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War of Secrets: Cryptology in WWII

Cryptology is the study of secret codes. Being able to read encoded German and Japanese military and diplomatic communications was vitally important for victory in World War II, and it helped shorten the war considerably.

Vital to Victory
In WWII, wireless radio communication was very important for directing military forces spread all over the world. But radio messages could be intercepted, so secret information -- plans and orders -- had to be transmitted in secret codes. All the major powers used complex machines that turned ordinary text into secret code. A German machine called Enigma and an American device known as SIGABA are on display in an exhibit in the museum's Air Power Gallery. 

The Allies were able to read German messages very early in the war thanks to brilliant work by Polish and British mathematicians. In the 1930s, Polish cryptanalysts (code-breaking experts) copied the German Enigma machine with the help of a German traitor, and solved its letter-scrambling patterns. They later shared this knowledge with France and Britain. Intelligence from decrypted Enigma messages, code-named "ULTRA," was extremely secret, and very few people knew about it. While the Germans never found out the Allies could solve their codes, they suspected it as their ability to sink Allied shipping slipped dramatically in 1942. This led the German Navy to add an additional rotor to their Enigma machines, and the submarine "wolf packs" once again started taking their toll of shipping. 

Cryptanalysts also exploited Japanese codes. By late 1940, the U.S. Army and Navy could read Japanese diplomatic messages between Tokyo and embassies in London, Washington, Berlin and Rome. American experts named the Japanese code PURPLE, and they called intelligence from these messages MAGIC. Unfortunately, the PURPLE diplomatic code did not provide specific military information, so Americans had no advance knowledge of the Pearl Harbor attack of Dec. 7, 1941. 

As the war went on, Allied analysts combined MAGIC and ULTRA intelligence. Japanese communication ironically played an important role in the war in Europe, since Tokyo wanted information from its diplomats about German and Italian progress. Intercepting these Japanese messages gave Allied commanders vital information about Nazi weapons production and German plans to defend Europe from invasion. Allied leaders also knew from MAGIC that Japan would not surrender unconditionally unless forced. 

Code-breaking Heroics
Capturing secret code books was a key to breaking Axis codes. In 1940 the crew of a captured German ship threw their code books overboard, but the British Royal Navy managed to recover some of them. In 1942 British sailors recovered code books from a sinking U-boat in the Mediterranean Ocean. Two British sailors died, but the books they rescued allowed cryptanalysts to solve German codes used to communicate with submarines in the Atlantic. 

With captured code books and skilled code breaking, the Allies were reading up to four thousand Enigma intercepts every day by the end of 1942. These and similar technological victories helped the Allies stem the tide of U-boat attacks on vital supply convoys. 

On the home front, cryptologists built special equipment to attack Axis codes. British experts at Bletchley Park, near London, and American teams in Washington, D.C., and Dayton, Ohio, worked in extreme secrecy to decipher Enigma messages. They used early computers and raw mathematical talent to work through millions of scrambled possibilities in each German message until they found the right solution. 

Breaking German and Japanese codes gave the Allies an important advantage in WWII -- it saved many lives and shortened the war, by some estimates as much as two years. 

Enigma and the Air War
ULTRA gave the Allies critical information in the European air war. During the Battle of Britain, the outnumbered Royal Air Force depended on ULTRA to counter German raids. Later, Enigma intercepts gave Allied planners detailed information on the effects of strategic bombing, and they allowed Allied air power to virtually halt Axis sea convoys in the Mediterranean. The Luftwaffe (German Air Force) also unwittingly confirmed Allied air tactics' effectiveness by using Enigma to transmit reports of their losses, along with plans and orders. 

Unbreakable American Codes
In contrast to German and Japanese codes, American codes proved unbreakable due to a superior code machine known as SIGABA, the most secure cryptographic machine used by any nation in WWII. 

The U.S. Army and Navy developed SIGABA before the war. "SIGABA" is not an acronym and does not stand for anything -- it is simply a code word. In 1935 Army cryptologists designed the basic machine, and they shared its design with the Navy. In 1940 the Army and Navy both adopted SIGABA, and the system became operational by August 1941. By 1943, more than 10,000 SIGABA machines were in use. 

SIGABA machines linked with British machines to let Presidents Roosevelt and Truman communicate securely with British Prime Minister Winston Churchill. SIGABA was so secret, though, that British personnel were not allowed access to the machine. 

The SIGABA system was used until 1959, when the speed of modern communications demanded new equipment. Most SIGABAs were destroyed to protect their design, and only a few exist today. The secret patent for SIGABA was declassified in 1996. 

Enigma vs. Sigaba
The Enigma and SIGABA machines on display have important similarities and differences. Both are "rotor machines," that is, they scrambled typed-in messages by sending electrical current through rotating wheels. Neither machine is a computer. They use electricity only to substitute letters in the alphabet with moving mechanical parts that are wired in a very complex fashion. 

Neither machine can send or receive messages like a radio or computer -- they can only encipher or decipher typed-in text. Only another machine with exactly the same settings can decode a message. Both Enigma and SIGABA depended on a secret daily "keylist" of machine settings to keep enemy cryptologists from decoding messages. 

The most important difference between the machines is in their complexity. Most Enigmas used three rotors and some used four; SIGABA used 15. This made SIGABA's letter scrambling much more complex, and practically unbreakable by cryptologists. Enigma is also older than SIGABA. Invented in Germany in 1918, Enigma at first was a commercial device to protect banking transactions. SIGABA was invented about 20 years later, and was exclusively military equipment. 

SIGABA was easier to use than Enigma. The German machine needed two people to operate -- one typed in the message, and another copied down the resulting lighted letters. SIGABA, however, printed the letters on a paper tape, allowing a single person to operate it. 

Both Axis and Allied forces had great faith in their code machines. With SIGABA, American confidence was justified. The Germans believed -- wrongly, it turned out -- that Enigma also was unbreakable.

Click here to return to the World War II Gallery.


Find Out More
Dr. Doug Lantry: "Code Machines" (00:44:31)
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