How British cryptanalysts solved the 'unbreakable' German Enigma code, shortening World War II

Routine weather observations in the North Atlantic played an unlikely role in the deciphering of the German encryption device known as the Enigma machine.

One of the pivotal turning points that led to the Allied forces’ eventual defeat of Nazi Germany and its fellow Axis Powers in the second World War came not on a battlefield, nor in the skies nor on the open seas, but rather in a stable yard of a countryside estate in South East England.

In 1939, a secret intelligence-gathering operation coordinated by the British Government Code & Cypher School began in Bletchley Park in Buckinghamshire. Here a team of gifted codebreakers worked on the all-important task of trying to decipher messages that had been encoded via the Germans’ Enigma machine.

The enigma machine enciphered messages so the Germans could securely send sensitive information such as troop movements or where shipments of supplies were headed throughout its Air Force, Army and Navy.

A typewriter-like device, it was created shortly after World War I by German engineer Arthur Scherbius. The Enigma used a combination of rotors, plugs and wiring to code messages and was said to have as many as 103 sextillion possible settings, which is one of the reasons the Germans thought their code was unbreakable, according to the Bletchley Park Museum.

Polish mathematicians had been the first to decipher the Germans’ code early in the 1930s, but as war efforts ramped up, the Germans increased the security of the system by changing the cipher system daily, according to the British Imperial War Museum.

Among the team of codebreakers at Bletchley Park were Alan Turing, a mathematician, and Gordon Welchman, a mathematician who also served as the dean of Cambridge College. Because the code would change every day, the team would find themselves in a battle against time.

Turing and Welchman would create a device that would counter the Enigma, known as the "Bombe," which imitated the functions of the Enigma machines and helped reduce the amount work for cryptanalysts.

One of the biggest developments came during the punishing Battle of the Atlantic, which was the longest-running battle of the war. While German U-boats prowled the waters of the North Atlantic, lying in wait for convoys of Allied ships carrying needed supplies to Great Britain, they were also conducting scientific research for the war effort.

“One of the things that [the Allies] realized that the Germans were doing with their U-boats was they were actually collecting weather data and they were sending it back home,” said Dr. Paul Thomsen, visiting professor of history at the United States Merchant Marine Academy in Kings Point, New York.

“That was one of the telltale signs that [the Allies] were able to help in the breaking of enigma.”

Conditions such as air temperature and pressure, wind direction, visibility and wave height were sent back to German high command to be aggregated. Each weather condition was represented by a different letter but always in the same order.

Thomsen added that when it comes to cryptography, there are two ways to break codes, monitoring for a pattern or looking for an exception to the rule.

According to an excerpt of the book “Enigma: The Battle for the Code” written by British author Hugh Sebag-Montfiore, the British had captured a number of German vessels which unearthed codebooks that were put to use in Bletchley Park to solve Enigma. However, after the Germans modified the naval enigma machine to include a fourth rotor, rather than three, messages from U-boats were unable to be read from February to October 1942, putting the pressure back on the the cryptanalysts.

During this time the Germans sunk hundreds of tons of Allied shipments. But according to Sebag-Montfiore, the capturing of U-Boat 559, on Oct. 30, 1942, in the Mediterranean, brought more German intelligence, this time codebooks that contained weather observations, into the hands of the codebreaking team at Bletchley Park.

Soon enough, Turing and his team were able to solve the code once more, after discovering a pattern in how the Germans sent weather information each day. The danger areas where these U-boats were positioned could now be avoided.

"There were clippings that certain [German] individuals were doing through Enigma from office to office talking about the weather," said Thomsen. "And some of these clippings were literally from the news services of where these officers were stationed, including Portugal, that were fed through the Enigma machine. And you see the pattern over and over again and all of a sudden you've got something that you can match to the outside source and it makes the codebreaking much easier."

Once the Allied forces had broken enigma, the key became not letting the Germans know their code had been broken, said Tom Czekanski, senior curator and restoration manager for the World War II Museum in New Orleans.

For example, intelligence could be gathered on when a German freighter would embark on a route, according to Czekanski. Rather than come out of the blue with fighter planes, the U.S. and its allies could send a reconnaissance plane to the area around the date and time of where the ship would be in transit.

"[The Allied forces] were careful to react to the code in such a way that [they] didn’t give away the idea," he said.

The work of the cryptanalysts has been credited with saving countless lives and shortening the war, with some estimates by as much as two to four years. According to a 2012 article by the BBC, analysts for British Prime Minister Winston Churchill said German torpedoes were sinking so many ships, that "Britain would soon be starving."

"The only thing that ever really frightened me during the war was the U-boat peril," Churchill would say later, according to the BBC.

Turing’s vital role in the war was portrayed by Benedict Cumberbatch in the 2014 film, “The Imitation Game,” which won best adapted screenplay at the 2015 Academy Awards and was nominated in several other categories.

The legacy of the top-secret work that team at Bletchley Park conducted is preserved at the Bletchley Park Museum and the estate has been declared a conservation area. Each year, over 250,000 visitors travel to the site to learn the story of how these scholars and academics worked to help change the course of history.