Images of Enigma and SIGABA courtesy national museum.af.mil, M-209 photo credit mike newton, Wikimedia Commons.

One encryption method used by the U.S. armed forces during World War II was the SIGABA machine, based on the German Enigma machine. Both use a series of rotors to encipher messages. Unlike Enigma, SIGABA was never broken during WWII, but it was time consuming to send a message. There were other cipher machines used for communication like the M-209, which could be used in the field because it was fast and efficient, but it wasn't as secure.

"In the early 1900s, electro-mechanical rotor cipher machines were invented. Invented too late to be used in World War I, they were used heavily during World War II and even as late as the 1970s...Using a series of three rotors, or wheels, one or more wheels would advance after each letter was pressed, changing the letter substitution each time. This made encryption very complex and decoding difficult."

~ Susan Borowsk, American Association for the Advancement of Science

SIGABA machine, Smithsonian National Air and Space Museum.

Image courtesy of National Archives.            

Civil engineer Philip Johnston, who lived on the Navajo reservation as a child with his missionary parents, is credited with suggesting the use of the Navajo language to create an unbreakable code. He pointed out that the Navajo language was unwritten, it couldn’t be learned from a book, and fewer than 30 non-Navajo spoke the language.

"What would you think of a device that would assure you of complete secrecy when you send or receive messages on the battlefield?...suppose we develop a code from an Indian language...One that would always be used orally, by radio or telephone, and never reduced to writing that would fall into enemy's hands?"

~ Philip Johnston, conversation with Lieutenant Colonel James E. Jones.

Philip Johnston, 1942, Northern Arizona University Cline Library.