Bombe

Bombe, electromechanical code-breaking machine created by cryptologists in Britain during World War II to decode German messages that were encrypted using the Enigma machine. The Bombe was derived from a device called the bomba—Polish for “bomb”—that was invented in Poland during the 1930s. Alan Turing developed the Bombe in 1939 at Bletchley Park, and the first Bombe machine was installed there in March 1940. Hundreds of Bombes were eventually built, including versions by the U.S. military. The Bombes were an important intelligence tool for the Allies, who used the machines to turn the tide of World War II against Nazi Germany.

Enigma and the Polish bombas

In the years leading up to World War II, the German military command had started using Enigma to encode strategic messages. Enigma encrypted messages via a polyalphabetic substitution cipher generated through a plugboard and a scrambler. Enigma operators received a new codebook every day, which was used to encrypt messages for that day. The drawback of using a single cipher for an entire day was that it generated numerous messages encrypted in the same way, making it easier for a cryptanalyst to crack the code through brute-force methods. To guard against these deciphering efforts, the Germans added a new message key for each individual message; this message key was encrypted using the day key for that particular day. The message itself was then encrypted using the newly generated message key, which used a different scrambler orientation. Thus, the day keys were employed to transmit only the message key twice for each message, and in turn the message was encrypted using the message key unique to the message. This complex system reduced the ability of a cryptanalyst to crack the code, and the Enigma code was for some time believed to be unbreakable.

Poland, at risk of invasion from Germany, had formed a bureau called the Biuro Szyfrów after World War I. In response to Germany’s use of Enigma, the bureau trained 20 mathematicians in cryptography and unleashed them on the Germans’ encrypted messages. These mathematicians included Marian Rejewski, who identified patterns in the working of the Enigma machine and was able to reduce the day key combinations from, potentially, 10 quadrillion to only 105,456 (17,576 scrambler settings times 6 possible scrambler arrangements). Replica Enigma machines were available to Rejewski and others at the cipher bureau, and they spent a year cataloging all 105,456 scrambler configurations. This catalog simplified decryption of the scrambler settings, and Rejewski then decoded the easier plugboard settings. By 1932, the Poles had cracked the Enigma code.

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The Germans, however, continued to make changes to Enigma, and Rejewski responded with a solution that would have far-reaching, and lasting, effects: he transformed the catalog that he and his team had compiled into a mechanical system—a machine—that could quickly find a match amid all 17,576 scrambler settings. Six of these machines working in parallel, one for each scrambler arrangement, constituted a unit that came to be known as a bomba. (There are several theories about the origin of the name, among them being the units’ bomblike clicking as they did their work.) Each bomba took about two hours to identify the day key, allowing the communications for the rest of the day to be easily decoded. It was a revolutionary solution. Strictly speaking, however, it was unnecessary: the Polish cipher bureau’s chief, Gwido Langer, was receiving each month’s worth of day keys from his spies. But Langer allowed Rejewski to keep working, unaware of the spies’ work, so that the bureau would be able to function without them.

In December 1938 the Germans added two new scramblers to Enigma, which upped the number of possible scrambler arrangements to 60 and, yet again, increased the level of encryption. To respond, the Poles needed many more bombas—10 times what they had, which was far beyond what the bureau could afford. The next month, the Germans increased the number of plugboard cables from 6 to 10, boosting the total number of possible keys to 159 quintillion. The Poles were overmatched, and they were now vulnerable. With an invasion imminent after Germany’s withdrawal, in April 1939, from its nonaggression treaty with Poland, Langer took action to ensure that what his bureau had accomplished would not be lost: he offered the British and the French spare Enigma replicas as well as blueprints for the Polish bombas. These reached England just weeks before Adolf Hitler’s invasion of Poland on September 1, 1939.

Turing’s Bombe, the industrialization of cryptanalysis, and Allied success

The progress made by Rejewski and his colleagues in the Polish cipher bureau made clear to the Allies that mathematicians were their most valuable cryptographic asset, not scholars working in the humanities and social sciences—classicists, linguists, and the like—who had previously been pressed into service. The Government Code and Cypher School at Bletchley Park in Buckinghamshire, England, became the centre of British code-breaking efforts; there, after the start of World War II in late 1939, newly recruited mathematicians and scientists, among them Alan Turing, studied Enigma as well as the Polish strategies for breaking it, and they set to work decrypting Enigma-generated messages on a daily basis. They also discovered new rules that enabled them to streamline the decoding process, which were necessary because Nazi Germany continued to make Enigma more and more complex. Those working at Bletchley Park expected the Germans to stop using repeated message keys, and Turing was the person who tackled that new challenge. Like Rejewski, he focused on simplifying the decryption problem so as to avoid running through 159 quintillion settings to check each message.

Turing’s method was based on the use of “cribs”—comparing patterns of the encrypted message and a known portion of plaintext, such as a weather update—to break the key. He was also aided by the fact that no letter was ever coded as itself. He decided not to replicate one Enigma machine so as to decode a message, but, rather, he designed an electrical circuit consisting, effectively, of three of these machines, each running all 159 quintillion combinations to arrive at the right setting. His circuit also eliminated the layer of encryption generated by the plugboard. By connecting the scramblers in series, each scrambler fed off the combinations from the earlier one, and only 17,576 scrambler combinations would need to be checked. However, this process needed to be run 60 times for each of the possible scrambler combinations, and the plugboard settings still had to be deciphered once the correct scrambler settings were identified. Turing’s previous academic research into mathematical machines helped him design this very practical method to crack the Enigma code.

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Once it had convinced the British government of the importance of its work and thus received sufficient funding, Bletchley Park built machines that put Turing’s theories into practice. They were called Bombes (or bombes), due to their resemblance to Rejewski’s bombas. Each of Turing’s Bombes consisted of 12 sets of Enigma scramblers, all connected so that the Bombe could process more letters. The complete unit was designed to be 2 metres tall, 2 metres long, and 1 metre wide (about 6.5 feet by 6.5 feet by 3.2 feet). The first Bombe was delivered to Bletchley Park on March 14, 1940. Built by the British Tabulating Machine Company, it was named Victory, but it proved to be too slow. A new design was created, and the new Bombe machine, called Agnus Dei, arrived in August. The Germans had, as expected, already stopped repeating the message key, but Agnus Dei proved to be the machine capable of facing the new German challenge, and many more were built during the following months. This version of the Bombe consisted of about 100 rotating drums, 16 km (10 miles) of wire, and about one million soldered connections. Hundreds of Wrens (members of the Women’s Royal Naval Service, or WRNS) operated the Bombe machines, working long shifts in dark, stuffy rooms. The Bombes began finding keys in less than an hour.

Developing and using the Bombes, Turing and others working at Bletchley Park pioneered machine-driven cryptanalysis and the industrialization of the code-breaking process. These accomplishments had a lasting impact on modern cryptography and computer science. The Bombe machines played a key role in deciphering the Enigma codes and were ultimately crucial to the Allies’ victory over Nazi Germany in World War II.