“Once we have the command structures all mapped out, everything becomes maths. You have a chart, neat circles and arrows pointing at each other, showing the information cascade. Who can argue with math? Numbers don’t lie. Having figured out their command structures from their chat rooms, we were able to map them over to their mobile communications, using the session identifiers the algorithm worked out.
“These twerps were half-smart, just enough to be properly stupid. They’d bought burner phones from newsagents with prepaid SIMs and they only used them to call each other. People who try that sort of thing, they just don’t understand how data-mining works. When I’ve got a visualization of all the calls in a country, they’re mostly clustered in the middle, all tangled up with one another. You might call your mum and your girlfriend regular, might call a taxi company or the office a few times a week, make the odd call to a takeaway. Just looking at the vis, it’s really obvious what sort of number any number is: there’s the ‘pizza nodes,’ connected to hundreds of other nodes, obviously takeaways or minicabs. There’s TKs—telephone kiosks, which is what we call payphones—they’ve got their own signature pattern: lots of overseas calls, calls to hotels, maybe a women’s shelter or A&E, the kinds of calls you make when you don’t have a mobile phone of your own.
“It makes detecting anomalies dead easy. If a group of people converge on a site, turn off their phones, wait an hour and then turn ’em on again, well, that shows up. You don’t have to even be looking for that pattern. Just graph call activity, that sort of thing jumps straight out at you. Might as well go to your secret meeting with a brass band and a banner marked UP TO NO GOOD.
“So think of the network graph now, all these nodes, most with a few lines going in and out, some pizza nodes with millions coming in and none going out, some TKs with loads going out and none coming in. And over here, off to the edge, where you couldn’t possibly miss it, all on its own, a fairy ring of six nodes, connected to each other and no one else. Practically a bullseye.
“You don’t need to be looking for that pattern to spot it, but the lads from the uni and their GCHQ minders, they knew all about that pattern. Soon as they saw one that the persistence algorithm mapped onto the same accounts we’d seen in the chat rooms, they started to look at its information cascades. Those mapped right onto the cascade analysis from the chat intercepts, same flows, perfect. Course they did—because the kid who told the best jokes was the most sociable of the lot, he was the one who called the others when they weren’t in the chat, desperate for a natter.”
I stopped him. “Thinking of your example of a group of phones that converge on a single location and all switch off together,” I said. “What about a group of friends who have a pact to turn off their phones whilst at dinner, to avoid distraction and interruption?”
He nodded. “Happens. It’s rare, but ’course, not as rare as your actual terrorists. Our policy is, hard drives are cheap, add ’em all to long-term retention, have a human being look at their comms later and see whether we caught some dolphins in the tuna-net.”
“I see.”
“We have their ‘command structure,’ we have their secret phone numbers, so the next step is to have a little listen, which isn’t very hard, as I’m sure you can appreciate, Mr. Holmes.”
“I make it a policy never to say anything over a telephone that I would regret seeing on the cover of the Times the next morning.”
“A good policy, though one that I think I might have a hard time keeping myself,” I said, thinking of the number of times my poor Mary and I had indulged ourselves in a little playful, romantic talk when no one could hear.
“Watson, if you find yourself tempted to have a breathy conversation with a ladyfriend over your mobile, I suggest you cool your ardor by contemplating the number of my brother’s young and impressionable associates who doubtlessly personally review every call you make. You’ve met my brother on a few occasions. Imagine what sort of man he would surround himself with.”
I shuddered. I had no interest in women at that time, and memory of Mary was so fresh and painful that I couldn’t conceive of a time when that interest would return. But I had cherished the memories of those silly, loving, personal calls, times when it had felt like we were truly ourselves, letting the pretense fall away and showing each other the truth behind our habitual masks. The thought that those calls had been recorded, that someone might have listened in on them—“just to check” and make sure that we weren’t up to no good . . . It cast those cherished memories in a new light. I wouldn’t ever be able to think of them in the same way again.
I was sure that Holmes had intuited my train of thought. He always could read me at a glance. He held my eye for a long moment and I sensed his sympathy. Somehow that made it worse.