Red was a Japanese naval code created during World War I and used until the outbreak of World War II. The Red code used the additive encryption method. The code assigned words and syllables numerical values. Before transmissions, these numbers were encrypted a second time using an additive codebook. The book contained a series of numbers that were added to the original numerical message in sequence. Each message contained a key that told the receiver where to begin the additive sequence in the book to decode the message. Cryptologists named the code Red after the color of the folder in which deciphered codes were bound.
In 1923, a United States Navy intelligence officer located a copy of the 1918 Imperial Japanese Navy secret operating code in the luggage of a visiting Japanese attaché. The codebook was clandestinely photographed and a special cryptology unit, known as the Research Desk, was created to begin the task of monitoring and deciphering intercepted messages. At the time, U.S. Navel Intelligence monitored only ship-to-ship communications and some radio transmissions in Asia and the Pacific. The Research Desk team established intercept stations throughout the Pacific and increased monitoring of Japanese diplomatic and military transmissions.
Cryptologists worked for five years to fully translate and break Red, the additive cipher that the 1918 codebook contained. Intercepts continued to use the aging code, facilitating the work of U.S. code breakers. In 1926, Lieutenant Joseph J. Rochefort accepted the directorship of the Research Desk. Rochefort was a skilled code breaker, but also fluent in the Japanese language and undertook much of the translation work for Red himself. Repeated messages and phrases that appeared in several transmissions helped code breakers recognize various additive decipherments. Three years after the analysis of Red began, cryptologist Agnes Meyer Driscoll cracked the code's additive encryption key. With the additive key, and the photographs of the original code book, any Red code message could be deciphered.
The Japanese replaced Red with a more sophisticated code on December 1, 1930. However, the new code, called Blue, contained numeric patterns that so closely resembled Red that Driscoll and her team were able to decipher and translate Blue in only two years.
█ FURTHER READING:
Budiansky, Stephen. Battle of Wits: The Complete Story of Codebreaking in World War II. New York: Touchstone Books, 2002.
Matthews, Tony. Shadows Dancing: Japanese Espionage Against the West. New York: St. Martin's Press, 1993.