Cryptologists, the Purple Machine, and Pearl Harbor

July 06, 2017

Behind the scenes of military conflicts throughout time, there was a team or individual dedicated to the often difficult task of determining what the enemy was doing or planning. While it seems like an impossible effort to pull off, these individuals sometimes worked day and night, picking apart their enemy’s coded messages in a way that would be virtually impossible without constant advances in technology.

These individuals are known as cryptologists, and their job is a difficult one – intercept, study, and correctly understand the often-changing codes of enemy military powers. During the lead-up to their entry into World War II, much of the American efforts were geared towards cracking Japan’s code, especially as tensions started to rise between the two nations.

Purple Machine

System-97 Alphabetical Typewriter

Shortly before the outbreak of the war in the Pacific, the United States started to look into and decipher what was known as the Japanese Purple Machine. A more powerful variant of the German Enigma Machine—an encryption method that the Allies were able to crack early in the war—the Purple Machine was a System-97 Alphabetical Typewriter that Japan created in 1937.

Surpassing the Enigma Machine, Japan developed a new type of decryption method that was far more complicated than the Nazi device. Using four rotors and a switchboard, this one-man device was more convenient to use as it reduced human error from the two-person Enigma Machine, though its bulkier size made it difficult to transport.

If the machine was ever stolen, it would be impossible to decode without the accompanying secret key, but even that was changed daily. It seemed the Japanese had a foolproof means of keeping their diplomatic communications secure, but the Purple Machine’s switchboard and over 70 trillion key combinations weren't quite enough. By 1939, the United States had its eye on the Purple Machine and hired William Friedman, a well-known cryptography expert.

Frank Rowlett

After 18 months of working on the machine, Friedman suffered a mental breakdown. With the leader of the deciphering team in an institution, other cryptologists were brought in to continue his work. With Friedman’s notes in hand, the team—led by his assistant Frank Rowlett—created eight Purple Machine replicas and before long, the encryption method was unlocked. That still left determining the daily key, which was cleverly cracked by Lt. Francis A. Raven, who caught onto a pattern in the use of the keys.

With the Purple Machine unlocked, the United States had front line access to Japan’s Foreign Office communications. Sadly, it wasn’t enough, and despite having access to communication between Japan and its diplomatic missions around the world, no data collected was used to prepare for the attack on Pearl Harbor. The cryptologists watching the code and deciphering Japanese intelligence caught wind of a potential attack on the United States, but that Pearl Harbor was the target and December 7th, 1941 was the intended date never came to light. The Japanese were careful, despite their overconfidence in the Purple Machine, to not divulge too much information through communication.

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