generated a different hash value. It didn’t make sense.
Carter wondered if opening an image in a photo-editing software program, such as Photoshop, could have altered the pixels in some way. They tested Carter’s theory, but without success. This was shaping up to be the sort of outlier Marvin Pressman would have jumped all over. It was the sort of curiosity that demanded an explanation.
Rainy and Carter worked late in the Lair trying to solve what was shaping up to be an unsolvable puzzle.
Tomlinson showed up an hour later. “Agent Miles, I need you to do a PowerPoint presentation for me,” he said.
Rainy groaned. Years ago she had made the tragic mistake of demonstrating to Tomlinson her mastery of PowerPoint. The ability to make effective slides was a skill management coveted.
“When do you need it, sir?” Rainy asked.
“Yesterday.”
“What about this evening? By eight?”
“Why? What do you got going on here?”
“We’re trying to figure out why the images don’t generate identical hash values. And we’re not having much luck.”
“Is it important?”
“Yes, I believe it is, sir.”
“In that case, eight will be fine.”
Tomlinson left. Rainy and Carter returned to their work.
“Can you magnify this one?” she said. She pointed to a copy of Lindsey Wells’s picture, one of the many copies that had begun populating the Web soon after she’d texted it to Tanner.
Carter magnified the image three hundred times. Rainy kept staring at the screen.
“What are you looking for?” asked Carter.
“Something I noticed when Clarence Stern was helping me ID the Lindsey Wells photograph.”
“And that something would be?”
“He saw things at a high magnification level. Just by looking at the color gradation, he was able to add missing pixels to form a complete image. You can see it only when the image is magnified.”
“It just looks like a bunch of colored squares,” Carter said.
“But there’s a smoothness to how those squares are stacked together. That smoothness is the logical next color variant to complete the picture. It’s how Clarence was able to guess which pixels were missing.”
“Are you looking for that same smoothness on this image?” asked Carter. He’d magnified the image so that all Rainy could see were rows and columns of colored blocks no more than an inch tall and wide.
“I’m looking for the out of the ordinary,” said Rainy. “Something that shouldn’t be there. Something we can’t easily see with our eyes. Look. There.” Rainy pointed to a section of the image. “The squares here go from light to dark without any gradation,” she said. “It’s jarring. It happens almost too quickly. Can you show me the same section, same magnification, but for a different image? I want to compare them.”
Carter did, and Rainy saw it right away. “We’ve got the same jarring transition in the same section of both images,” she said.
“The unusual shading pattern looks similar, but they’re not identical,” Carter said. “The pixel colors are different, too.”
“But it’s something,” Rainy said. She was feeling breathless. “Each image looks identical. Only at magnification can we see the actual location of pixel color variation. Why?”
“It’s probably a watermark,” said a voice from behind them.
Rainy turned, and her eyes went wide with delight. Clarence Stern had just entered the Lair.
“Tomlinson said he’ll need that PowerPoint deck by six,” Stern announced. “Now, move over, Carter. Let me figure this out.”
Chapter 72
“You think it’s an invisible watermark?”
“Seems like it to me,” Stern said to Rainy. “Watermarks are nothing more than embedding information into a digital media. Could be audio. Could be a picture.”
“Could be spinning the Beatles’ ‘I’m So Tired’ backward and hearing Paul is dead,” Carter said.
“Well, that’s a watermark of sorts, I guess,” Stern said. “It’s used a lot in copyright protection. It’s also used in source tracing.”
Rainy nodded. “Of course. The movie industry has been using source trace watermarks for ages. They can identify who downloads their intellectual property and then create a map of the distribution network. We’ve been exploring applications for them as well.”
Carter nodded enthusiastically. “If each of the images Mann gave us has a unique watermark, it would explain why they weren’t generating the same hash value. The watermark is what makes each image unique from the other. But it’s hidden, so we can’t easily see the difference with our eyes.”
“The question now is,” Stern said, “how do we reveal the watermark?”
Stern picked one image to work with. He spent a half an hour bumping up the contrast and adjusting the image levels.
“I’ve got the contrast here set to one hundred percent.”
Rainy looked. “See anything?”
“I’ve got to run the contrast filter a bunch of times over before I can say.”
Stern was back to his Stern ways. Grunting. Sighing. Pouting. He picked up a pencil and prepared to throw it at the monitor.
“That’s my monitor, Clarence,” Carter said. “I trust you. But not that much.”
Stern set down the pencil. He looked over at Rainy. “Do you have an original?” he asked.
“What do you mean?”
“An original source. One that hasn’t been moved from a point A to a point B. One that wouldn’t have a watermark applied.”
Rainy thought a moment. “Lindsey Wells,” she said. “After my seminar she gave me her cell phone. She deleted the sent messages, but not the pictures. She thought they might somehow be helpful.”
“Well, she just might be right,” said Stern. “Let me have it.”
Rainy returned to the workstation with Lindsey’s cell phone. It took only a few minutes for Stern to download the pictures to Carter’s machine.
“What are you going to do?” Rainy asked.
“I’m going to run a difference filter,” Stern said.
“I’m not familiar with that,” Rainy said.
“The difference filter compares the original to a copy. Look, I’ll compare the original to itself.”
Stern did just that, and all Rainy saw afterward was a black square on the screen.
“A black square means the images are identical,” Stern explained. “All pixels turned a pure black color. Now let’s run the difference filter on the original and one of the matching images.”
Rainy examined the completed output. “It still looks like a black square to me,” she said.
“But some of the pixels are not quite pure black,” Stern said. “When I change the color levels to brighten all the very dark colors, I suspect our hidden watermark will become visible.”
Stern adjusted the levels. The dark colors transformed to bright, almost neon shades. Rainy’s hand went to her mouth when she saw what appeared. Most of the image square was still black. But not all of it. At the bottom of the square, Rainy saw a series of numbers. Stern’s level adjustment had turned the color of those numbers a bright yellow.
“I bet those numbers are an IP address,” Stern said. “Whoever embedded this watermark wanted to track
