Cryptonomicon

On Tom's laptop, each second of time is divided into seventy-five perfectly regular slices, during which a full grave-rubbing is performed followed by a vertical retrace interval. Randy can follow Pekka and Cantrell's conversation well enough to gather that they have already figured out, from analyzing the signals coming through the wall, that Tom Howard has his screen set up to give him 768 lines, and 1,024 pixels on each line. For every pixel, four bytes will be read from the video buffer and sent on down the line to the screen. (Tom is using the highest possible level of color definition on his screen, which means that one byte apiece is needed to represent the intensity of blue, green, and red and another is basically left over, but kept in there anyway because computers like powers of two, and computers are so ridiculously fast and powerful now that, even though all of this is happening on a timetable that would strike a human being as rather aggressive, the extra bytes just don't make any difference.) Each byte is eight binary digits or bits and so, 1,024 times a line, 4 x 8 = 32 bits are being read from the screen buffer.

Unbeknownst to Tom, his computer happens to be sitting right next to an antenna. The wires Pekka taped to the wall can read the electromagnetic waves that are radiating out of the computer's circuitry at all times.

Tom's laptop is sold as a computer, not as a radio station, and so it might seem odd that it should be radiating anything at all. It is all a byproduct of the fact that computers are binary critters, which means that all chip-to-chip, subsystem-to-subsystem communication taking place inside the machine-everything moving down those flat ribbons of wire, and the little metallic traces on the circuit boards--consists of transitions from zero to one and back again. The way that you represent bits in a computer is by switching the wire's voltage back and forth between zero and five volts. In computer textbooks these transitions are always graphed as if they were perfect square waves, meaning that you have this perfectly flat line at V 0, representing a binary zero, and then it makes a perfect right-angle turn and jumps vertically to V 5 and then executes another perfect right-angle turn and remains at fivevolts until it's time to go back to zero again, and so on.

This is the Platonic ideal of how computer circuitry is supposed to operate, but engineers have to build actual circuits in the grimy analog world. The hunks of metal and silicon can't manifest the Platonic behavior shown in those textbooks. Circuits can jump between zero and five volts really, really abruptly but if you monitor them on an oscilloscope, you can see that it's not a perfectly square wave. Instead you get some thing that looks like this:

The little waves are called ringing; these transitions among binary digits hit the circuitry like a clapper striking a bell. 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 Van Eck phreaking.

That's what Randy knows. When it comes to the details, Cantrell and Pekka are way out of his league, so after a few minutes he feels himself losing interest. He sits down on Cantrell's bed, which is the only place left tosit, and discovers a little palmtop computer on the bedside table. It is already up and running, patched into the world over a telephone wire. Randy's heard of this product. It is supposed to be a first stab at a network computer, and so it's running a Web browser whenever it is turned on; the Web browser isthe interface.