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Stealing the Japanese Code

Converted for the Web from "Battle Of Wits: The Complete Story Of Codebreaking In World War II" by Stephen Budiansky

Rochefort's first dose of cryptanalysis left him decidedly disinclined for another. It was not that there was any particular pressure on him to produce results. No one in the Navy had much of an idea what he was up to anyway, and no one would have understood it if he had. But the work had a way of generating its own compulsive pressures. Rochefort would come home every evening at five or six o'clock with his stomach in knots from the tension of the problem he was tackling. It would be eight or nine at night before he could manage to force down his supper. He developed an ulcer and greeted his recall to sea duty in 1927 with unfeigned relief.

But in those two years Rochefort scored America's first victory in a long shadow war with the Japanese Navy. Left over from 1918 was most of a $100,000 secret naval intelligence slush fund. To conceal it from Congress, the money was deposited in a Washington bank in a personal account belonging to the Director of Naval Intelligence. Whenever a new DNI took over, his predecessor just handed the money over to him along with the keys to the office. The money had begun to burn a hole in the pockets of successive DNIs, and in the early 1920s the incumbent decided to get rid of some of it by financing a series of break-ins at the Japanese consulate in New York City. The Japanese Navy's "Red" code book was secretly photographed and, over the course of several years, laboriously translated by linguists hired with more of the DNI's secret funds. (Just how hard it was to use up $100,000 was shown in 1931, when an acting DNI, in a fit of conscience for which his successors never forgave him, returned the money to the Treasury. The balance was $65,000.)

A complete code book was a windfall, but there was still one crucial piece missing. Like almost all of the Japanese Navy codes that Rochefort and his colleagues would encounter over the course of their long battle of wits with their Japanese counterparts, Red was an enciphered code. Every word or syllable likely to be used in a message was assigned a numerical value -- that was the "code" part. But such a simple one-for-one substitution would not hold up a team of Boy Scouts, much less a determined military foe, for very long. So before the Japanese Navy sent any coded message over the airwaves, it was given a second disguise. The code clerk opened a second book, which contained page after page of random numbers; starting at the top of a page, he added the first of these random "additives" to the first code group of his message, the second to the second, and so on. An indicator buried in the message would tell what page in the additive book he had used for this "encipherment" of the basic code, so that the recipient could turn to that same page and strip off the additive before looking up the meaning of each code group.

Thanks to the DNI's black-bag jobs, Rochefort had the code book. What he did not have was the additive book. To make matters worse, the Japanese changed the additive book frequently. With nothing to go on but the raw traffic that the Japanese Navy put out over the airwaves, Rochefort's job was to reproduce an additive book that he had never seen.

Breaking a code when one has the underlying code book but no additive book is like finding a way across a strange country without a map or a compass. Breaking a code when one has neither code book nor additive book is like finding a way across a strange country with both eyes closed. Doing the former was what had given Rochefort his ulcer in 1926. His task in 1942 was to do the latter.

Copyright © 2000 by Stephen Budiansky. All rights reserved. Converted for the Web with the permission of Simon & Schuster.

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