Magic (cryptography) - Decryption Process

Decryption Process

There were several prior steps needed before any decrypt was ready for distribution:

1. Interception. The Japanese Foreign Office used both wireless transmission and cables to communicate with its off shore units. Wireless transmission was intercepted (if possible) at any of several listening stations (Hawaii, Guam, Bainbridge Island in Washington state, Dutch Harbor on an Alaska island etc.) and the raw cypher groups were forwarded to Washington, DC. Eventually, there were decryption stations (including a copy of the Army's PURPLE machine) in the Philippines as well. Cable traffic was (for many years before late 1941) collected at cable company offices by a military officer who made copies and started them to Washington. Cable traffic in Hawaii was not intercepted due to legal concerns until David Sarnoff of RCA agreed to allow it during a visit to Hawaii the first week of December 1941. At one point, intercepts were being mailed to (Army or Navy) Intelligence from the field.
2. Deciphering. The raw intercept was deciphered by either the Army or the Navy (depending on the day). Deciphering was usually successful as the cipher had been broken.
3. Translation. Having obtained the plain text, in Latin letters, it was translated. Because the Navy had more Japanese-speaking officers, much of the burden of translation fell onto the Navy. And because Japanese is a difficult language, with meaning highly dependent upon context, effective translation required not only fluent Japanese, but considerable knowledge of the context within which the message was sent.
4. Evaluation. The translated decrypt had to be evaluated for its intelligence content. For example, is the ostensible content of the message meaningful? If it is, for instance, part of a power contest within the Foreign Office or some other part of the Japanese government, its meaning and implications would be quite different from a simple informational or instructional message to an Embassy. Or, might it be another message in a series whose meaning, taken together, is more than the meaning of any individual message. Thus, the fourteenth message to an Embassy instructing that Embassy to instruct Japanese merchant ships calling at that country to return to home waters before, say, the end of November would be more significant than a single such message meant for a single ship or port. Only after having evaluated a translated decrypt for its intelligence value could anyone decide whether it deserved to be distributed.

In the period before the attack on Pearl Harbor, the material was handled awkwardly and inefficiently, and was distributed even more awkwardly. Nevertheless, the extraordinary experience of reading a foreign government's most closely held communications, sometimes even before the intended recipient, was astonishing. It was so astonishing, someone (possibly President Roosevelt) called it magic. The name stuck.