Let’s Get Cracking: The Bletchley Park Bombe
By Warren Seeds
During World War II Germany implemented a strategy called Blitzkrieg, which meant “Lightning War”. The idea behind this strategy was for Germany to fight and advance as quickly as possible and strike without warning, so they could have both momentum and surprise on their side when the first shot was fired, allowing them to triumph over superior numbers (Reilly 254). In order to maximize the potential of this strategy, all attacking German commanders must be on the same page about when and where to make calculated blows. Because the German Army was spread throughout Europe, the commanders could not gather in a secure location; nor could they send their plans plainly by telegraph because they would be intercepted. The German commanders solved this communication problem by using a secret code to relay important information. They would use a machine called “Enigma” to encode and decode their messages.
The machine resembled a typewriter with a keyboard, three rotors, and a set of lettered lightbulbs would light up according to which key was typed (Fig. 1). For example, if the letter “a” was pressed on the keyboard, an electric current would be sent through the “a” wire, then through the three rotors which would each redirect the current into a different letter’s wire. Then the current would be reflected back through the rotors the opposite way and sent to the lightbulb. The genius behind the machine were the rotors. After a key is pressed, the rotors would spin, meaning if one key is pressed twice in a row, the encrypted letters would not be identical. Furthermore, if Allied forces were able to capture one of the enigma machines, they would know how the encryption worked, but they would not know the machine’s proper settings for that specific day to decode the message. The machine used three of a possible five to eight uniquely wired dials (depending on which version the machine was) each with 26 positions, meaning there were 17,576 possible settings for any given day meaning brute forcing was out of the question (Bateman 7). If the allied forces stood a chance against cracking the enigma cipher, they would have to use a different method.
Just as the Germans turned toward machines to encode their messages, the Allied forces would need to use them to decode intercepted messages. Prior to World War II, when the Enigma machines were less complex, only using three possible rotors, Polish cryptanalysts were able to design a machine they called “Bombe” because of the ticking noise it made that could crack the code given the daily settings found in the encrypted daily Enigma settings broadcast (Dewdney 146). However, in the months leading up to the outbreak of the war, Germany made more complex and secure versions of the Enigma machine with more rotors and a plugboard on the reflector that would scramble six or seven letters again before their second pass through the rotors. The plugboard complicated things as there were over a trillion different wiring patterns that the cryptanalysts would have to know before decoding the message. After learning of the new complexities of the Enigma machine, the Poles gave realized there was little more they could do and turned their progress over to the British (Dewdney 146).
The British first assembled a group of gifted mathematicians and cryptanalysts, including Alan Turing, in a secret headquarters at a manor in Buckinghamshire called Bletchley Park. They used this are to begin their research on methods to chip away at the cipher. One of the methods they used to help with the problem of the plugboard was called the probable-word method in which analysts would examine many letters and search for repeated blocks of letters with the same length, then hypothesize the word based on probability. For example, if many letters were intercepted with a block of four letters followed by another block and a comma at the top of the letter, one could assume (if the letters were known to be written in English) that the block of four letters was the word “Dear” followed by a name. Through using this method and correctly guessing many words, the Bletchley Park analysts were able to find out the plugboard’s daily wiring by reworking the polis Bombe to be able to run all the possible plugboard combinations until the correct combination was discovered. After further research and much more difficult calculations, the analysts were able to reliably determine the positions of the three rotors in the machine allowing them to decode any message encrypted by the Enigma machine (Dewdney 147).
Turing’s version of the Bombe (Fig. 2) was one of the very first milestones in machine intelligence as it was able to solve problems by systematically searching through millions of possible solutions and return the correct solution in a reasonable amount of time (Copeland). However, while the Turing Bombe was both an invaluable resource in the conclusion of World War II and the evolution of modern-day computers, it is only part of the reason why Alan Turing is considered one of the fathers of modern Computer Science. Fueled by his research in machine learning and intelligent machines, Turing’s arguably most widely known contribution to the computer science world was his “Turing Test” which is the way one is able to determine if an AI is truly able to think for itself (Copeland). The Turing test states that if an innocent person is able to have a conversation with a machine without being able to tell that it is a machine, the AI passes the test (Godwin).
The Turing Bombe is undoubtedly one of the most impressive feats of wartime technological development. An interesting thought I came across while researching this topic was the correlation between war and technological development. It is no wonder that much of the advances in technology are seen during times of war as both countries are funding the research of new technologies to give them an edge over their opponent. The moon landing was a result of the Cold War era “Space Race” between the United States and the USSR. Both countries were fearful of what might happen if their opponent were able to conquer the vast battlefield of space before they were able to themselves (History.com). It is interesting to ponder what kind of new technologies might emerge as the result of a future conflict between two countries.
Figure 1. - Enigma machine on display in Museo Scienza e Technologia in Milan, Italy
Figure 2. - A wartime picture of a Bletchley Park Bombe
Works Cited
Bateman, Gary M. “THE ENIGMA CIPHER Machine.” American Intelligence Journal, vol. 5, no. 2, 1983, pp. 6–11. JSTOR. Accessed 8 May 2020. This source allowed me to explain the complexity of the Enigma machine. This source is from the American Intelligence Journal, a credible source kept by the National Military Intelligence Foundation.
Copeland, Jack, and Diane Proudfoot. “Alan Turing Father of the Modern Computer.” Rutherford Journal, Rutherford Journal. This source enabled me to elaborate on the impact of Turing’s Bombe to the world of computer science. The Rutherford Journal is a peer-reviewed scholarly journal from New Zealand and is a reputable source.
Dewdney, A. K. “COMPUTER RECREATIONS.” Scientific American, vol. 259, no. 4, 1988, pp. 144–147. JSTOR. Accessed 8 May 2020. This source allowed me to continue explaining the complexity of the Enigma machine, along with the way the allies first sought to solve the cipher. This article comes from the Scientific American journal, a scholarly and credible source.
“Enigma Machine.” Wikipedia, Wikimedia Foundation, Milan, 7 May 2020. Photograph of an Enigma Machine on display at Museo Nazionale Scienza e Tecnologia Leonardo da Vinci in Milan, Italy.
GODWIN, DWAYNE, and JORGE CHAM. “TURING’S TEST.” Scientific American Mind, vol. 25, no. 6, 2014, pp. 76–76. JSTOR. Accessed 8 May 2020. This source allowed me to explain the Turing Test, one of Alan Turing’s most recognizable contributions to the field of computer science. The source was authored by Dwayne Godwin, a reputable professor of Neuroscience at Wake Forest University.
History.com Editors. “The Space Race.” History.com, A&E Television Networks, 22 Feb. 2010. This source was used in order to explain an example of wartime technological development. While History.com’s reliability is widely speculated, the small amount of information used from this article is credible.
Reilly, Henry J. “Blitzkrieg.” Foreign Affairs, vol. 18, no. 2, 1940, pp. 254–265. JSTOR. Accessed 8 May 2020. This source allowed me to explain the German Blitzkrieg tactic and how it was so deadly to Allied forces. Written by an American soldier and journalist, this article is distributed by the Council on Foreign Relations, a reputable source on Foreign Affairs.
Unknown. “A Wartime Picture of a Bletchley Park Bombe.” Wikipedia, Wikimedia Foundation, 27 Mar. 2020. Photograph of the Turing Bombe at Bletchley Park during WWII.