Cryptonomicon

by Neal Stephenson

The voltage jumps, but after it jumps it oscillates back and forth around the new value for a little while. Whenever you have an oscillating voltage in a conductor like this, it means that electromagnetic waves are propagating out into space. Consequently each wire in a running computer is like a little radio transmitter. The signals that it broadcasts are completely dependent upon the details of what’s going on inside the machine. Since there are a lot of wires in there, and the particulars of what they are doing are fairly unpredictable, it is difficult for anyone monitoring the transmissions to make head or tail of them. A great deal of what comes out of the machine is completely irrelevant from a surveillance point of view. But there is one pattern of signals that is (1) totally predictable and (2) exactly what Pekka wants to see, and that is the stream of bytes being read from the screen buffer and sent down the wire to the screen hardware. Amid all the random noise coming from the machine, the ticks of the horizontal and vertical retrace intervals will stand out as clearly as the beating of a drum in a teeming jungle. Now that Pekka has zeroed in on that beat, he should be able to pick up the radiation emanating from the wire that connects screen buffer to video hardware, and translate it back into a sequence of ones and zeroes that can be dumped out onto their own screen. They will be able to see exactly what Tom Howard sees, through the kind of surveillance called ...