A bombe was a mechanical device used for the rapid decryption and transcription of complex ciphers. Developed during World War II, the multiple bombes employed by British and United States military intelligence code breakers aided the allied war effort by providing access to German and Japanese military secrets. The most famous bombe, employed by British code breakers at Bletchley

Park against the German Enigma cipher, could break messages 72 times faster than the first Pentium computer.

The bombe derived its name from the loud, rhythmic, and somewhat ominous ticking noise it made while computing code permutations. The machine itself was highly complex, requiring skill in mathematical code breaking and engineering to construct. Throughout World War II, the form of the bombe changed many times. Each improvement added to the machine's ultimate effectiveness and efficiency.

Enigma and the development of the bombe. Most of Germany's high-level military messages were encoded using a cipher machine called Enigma. The complex code used not only a cipher, but also an overlaying encryption to disguise the original text. The series of rotor wheels on the Enigma teleprinter gave the machine an extraordinary number of code combinations. The Germans were confidant that the machine code was perfectly random, and therefore mathematically unbreakable. However, both Polish and Swedish intelligence made significant progress breaking Enigma even before the outbreak of World War II.

In the months preceding the German invasion of Poland in 1939, Polish intelligence gave British intelligence information on their efforts to break Enigma. Most helpful was the information Polish spies gathered on how the cipher machine operated, including sketches of the teleprinter and some of its components. The Poles also included blueprints for a code-breaking device that they had not yet been able to construct, the first bombe decoder. At the time the Poles broke Enigma using longhand mathematics, the Enigma machine had only three rotors. On the eve of war, the Germans replaced most of the three rotor machines with new a new five rotor model, making Enigma more difficult to break, and sending British engineers back to the drawing board to redesign the bombe.

Before the mechanical device could be designed and constructed, however, Bletchley Park cryptologists had to break the new version mathematically. With the information provided by Polish intelligence, Bletchley Park cryptologists found two key weak links in the Enigma code. Enigma code prohibited that any letter be encrypted as itself, and German standards of diplomatic communication dictated that the same phrase begin many transmissions. Exploiting these two weaknesses, British cryptologists broke Enigma in 1940. Within a year, they had broken two other major German codes, including the perplexing Lorenz cipher used by Hitler's High Command. Bletchley Park engineers then set out to adapt original bombe designs to operate against the new codes.

British engineer Alan Turing designed and constructed the first successful bombe. The Turing Bombe, or "Tabs," as it became known, operated against the German Enigma code, but could be adapted to decipher other codes. The Turing Bombe was the main device used against Enigma, but its complex operation required the work of several operators. During the course of the war, women were the predominant operators of Bletchley Park bombes, decoding and translating intercepts for intelligence service use. Even with the operation of several bombes, Enigma intercept information could not be used in "real time" but military field command or forward intelligence units. A series of improvements aided computational time, including a diagonal switchboard and "machine gun" voltage regulator, which were added to eliminate processing errors that stopped the bombe's computation. A teleprinter was added to the device to allow for simultaneous transcription of messages into the original German, ready for translation.

British intelligence shared some of their cryptanalytic work with United States forces, even before the U.S. entered the war in 1941. However, after the bombing of Pearl Harbor, President Roosevelt acknowledged that the cryptanalytic efforts of military intelligence needed additional aid. Some Bletchley Park personnel went to America to train new code breakers, most of whom were members of the Women Accepted for Voluntary Emergency Service Corps (WAVES). WAVES assembled and trained to operate various bombes, eventually producing 121 bombes for used against seven different Japanese and German codes. After the Germans began sharing Enigma code secrets and teleprinter construction secrets with the Japanese in 1942, U.S. intelligence became more able to decipher Japanese codes and could adapt Enigma bombe designs to fit Japanese Red and Purple codes.

How a bombe worked: The mechanics of code breaking. The Enigma teleprinter functioned by replacing plain text letters with random letters, chosen by the settings of a series of rotors individual to each letter and space in a plain text message. The Enigma machine had a possible 15 million, million (15 x 10 12 ) combinations, but within each rotor set, the combinations were far fewer. Repeated phrases, called "cribs," such as common greetings or the name and ranks of officers, gave cryptographers a clue about the mathematical cycle of the rotors and how they replaced plain text letters. Once a series of these cycles was mathematically determined, the logic equation could be used to painstakingly decipher intercepts. The bombe worked on the concept that these cycles, and the equations representing them, could be replaced with electrical circuits.

The Turing Bombe replicated the rotors of a German Enigma machine, replacing the center reflecting rotor with a standard rotor that could be handset. The rotors were connected by a set of 26 parallel wires. The wire selected by the rotor positions determined the passage of voltage to the plug board. The machine then searched for various combinations of loops and live wires, assigning each a value on the plaintext/cipher text rows of a diagonal board. A teleprinter decoded the messages on to synchronized paper tapes.

Legacy of bombes. By the end of the war, the bombe was still being used to decode enemy intercepts in the United States. British code breakers and engineers at Bletchley Park, however, invented a new machine, Colossus, that decoded messages more rapidly and with greater accuracy than the bombes. Colossus was the world's first programmable computer, capable of decoding and transcribing messages without the cumbersome synchronization of paper tapes. The advent of punch-card computer processing ended the era of the code breaking bombe.

After the end of the war, British intelligence dismantled its operations as Bletchley Park. The numerous bombes, and Colossus, were disassembled or destroyed. The entire code breaking operation remained secret until the late-1980s, but after the news of Bletchley Park operations was broken to the public, historical preservationists sought to restore Bletchley Park and its code breaking apparati. The British Computer Society's Computer Conservation Society embarked on an ambitious endeavor to reconstruct Colossus and the Turning bombe in 1999.