A Conspiracy of Bones (Temperance Brennan #19)(107)
She did. I pointed to the bundled podcasts. Explained how they’d appeared on my Mac earlier and not on Mama’s. Peppers logged onto one of the lab’s computers, joined Body Language as a thirty-three-year-old female, promising to bill Slidell at the CCU for reimbursement, and navigated to the proper aisle in the general store. No bundled podcasts. Peppers agreed that an algorithm in the malware was sending some users to a rogue server, then to a modified version of the page, which was hawking the bundles to what the site’s operators perceived as a specialized slice of the market.
“Roll me through steganography,” I said. “How does it work?”
“Stick to the King’s English, ladies?” Slidell, churlish.
The ladies shared an eye roll.
“You’ve heard of encryption, right?” Peppers began.
“Coding,” I said.
Peppers nodded. “When we talk about encrypting, we mean making something indecipherable. It’s obvious the secret code is there, but no one can read it without knowing the key. Steganography is all about hiding a message so that no one even knows it’s present.”
“Like writing with invisible ink.” Mama had used that analogy.
“Exactly. Say you want to hide some info in another document, maybe in an image. You do it by subtly adjusting individual pixels …”
Peppers stopped mid-sentence. Turned from the screen to see if we were following.
“Pixels are the tiny squares that make up a digital image.” Hearing Slidell grunt, I let it go at that.
“A pixel is barely noticeable to the human eye but easily detected by a computer,” Peppers continued. “By making very subtle adjustments, you can hide whole strings of text. For example, change the color or brightness values of three successive pixels, and you could invisibly code the word CAB. I am greatly oversimplifying.”
“You’re talking about hiding text,” I said. “Could you also conceal one image inside another?”
“Yes. Since the intensity values are changed only slightly, the steganography creates deviations so subtle they can’t usually be detected by the naked eye. Typically, the only way you could say that one pic is a steg would be to compare two seemingly identical images. Even then, if you suspect one may be modified, there’s no quick way to tell which is the innocent and which is the carrier.”
“Come on, come on.” Slidell flicked a hand at the screen. “If Body’s dirty, I got to nail his ass fast.”
“Can you hide text or images in audio files?” I asked, feeling the same blunt-force dread as when Mama first proposed the idea.
“Yes. You take advantage of the way the algorithm for MP3—that’s code for mathematical process—converts and compresses analog audio into digital form. Your secret information would not only be hidden, it would be encrypted as well, so very hard to detect and decrypt.”
“There are programs that do this?” I was so pumped I was asking dumb questions.
“The web is lousy with apps. For example, MP3Stego hides things in music files. SkyDe is a steganographic add-on for Skype. There’s COAGULA—”
Slidell cut us off. “Brennan’s thinking this jerkweed Body might be floating kiddie porn in stuff he sells on his site. Maybe hiding it in these podcasts.” Jabbing a thumb at the monitor.
“Is that possible?” I asked, wanting confirmation. Not wanting confirmation.
“A lot of steganography is done with apps on mobile devices. What you’re suggesting would require computers on both ends. The buyer would download the podcast—the MP3 file, one of the most common audio files out there, by the way. Normally, he or she would play it using an audio player on his or her computer, iTunes on Mac, for example. But instead, with the setup I’m picturing, the buyer would open the file in a special program that is shared by the bad guy and the buyer, let’s call it Play Inside.”
“Which Body has used to hide images or videos.”
“Yes. The buyer opens Play Inside and selects the downloaded podcast from within that program. Play Inside then decodes the hidden porn, or whatever it is. Additional security might be to require the use of a password to open the file from within the special program.”
“Body’s got tech people could figure shit like that.” Slidell was pressing way too close to my back. “Maybe that freak Unger.”
“A savvy undergrad could set it up.” Peppers turned raised brows to me, then swiveled them to Slidell. “You in a spending mood, detective? Shall we put one of these in our shopping basket?”
“Fuckin’ A.”
We cruised the inventory, chose a podcast collection labeled “Our Children Are in Danger,” the only suggestive title of the half dozen offered. Peppers purchased and downloaded the bundle to one of the lab’s computers. Then, with a flurry of typing I couldn’t begin to follow, she launched a program and opened the first MP3.
“The DSSS steg-hiding algorithm—” she began.
“DSSS?” Wanting to be sure I understood.
“Direct sequence spread spectrum. It’s a technical method used for cell phones and other digital signals, which can also modify MP3 files to hide bits of a message.”
“Because an MP3 file is nothing more than a digital signal.”
“Exactly. The DSSS steg-hiding algorithm can enter random noises into audio files.” More keystrokes. “The software I’m using now analyzes sound, checking for extremely subtle variations, random noises like blips, barely detectable, but clues that might suggest a steg version of an original file.”