The American Garden at Bletchley Park — part of which can be seen in the background — is dedicated to the hundreds of American code breakers stationed there during World War II. (Ben Murray / S&S)
The American Garden at Bletchley Park — part of which can be seen in the background — is dedicated to the hundreds of American code breakers stationed there during World War II. (Ben Murray / S&S)
Headphones and radios in a recreated "Y station" at Bletchley Park National Codes Center (Ben Murray / S&S)
A preserved version of a specialized, four-rotor Enigma machine used by the Abwher, or German military intelligence branch, to send encoded messages to each other during the war. (Ben Murray / S&S)
A technician at Bletchley Park's Block B building works on a restored version of a Bombe machine. Banks of these machines were built to copy the possible ring settings on Enigma machines that were a key to breaking the codes. (Ben Murray / S&S)
Built to crack the more complicated Lorenz cipher system used by the Germans to send high-level messages between commands, the Colossus was created to read vast sets of binary code very quickly. This process is the foundation of modern computing. (Ben Murray / S&S)
BLETCHLEY PARK — At first glance, the task in front of the mathematicians and cryptologists at this humble collection of huts and industrial buildings during World War II seemed nearly impossible.
As Germany threatened Europe in the late 1930s, men and women of the Government Code and Cipher School were told to break a German code system generated by machines they had never seen and which created ciphers with nearly limitless permutations.
It was called the Enigma system, and the Germans believed it could not be cracked.
All it produced were, to most eyes, streams of random letters plucked from intercepted Morse code transmissions that held vital information about troop strengths, aircraft movement and U-boat positions.
A detailed exhibit based on the code-breakers’ efforts may sound dry for some, but the results come together in an engaging set of displays at what was once one of Britain’s most secret facilities.
About halfway between the intellectual centers of Cambridge and Oxford, Bletchley Park takes visitors through a series of buildings used by the code-breakers. They include various systems and machines they invented to solve the increasingly convoluted cipher systems on display.
Because many of the exhibits explain the effort to break the Enigma, a good place to start is the floor above the main admissions desk, which explains how the German machines worked.
But the more fascinating aspects are downstairs. Those exhibits explain the numbers systems, machines and incredible leaps of logic employed by code-breakers to solve the Enigma code.
The first break came when a Polish mathematician gave the Allied staff information about how the Enigma physically functioned. Using electrical impulses and sets of rotating rings, it could create codes virtually unbreakable to anyone without the key.
But a series of breakthroughs provided a solution. First the machine’s design revealed it could not encrypt a letter as itself, reducing somewhat the numbers of possible combinations. Not long after, British code-breakers realized that, with educated guesses based on words that they suspected were in the German messages, they could reasonably deduce what one or several of the encrypted letters actually were.
But even with that hint, the thousands of potential settings for the rotating rings had to be tested to see which one made the code work for that day. To do that, the code-breakers designed and built massive calculating machines — the first precursors to the modern computer.
Those machines — both the increasingly complex versions of the Enigma and its code-breaking nemeses, the Colossus, Bombe and Tunny machines — are some of the highlights of the Bletchley Park tour.
But the organizers at Bletchley seem to realize that code-breaking and numbers systems aren’t exactly for everyone, and the result is a sprawling complex of World War II-related exhibits.
Along with the deciphering stations are displays on D-Day, an extensive collection of Winston Churchill memorabilia, a vintage car garage, toy museum, model ship and railway collection and the park’s central mansion, among other exhibits.
There’s a manicured garden dedicated to the American code-breakers who worked there, and an extensive cafeteria, plus a souvenir shop and playground.
But the heart of the complex is the code-breaking system, and a tour of all its related parts can take several hours. For those interested, however, it doesn’t all have to be digested at once — the 10 pound fee includes unlimited visits to Bletchley Park for a year.
Getting thereLocation: In Milton Keynes, near the Bletchley train station. The park is somewhat hard to find, and the best maps to the museum are on the Bletchley Web site.
Cost: 10 pounds for adults, 6 pounds for youths 12-16, children younger than 12 are free. Tickets bought online receive a discount. Parking is 3 pounds.
Hours: 9:30 a.m. to 5 p.m. April-October and 10:30 a.m. to 4 p.m. November-March.
Food: A restaurant is open for lunch with sandwiches, pies and hot food.
Web site:www.bletchleypark.org.uk.
A war fought in letters
BLETCHLEY PARK — German intelligence officers in World War II had good reason to think their Enigma cipher system could not be cracked without a key.
To create the secure message system, they used sets of special typewriter-looking machines that generated apparently random letters for those used in a message.
To create the encryption, an operator pressed the letter he wanted to type on an Enigma machine. The pressure created an electrical impulse that traveled from the keyboard through a series of up to four adjustable rings, set to reroute the signal in a complex pattern before returning it to another part of the machine. There it would light up a different letter, which was used as the code letter to be sent in the message — via Morse code to a decryption officer with another Enigma machine.
The Enigma was difficult to crack because the solution depended on the exact setting for the adjustable rings and the way in which a series of plugs inserted into the front of it were applied.
Only another person with an Enigma machine and the specific plug and ring settings — which the Germans changed every 24 hours — could reverse the cipher and read the message. Or so the Germans thought.
What the Bletchley Park code-breakers did was to build a machine, called a Bombe, that could quickly copy all of the Enigma’s possible ring settings. After guessing part of a message based on common words used at the start of many German transmissions, they could use the combination of guessed letters and the Bombe machine to crack the day’s code for a network of Enigma machines within hours. It was a system that would last for most of the war.
But the Germans weren’t content with simple Enigma security. They later created other versions, including a more complex version called the Lorenz that had up to 12 rotating rings and was used for high-level transmissions. For this, the Bletchley Park code-breakers needed to test far more possible ring settings to guess a cipher, and thus had to build a bigger Bombe.
To defeat the Lorenz’s dual-impulse system, they created a machine that could, essentially, test massive streams of binary code. The result was Colossus, one of the world’s first computers.
— Ben Murray
