Prisoners of Tomorrow Read Free Page A

problems to their designers. Speed is important, of course, and so is safety, which means essentially the same in communications as it does on highways: what arrives at a destination should bear as close a resemblance as possible to whatever left the departure point.
    Also important in both fields is using system resources efficiently, which means avoiding situations in which some channels become choked while others are not being used at all. Thus, morning commuters seek out alternative routes for getting to work, which spreads traffic out over all the available roads, to come together at a common destination. A standard technique for sending large files of information from one computer to another through a communications network works the same way. The sending computer breaks the file up into data “packets,” which follow different routes through the network to the destination, with different computers along the way deciding from moment to moment which way to route any given packet, depending on the conditions at the time.
    To guard against errors due to interference, equipment faults, or other causes, the computer at the sending end uses the data content of a packet to compute a mathematical function known as a “checksum,” which it sends along with the message. The receiving computer performs the same calculation on what should be the same data and compares its checksum with the one that has been sent. If they match, then the message is clear; or more precisely, the chances against it are astronomically remote.
    Raymond went on, “We figured out a way to transmit Magician’s Tangerine file down, using the Russians’ own Earthlink. Basically the idea was very simple: rig the packets to carry a bad checksum, which means that the receiving Soviet groundstation throws them out as garbage. But NSA is watchlisting the mismatches. Get it?”
    Paula was already nodding and smiling faintly. It was neat. When the checksums failed to match, the receiving computer would simply assume that the message it had assembled was corrupted, disregard it, and signal for a repeat transmission. What Colonel Raymond was saying was that the checksums for the packets containing the Tangerine file would be deliberately miscalculated. Therefore they would, in effect, be invisible to the message-processing computers at the Soviet groundstation. But the computers in the US National Security Agency’s receiving posts in Japan, Australia, Britain, and elsewhere, which eavesdropped on the Soviets’ communications all the time—and just about everyone else’s, too, for that matter—would be programmed to look for just those mismatches. Thus they would be able to intercept the information that the Soviet system ignored. (It went without saying that it would be a simple matter to abort the retransmission attempts for each packet after a couple of tries, to avoid getting the system into a loop that would otherwise go on forever.)
    “Tricky, though,” Paula said. “Magician would have to get inside the communications center up there.”
    “He was a maintenance supervisor,” Raymond reminded her. “That part was okay.”
    “Yes, but it would involve actually getting into the system software somehow, and tampering with it. Was that really Magician’s field?”
    Foleda gave a heavy sigh. “You’ve hit it, right on the nail.”
    Paula glanced around quickly. “I take it from the way we’re talking that this didn’t work out.”
    “We worked with what we had,” Foleda said. “Magician wasn’t an expert on Soviet software. But we got the job down to what seemed like a straightforward procedure, and he was confident he could hack it. . . . But something went wrong. He got caught. The last we heard he was back in Moscow—in the Lubyanka jail.”
    “The Tangerine file wasn’t transmitted?” Paula said.
    Foleda shook his head. “Nothing ever came through.”
    “Presumably they got him first,” Kehrn said, still below the wallscreen.