Dead or Alive - Tom Clancy [155]
“Which begs the question,” Jack said, “are the couriers carrying just messages, or onetime pads, too—if that’s what they’re using—”
Rounds interrupted. “Jack, bring everyone up to speed on this guy. …”
“Shasif Hadi,” Jack replied. “He was on an e-mail distribution list we’ve had our eye on. His ISP account wasn’t as well insulated as the others. We’re trying to peel back his financials. Whether that’ll lead to anything but which grocery store he shops at, I don’t know.”
“About the couriers,” Chavez said. “Doesn’t the FBI look at frequent travelers on the airlines? Any way of sorting a pattern that way? Find some link between URC e-mail traffic and travel patterns.”
Dominic answered this. “You have any idea how many people regularly hop the Atlantic? Thousands, and the Bureau’s looking at all of them. It’ll take a long time to check out as many as a quarter of them. It’s like reading through a phone book eight hours a day. And for all we know, the bastard’s sending his CD-ROMs by FedEx or even regular mail. A mailbox is a great place to hide something.”
Jerry Rounds’s laptop chimed, and he checked the screen. He read for a full minute, then said, “This complicates things.”
“What?” Jack said.
“We got an info dump from the Tripoli embassy thing. Ding inadvertently pocketed a flash drive from one of the tangos. The drive had a bunch of JPEG files on it.”
“Pictures of the Emir’s bolt-hole?” Brian asked.
“Not so lucky. The bad guys are upping their game. They’re using steganography.”
“Come again?”
“Steganography. Stego, for short. It’s a method of encryption—essentially, hiding a message inside an image.”
“Like invisible ink.”
“More or less, but it’s even older than that. In ancient Greece they used to shave a portion of a servant’s head, tattoo a message on the skull, then wait for the hair to grow back and send him through enemy lines. Here we’re talking about digital pictures, but the concept is the same. See, a digital image is nothing more than a whole bunch of colored dots.”
“Pixels,” Chavez offered.
“Right. Each pixel is assigned a number—a red, blue, and green value, usually ranging from zero to two fifty-five, depending on the intensity. Each of these are, in turn, stored in eight bits, starting at one twenty-eight and jumping down to one, halving as they go, so one twenty-eight to sixty-four to thirty-two, and so on. A difference in one or two or even four in the RGB value is imperceptible to the human eye—”
“You’re losing me,” Brian said. “Bottom-line it.”
“You’re essentially hiding characters inside a digital photo by slightly altering its pixels.”
“How much information?”
“Say, a six forty by four eighty image … half a million characters, give or take. A good-sized novel.”
“Damn,” Chavez muttered.
“That’s the hell of it, though,” Jack said. “If they’re using stego, they’re probably smart enough to keep the messages short. We’re talking about a dozen or so altered pixels in an image containing millions. It’s the proverbial needle in a haystack.”
“So how hard is it to do the encoding?” Chavez asked. “Any way we can track it that way?”
“Not likely. There are tons of shareware and freeware programs out there that can do it. Some are better than others, but it’s not a specialized thing. Doesn’t have to be, when only the sender and recipient have the decryption key.”
“How about pulling the messages out? Can it be done? What’s that involve?”
Rounds answered, “It’s essentially reverse-engineering each image—deconstructing it, figuring out which pixels have been altered and by what amount, then pulling out the message.”
“This sounds right up the NSA’s alley,” Brian said. “Can we tap—”
“No,” Rounds replied. “Love to, believe me, but intercepting their traffic is one thing. Trying to hack into their systems is another. Anyway, we might not need something that strong. Jack, are there commercial programs out there?”
“Yeah, but whether they’ve got the horsepower we need, I don’t know. I’ll start looking around. If nothing else,