarine war, as they were now able to read messages to and from the submarines.
United States Navy bombes
By late 1941 the change in German Navy fortunes led Admiral Karl Dönitz to become convinced that the Allies could read Navy communications and a fourth thin rotor with unknown wiring was added to German Navy systems to produce the Triton system, with a lock-out which would allow them to remain compatible with three rotor machines when necessary. As before, the unknown wiring would prevent the reading of the messages. Fortunately for the Allies, in December 1941, before the machine went into official service, a submarine accidentally sent a message with four rotors, then sent the same message using only three, disclosing the wiring of the extra rotor. In February 1942 the change became official and the ability to read the critical messages for the submarines largely ceased until new equipment became available which could use the infromation about the fourth rotor wiring to decrypt messages.
That spring was known as the "Happy Time" for the submarines, with renewed success in their attacks on shipping due in part to the security of their communications and the German ability to read the convoy messages sent in Allied Naval Cipher No. 3 and find out where the convoys were. Between January and March 1942 submarines sank 216 ships off the US East Coast. In May 1942 the US switched for the first time to using a convoy system and to requiring blackouts of coastal cities so ships wouldn't be silhouetted against their lights but the improvement was small.
An urgent work plan to design bombes which could decrypt the four rotor system, with delivery scheduled for August or September of 1942, was begun at Bletchley Park. The urgent need, doubts about the British design and slow progress with it prompted the US to start investigating designs for a parallel effort, based in part wiring diagrams provided to US Navy officers during a visit to Bletchley park in July 1942. Funding for a full US development effort was requested on 3 September 1942 and approved the following day.
The last manufactured United States bombe, with four wheel sets, is on display at the National Cryptologic Museum. Jack Ingram, Curator of the museum, describes being told of the existence of a second and searching for it but not finding it whole. Whether it remains in storage in pieces, waiting to be discovered, or no longer exists, is unknown.
