A whole section of Arlington Hall was devoted to “protective security”—and this section, like others, was staffed mostly by women, many of them WACs. The women operated the SIGABA machines, which were America’s version of the Enigma. The SIGABA was initially conceived by William Friedman to encipher U.S. Army traffic, its design then improved by Frank Rowlett; the reason it was never as famous as the German Enigma was because unlike the German Enigma, it was never cracked. The simple reason it was never cracked was because Rowlett designed it so well (also it was heavier than Enigma, not quite so portable, and did not get overused as Enigma did). But it was also thanks to the care and competence of the people who used it. The WACs at Arlington Hall produced manuals instructing field soldiers how to use the machine; they maintained the Arlington Hall SIGABAs and tested them to make recommendations for operational and mechanical changes. They monitored their use to see if operators in the field were violating security. (The United States was rather notorious for poor radio security, from which the Germans learned a lot, so it was a ceaseless job.) They closely studied American message traffic, looking for “cryptographic error.” They cryptanalyzed American traffic, to see how easy or hard it was to break. They looked for code-room errors and insecure practices.
Today all of this would be known as “communications security,” and it went far beyond sending encrypted messages. One unit of women bird-dogged the American military units to make sure their radio traffic did not reveal too much about their whereabouts. The women intently studied the flow of U.S. military traffic to make sure that the Allies were not revealing the kinds of things that the enemy was revealing to them. They made charts and graphs to study American communications in specific regions, at specific times, during specific conflicts and events, to see what—if anything—might have been disclosed to the enemy. And they studied the characteristics of certain circuits.
These same skills enabled the WACs to create dummy traffic: fake radio traffic that so exactly resembled real American traffic that it persuaded the enemy that the fictitious units existed and were on the move. Such transmissions were useful for concealing military as well as political movements, and not just during the Normandy invasion. Exactly as Churchill described, the effect was to create a bodyguard, or protective area, around troops or leaders. In the Pacific theater, when a real attack was planned against Guam and Saipan, Arlington Hall created fake traffic to divert Japanese attention to Alaska. They created fake traffic to enable the deployment of the Fifth Infantry Division from the Iceland Base Command to the United Kingdom. They used fake traffic to disguise movements to and from the Yalta Conference.
The women analyzed Allied traffic, so as to be able to convincingly re-create a fake version of it. The calculations unit worked to determine “the various circuit characteristics, such as group-count frequency distribution, percentage in each precedence and security classification, filing time distributions, address combination, and cryptonets employed for each station of which the traffic is to be manipulated,” as one report put it.
In order to create fake traffic, the women had to understand every last thing about the real traffic that went out, and the circuits and stations it traveled through. Once created, fake traffic had to be routed. The women had to create a plausible schedule; release prearranged dummy traffic to be transmitted during times when such traffic might be expected; maintain circuit flow; and monitor what went out. They had to understand the circuits, the call signs, the frequency, the peak volume times: everything.
Meanwhile, the women also were conducting analysis of German radio traffic. “They were really pushing things very hard at that time, all the way around,” recalled Ann Brown, a WAC working in traffic analysis at Arlington during the Normandy invasion.
The bogus force included a dummy landing-craft tank, a fake headquarters, and two assault forces with associated ships and craft. The Allies began sending out dummy traffic months before the D-Day landing, as Patton’s fictitious Army traveled around England and began to gather. Meanwhile, the double agents were hard at work communicating with Germany, ably encouraging the notion that FUSAG was poised to assault the Pas de Calais.
The Purple machine delivered the happy news that the deception plan had worked. On June 1, 1944, the tireless and always obliging Baron Oshima crafted a message to Tokyo and sent it over the Purple circuit. The encrypted message revealed that Hitler, anticipating an Allied invasion, expected that diversionary landings would take place in Norway and Denmark and on the French Mediterranean coast.
Oshima added—and this was crucial; this was exactly what the Allies had hoped—that the Führer expected the real Allied attack, when it came, to come sailing through the Strait of Dover, toward the Pas de Calais.
“No invasion tonight,” thought Wellesley’s Ann White as she walked along the peaceful streets of northwest Washington late in the evening of June 5, 1944. It was warm in the capital by this time of year, and rosebuds were nearing bloom in the manicured neighborhood where the code-breaking facility was located. It was nearing midnight as Ann, wearing her uniform, turned off Nebraska Avenue and entered the Naval Communications Annex compound. She passed the now familiar double line of Marine guards, showed her badge, saluted the men as she went past, and made her way into the rooms that held the German Enigma section, where she relieved the officer heading the evening watch. Ann was a watch officer, now—a lieutenant—and supervised the midnight watch of women reading Enigma traffic. She had trained on the pistol range and knew how to shoot the gun that was always kept lying on the watch officer’s desk. The room was so secure that buzzers controlled the entry, and people wore badges displaying the level of confidential material they could access.
The women working in the Enigma unit knew an invasion of France was in the offing, though those elsewhere in the Annex did not. The Enigma team had known this for several days and had been told to keep their mouths shut. The bombes were pounding full force to break the German traffic, and an air of suspense and tension hung heavy as the women waited, wondering when the landing would occur. It could happen anytime. Tonight, however, Ann had looked up and taken note of the gorgeous full moon as she walked into the unit, which was known as OP-20-GY-A-1.
Surely, she thought, the night would be too bright for a surprise landing across the English Channel.
But then, not quite two hours into her shift, her team received an intercepted message that suggested differently. “At 0130, messages on coastal circuits were translated which gave news to those U-boats of the invasion of France,” noted the log for the unit. There was a flurry of rapt activity as the women ran the message through the M-9, to get the text. Ann, knowing German, was able to read the words even before the Enigma message was taken upstairs and translated into English. It was a terse message, sent by central command to all U-boats on the circuit. “Enemy landing at the mouth of the Seine.” All up and down the coast of France, the Enigma machines were chattering, spitting out the same warning. “Enemy landing at the mouth of the Seine.” And here in northwest Washington, the women read the words as quickly as the U-boat crews themselves.