Cracking Code Purple

On a warm afternoon in September 1940, a young woman sat at her desk on the second floor of the Munitions Building in Washington, D.C. Her hazel eyes peered through rimless eyeglasses at strings of letters and numbers. She had been looking at symbols like these for more than a year and they still didn’t make sense.

All of a sudden, something jumped out at her, and she became very still. Al Small, her co-worker, noticed her concentration and walked over. The woman shared her discovery and they went to fetch their boss, Frank Rowlett. Another man joined them. All three crowded around the woman’s desk as she pointed out how some symbols stood at a certain interval from one another.

Rowlett jumped up and down. Small dashed around the room excitedly. Their cries of “That’s it!” “Whoopee!” “Hooray!” broke the silence of the room, which was usually as quiet as a library. To celebrate, the team sent out for Coca-Colas. It was the first one the woman had ever tasted. 

The Purple Solution

The woman’s name was Genevieve Grotjan and she was a code breaker, or cryptanalyst, with the U.S. Army’s Signal Intelligence Service (SIS). Many cryptanalysts were people who studied how languages worked. Some were mathematicians. Grotjan had wanted to be a math teacher.

At the time, Grotjan was one of just a few women working at the highest levels of code breaking. Her discovery involved a complicated Japanese diplomatic code nicknamed Purple. Japanese codes were often named after colors.

In 1940, World War II was raging in Europe, North Africa, and Asia. The United Kingdom and France were fighting against Germany and Italy. Japan had invaded China, and in September 1940 Japan signed a pact to join forces with Germany and Italy.

The United States had not entered the war, but relations with Japan were becoming more and more strained. To “listen to” secret Japanese communications, the United States used a complex system to intercept the communications and break the codes. 

How the System Worked

Here is how the Purple code and other Japanese diplomatic codes worked. The Japanese encoded a message using a special device called a cipher machine. Then they radioed the message forward. A U.S. radio station intercepted the message and sent it to Washington. There, the cryptanalysts went to work. When they had cracked the entire code, they built a machine just like the Japanese cipher machine and used it to decode other messages. Finally, translators turned these messages into English.

A coded message consisted of a pattern of numbers and letters. It might look something like this: 78232 RSECO. Each symbol stood for a letter, but the letter it stood for changed as the machine moved forward. Each time this happened, a symbol would come to stand for something different, which made the code incredibly difficult to solve.

The cryptanalysts had to do a lot of guessing and checking their guesses. For example, they knew that many Japanese diplomatic messages began with the Japanese words “I have the honor to inform Your Excellency,” so they tried that combination of letters first. 

Making Magic

Soon after Grotjan’s discovery, the SIS and U.S. Navy built a Purple cipher machine just like the decoding machines used in Japanese embassies. With it, the United States could read Japan’s most secret diplomatic messages and learn what the Japanese were doing and planning during the war. These translated messages soon became known as “magic” and were sent to high-ranking government officials, including President Franklin D. Roosevelt.

Grotjan was modest about her discovery. “Maybe I was just lucky in getting the right series of papers. I was elated up to a point but I didn’t think ‘This is it!’ because there was so much more to do.” Others celebrated it. A government committee said, “Magic contributed enormously to the defeat of the enemy, greatly shortened the war, and saved many thousands of lives.”