Code Breaking World War I through World War II
The most simple ciphers are one-to-one substitution codes – A=Z, B=Y. More difficult ciphers, such as the Playfair cipher, encrypted letter pairs, making them more difficult to break. The Vigenère cipher was a polyalphabetic substitution, which used multiple substitution alphabets, and was used for 300 years until it was broken in 1863.
In World War I, most communication was sent using simple substitution ciphers. Messages that were important used encrypted codes that required a book to decode, which was an issue to code safety.
The most famous decoding of the war was the Zimmerman telegram, sent from Germany to Mexico, suggesting an alliance between the countries. If Germany won the war, Mexico would get back Arizona, New Mexico, and Texas. The telegram was intercepted and decoded by British intelligence, and its message was one of the events that brought the United States into the war.
In the United States, Herbert Yardley led the cryptographic section of Military Intelligence during the war. Some of the code breaking for World War I was performed by the Riverbank Laboratory, where well-known cryptographers, husband-and-wife-team William and Elizebeth Smith Friedman worked, and trained other cryptographers. They applied statistics to cryptography, and the Riverbank Publications demonstrated their techniques.
The Coast Guard, then part of the Treasury Department, developed its own cryptography section after the passage of the Volstead Act, also known as Prohibition, in 1919. This section was led by Elizebeth Smith Friedman. She and a small team of code breakers intercepted and decrypted radio communications between alcohol smugglers; Elizebeth testified at several high-profile trials, including a large rum-running ring in New Orleans in 1933. After Prohibition was repealed, she focused her efforts on drug smuggling.
After Secretary of State Henry Stimson shut down State Department cryptanalysis office in 1929, Herbert Yardley, head of the office, wrote The American Black Chamber, about his experiences breaking code for the government. William Friedman disagreed with the publishing of the book, viewing it as sensationalist and divulging information that should stay confidential. The U.S. government instituted regulations barring the publication of items deemed to contraindicate national security.
After the war, cryptography became mechanized when Edward Hebern invented the rotor machine design. The design was improved by several inventors including Boris Hagelin and Arthur Scherbius, developer of the famous enigma machine.
In 1939, cryptanalysts led by William Friedman and Frank Rowlett reverse-engineered a machine to help read the Japanese diplomatic code, Purple. Unfortunately, the diplomatic messages did not give detailed information on the attack at Pearl Harbor on Dec. 7, 1941. Code-breaking efforts were focused in two locations led by William Friedman and Rowlett: Arlington Hall in Virginia, and a compound of buildings near the National Cathedral in Washington, D.C., which was pioneering the use of early computing, using large punch-card machines. They had numerous college girls and female Navy personnel on their staff, as it was found that women had a knack and the patience for code-breaking work.
Elizebeth Smith Friedman was still working for the Coast Guard during World War II. Her experience intercepting radio communications enabled her to track and decode spy communications in the Western Hemisphere, especially South America, and the information was then passed to the FBI. The contributions of her team were not acknowledged by FBI Director J. Edgar Hoover, however, who inferred that the code breaking was done by his staff.
The famous and supposedly unbreakable enigma machine, first developed for confidential business communication, was improved upon and used by Germany during the war. In fact, the machine was broken several times. First, a Polish mathematician, Marian Rejewski, broke an early version. The British used this information to build a Bombe machine, which automated the reading of German communications. The enigma was reverse-engineered by both the Arlington Hall team, and by the Coast Guard team, led by William and Elizebeth Smith Friedman, respectively. Neither knew the other was working on it.
Of important note: The United States began using Native American code talkers during World War I, mostly from the Cherokee and Choctaw tribes. The code-talker program was expanded in World War II, with the more-famous Navajo code talkers. The code-talkers were extremely valuable as their languages were in limited use in the United States and unknown in foreign countries.
The George C. Marshall Foundation holds the personal library and papers of William and Elizebeth Smith Friedman.
To find other items that the Marshall Foundation has on codes, search”code” “code breaking” “enigma” “William Friedman” “Elizebeth Friedman” or other names from this introduction in the library catalog: https://www.marshallfoundation.org/library/results/
Digitized items in the George C. Marshall archives: