People often imagine that espionage and Internet security involve rapid, endless cycles of code-craft. Security experts cook up secret codes. Clever adversaries soon break them. New codes are then needed.

But in computers today, good codes (i.e., encryption schemes) are actually strong, time-tested systems. Clever adversaries usually don’t even try to break them: They instead go around them. Sometimes they can learn secrets by exploiting what are known as “side channels.” Even the strongest encryption systems can succumb.

Here’s an analogy from everyday life. Your roommate, who is applying to graduate school, receives a letter from a prestigious university. You find this letter unopened on a table by your front door. (Envelopes are like encryption: They conceal messages). Now if you are the snooping type, you may want to know whether it’s an acceptance or rejection. But of course you don’t want to tear open the envelope.

So what do you do? If you were a government agent, you might use an x-ray machine to penetrate the envelope. If your resources are limited to a pedestrian set of shady skills, you might try to steam open the envelope. But there’s an easier approach. You can simply palpate the envelope to determine whether it’s thick (acceptance—forms to return) or thin (rejection—curt note). The thickness of the envelope is a side channel.

More sophisticated is the “pizza index” side-channel in Washington D.C.. Pizza deliverers in Washington D.C. have purportedly seen surges in pizza orders just before a major political or military events. In 1991, the L.A. Times reported that, “The one-night record for late-night deliveries at the CIA—21 pizzas—was set Aug. 1 [1990], the night before Iraq invaded Kuwait.” The CIA seems subsequently to have learned the discipline of pizza silence. The night before the Gulf War began, it didn’t receive any pizzas. The Pentagon, on the other hand, may have spelled out its intentions that night in red sauce…

In Tetraktys, the NSA uses side channels based on Internet routing delays in its efforts to locate the Pythagoreans.