Then the code system changed again, and this change was even bigger. On June 1, 1939, the Japanese fleet began using a code that the Allies came to call JN-25. The Japanese—who had moved to using numbers rather than characters—now employed a massive codebook containing about thirty thousand five-digit groups. They also had a new way of enciphering. Before the code was sent, each code group was enciphered by using math to apply an “additive.”
Here is how the additive method worked: When a Japanese cryptographer began encoding a single message, he would look in the codebook and find the five-digit group that stood for the word (or syllable or phrase or punctuation mark) he wanted. He would repeat that process until he got to the end of the message. Then he would get out a different book, called an additive book, turn to a page—selected at random—pick a five-digit number, and add that to the first code group. He would add the next additive to the second. And so on. The Japanese code makers used a peculiar kind of math called noncarrying or “false” addition. There was no carrying of digits, so 8 plus 7 would equal 5, rather than 15. If the code group for “maru” was, say, 13563, and the additive was 24968, the resulting group would be 37421 (1 + 2 =3; 3 + 4 = 7; 5 + 9 = 4; 6 + 6 = 2; 3 + 8 = 1). That was the group of digits that would be radioed. To crack a message, the Americans had to figure out the additive and subtract it to get the code group. Then they had to figure out what the code group stood for.
Once again, it was Agnes Driscoll who diagnosed the new system. Neither she nor anybody in the Navy operation had seen an additive cipher—everything up to then had been transposition, or switching—but she figured it out. It took her less than a year to make a dent. A March 1 status report for the unit “GYP-1” stated that for the “5-number system”—an early title for JN-25—“First break [was] made by Mrs. Driscoll. Solution progressing satisfactorily.” She worked on it for several more months before being transferred in late 1940 to German systems—a promotion in the sense that the Atlantic was beginning to emerge as the hot spot. The research team continued working their way through JN-25, using her methods.
The process of stripping additives and discerning the meaning of code groups was laborious and excruciating. Years after World War II ended, American code breakers who worked in Hawaii and Australia were still arguing with their D.C. counterparts over what certain code groups stood for. Much like the women who trained the men who would get to do the wartime flying, much like Elizebeth Friedman over at the Coast Guard, Agnes Driscoll taught the men in the field who did this. “In the Navy she was without peer as a cryptanalyst,” wrote Edwin Layton, who headed naval intelligence for Admiral Nimitz, the chief naval commander in the Pacific during the war. In December 1940, both code and cipher were changed, to a system the Allies called JN-25B; the team stripped the additives and built a partial bank of code words. Then, in early December 1941—days before Pearl Harbor—the additive books were changed. The codebooks were not. The U.S. Navy was able to recover a certain amount of the new system—but not enough—before the attack on Pearl Harbor happened and all hell broke loose.
“If the Japanese Navy had changed the codebook along with the cipher keys on 1 December 1941, there is no telling how badly the war in the Pacific would have gone,” said Laurance Safford.
As crushing as Pearl Harbor was, it was thanks in large part to Driscoll’s decades-long detective work—and to the example Elizebeth Friedman set for other women—that America did not enter the Second World War quite as blind as it might have seemed.
CHAPTER THREE
The Most Difficult Problem
September 1940
Poland was occupied and had been for a year. Czechoslovakia had put up little or no resistance to being partitioned. The Nazi war machine had overrun Norway and Denmark, defeated Belgium and so many others, and proceeded to march into Paris, where Nazi officers were drinking café au lait and popping champagne corks in the finest restaurants on the Champs-élysées. England was holding out, but barely, pummeled by German air raids and braced for an anticipated sea invasion launched from the shores of occupied France. Japan was shouldering its way through China and around the Pacific, chasing resources and seeking to establish a “new order” in which Asian nations would be rescued from Western domination and dominated, instead, by Japan. And here—in crowded U.S. Army offices in downtown Washington—a young civilian woman was patiently standing, waiting for a group of men to stop talking and notice her. She had something urgent she needed to tell them, but, shy and reluctant to interrupt, she waited for a pause.
The office was nothing to brag about—just a few rooms tucked away in a Washington eyesore known as the Munitions Building, erected in 1918 as a headquarters for the War Department. The Munitions Building and the U.S. Navy headquarters were side by side, as it happened, both constructed as “temporary” wartime structures during World War I and both still in service even now that the Great War was long over, together dominating the part of the capital city between Foggy Bottom and the National Mall. The twin buildings had concrete facades and a series of thin wings that stretched backward, perpendicular to the facade. Working in them was like working in a multistory warehouse.
On the first floor, the wings of the buildings had long narrow corridors and doors through which bicycle messengers often burst, so people had to be careful not to be knocked down by bag-wielding boys on bikes. On upper floors, wooden desks lined open workrooms, and large windows admitted the Washington air, for better or worse, depending on the season and humidity level. Technically the whole edifice was known as the Main Navy and Munitions Building. Washington’s physical wartime expansion had not—yet—been effected; the Pentagon did not yet exist, nor had Arlington Hall been requisitioned, and in 1940 these two squat ugly buildings housed, in effect, the country’s entire military brain trust.