Cryptonomicon

Waterhouse draws another circle-A in Manila itself, one in Tokyo, one in Rabaul, one in Penang, one in the Indian Ocean.

"What's that?" Comstock asks.

"We picked up an Azure transmission from a German U-boat here," Waterhouse says.

"How do you know it was a German U-boat?"

"Recognized the fist," Waterhouse says. "So, this is the spatial arrangement of Azure transmitters--not counting the stations in Europe that are making Pufferfish transmissions, and hence, according to Assertion One, are part of the same network. Anyway, now let us say that an Azure message originates from Tokyo on a certain date. We don't know what it says, because we haven't broken Azure yet. We just know that the message went out to these places." Waterhouse draws lines radiating downward from Tokyo to Manila, Rabaul, Penang. "Now, each one of these cities is a major military base. Consequently, each is the source of a steady stream of traffic, communicating with all of the Nipponese bases in its region." Waterhouse draws shorter lines radiating from Manila to various locations in the Philippines, and from Rabaul to New Guinea and the Solomons.

"Correction, Waterhouse," Comstock says. "We own New Guinea now."

"But I'm going back in time!" Waterhouse says. "Back to 1943, when there were Nip bases all along the north coast of New Guinea, and through the Solomons. So, let us say that within a brief window of time following this Azure message from Tokyo, a number of messages are transmitted from places like Rabaul and Manila to smaller bases in those areas. Some of them are in ciphers that we have learned how to break. Now, it is not unreasonable to suppose that some of these messages were sent out as a consequence of whatever orders were contained in that Azure message."

"But those places send out thousands of messages a day," Comstock protests. "What makes you think that you can pick out the messages that are a consequence of the Azure orders?"

"It's just a brute force statistics problem," Waterhouse says. "Suppose that Tokyo sent the Azure message to Rabaul on October 15th, 1943. Now, suppose I take all of the messages that were sent out from Rabaul on October 14th and I index them in various ways: what destinations they were transmitted to, how long they were, and, if we were able to decrypt them, what their subject matter was. Were they orders for troop movements? Supply shipments? Changes in tactics or procedures? Then, I take all of the messages that were sent out from Rabaul on October 16th--the day after the Azure message came in from Tokyo--and I run exactly the same statistical analysis on them."

Waterhouse steps back from the chalkboard and turns into a blinding fusillade of strobe lights. "You see, it is all about information flow. Information flows from Tokyo to Rabaul. We don't know what the information was. But it will, in some way, influence what Rabaul does afterwards. Rabaul is changed, irrevocably, by the arrival of that information, and by comparing Rabaul's observed behavior before and after that change, we can make inferences."

"Such as?" Comstock says warily.

Waterhouse shrugs. "The differences are very slight. They hardly stand out from the noise. Over the course of the war, thirty-one Azure messages have gone out from Tokyo, so I have that many data sets to work with. Any one data set by itself might not tell me anything. But when I combine all of the data sets together--giving me greater depth--then I can see some patterns. And one of the patterns that I most definitely see is that, on the day after an Azure message went out to, say, Rabaul, Rabaul was much more likely to transmit messages having to do with mining engineers. This has ramifications that can be traced all the way back until the loop is closed."

"Loop is closed?"

"Okay. Let's take it from the top. Azure message goes from Tokyo to Rabaul," Waterhouse says, drawing a heavy line down the chalkboard joining those two cities. "The next day, a message in some other crypto system--one that we have broken--goes from Rabaul to a submarine operating out of a base here, in the Moluccas. The message states that the submarine is to proceed to an outpost on the north coast of New Guinea and pick up four passengers, who are identified by name. From our archives, we know who these men are: three aircraft mechanics and one mining engineer. A few days later, the submarine transmits from the Bismarck Sea stating that it has picked those men up. A few days after that, our waterfront spies in Manila inform us that the same submarine has showed up there. On the same day, another Azure message is transmitted from Manila back up to Tokyo," Waterhouse concludes, adding a final line to the polygon, "closing the loop."

"But that could all be a series of random, unconnected events," says one of Comstock's math whizzes, before Comstock can say it. "The Nips are desperate for aircraft mechanics. There's nothing unusual about this kind of message traffic."