Cryptonomicon

But that is not what it looks like now. There is hardly a piece of land within half a mile that has not been recently paved or built upon. Once you get beyond the Mansion and its quaint little outbuildings, the park consists of one-story brick structures, nothing more than long corridors with multiple transepts: +++++++, and new +'s being added as fast as the masons can slap bricks on mud (Waterhouse wonders, idly, whether Rudy has seen aerial reconnaissance photos of this place, and deduced from all of those +'s the mathematical nature of the enterprise). The tortuous channels between buildings are narrow, and each is made twice as narrow by an eight-foot-high blast wall running down the middle of it, so that the Jerries will have to spend at least one bomb for each building.

"In that building there," Chattan says, pointing to a small building not far away--a truly wretched-looking brick hovel--"are the Turing Bombes. That's 'bombe' with an 'e' on the end. They are calculating machines invented by your friend the Prof."

"Are they true universal Turing machines?" Waterhouse blurts. He is in the grip of a stunning vision of what Bletchley Park might, in fact, be: a secret kingdom in which Alan has somehow found the resources needed to realize his great vision. A kingdom ruled not by men but by information, where humble buildings made of + signs house Universal Machines that can be configured to perform any computable operation.

"No," Chattan says, with a gentle, sad smile.

Waterhouse exhales for a long time. "Ah."

"Perhaps that will come next year, or the next."

"Perhaps."

"The bombes were adapted, by Turing and Welchman and others, from a design dreamed up by Polish cryptanalysts. They consist of rotating drums that test many possible Enigma keys with great speed. I'm sure the Prof will explain it to you. But the point is that they have these vast pegboards in the back, like telephone switchboards, and some of our girls have the job of putting the right pegs into the right holes and wiring the things up every day. Requires good eyesight, careful attention, and height."

"Height?"

"You'll notice that the girls who are assigned to that particular duty are unusually tall. If the Germans were to somehow get their hands on the personnel records for all of the people who work at Bletchley Park, and graph their heights on a histogram, they would see a normal bell shaped curve, representing most of the workers, with an abnormal bump on it--representing the unusual population of tall girls whom we have brought in to work the plug boards."

"Yes, I see," Waterhouse says, "and someone like Rudy--Dr. von Hacklheber--would notice the anomaly, and wonder about it."

"Precisely," Chattan says. "And it would then be the job of Detachment 2702--the Ultra Mega Group--to plant false information that would throw your friend Rudy off the scent." Chattan turns away from the window, strolls over to his desk, and opens a large cigarette box, neatly stacked with fresh ammunition. He offers one to Waterhouse with a deft hand gesture, and Waterhouse accepts it, just to be social. As Chattan is giving him a light, he gazes through the flame into Waterhouse's eye and says, 'I put it to you now. How would you go about concealing from your friend Rudy that we had a lot of tall girls here?'

"Assuming that he already had the personnel records?"

"Yes."

"Then it would be too late to conceal anything."

"Granted. Let us instead assume that he has some channel of information that is bringing him these records, a few at a time. This channel is still open and functioning. We cannot shut it down. Or perhaps we choose not to shut it down, because even the absence of this channel will tell Rudy something important."

"Well, there you go then," Waterhouse says. "We gin up some false personnel records and plant them in the channel."

There is a small chalkboard on the wall of Chattan's office. It is a palimpsest, not very well erased; the housekeeping detail here must have a standing order never to clean it, lest something important be lost. As Waterhouse approaches it, he can see older calculations layered atop each other, fading off into the blackness like transmissions of white light propagating into deep space.

He recognizes Alan's handwriting all over the place. It takes a physical effort not to stand there and try to reconstruct Alan's calculations from the ghosts lingering on the slate. He draws over them only with reluctance.

Waterhouse slashes an abscissa and an ordinate onto the board, then sweeps out a bell-shaped curve. On top of the curve, to the right of the peak, he adds a little hump.

"The tall girls," he explains. "The problem is this notch." He points to the valley between the main peak and the bump. Then he draws a new peak high and wide enough to cover both:

"We can do that by planting fake personnel records in Rudy's channel, giving heights that are taller than the overall average, but shorter than the bombe girls."