Proof of concept: iPhone captures keystrokes via 'thump phreaking'

Researchers at Georgia Tech have worked up a proof-of-concept demonstration of using an iPhone 4's accelerometer as a keylogger. After setting the iPhone near a computer keyboard, the device's built-in accelerometer and gyroscope were able to decipher entire sentences "with up to 80 percent accuracy."

Similar keyloggers have already been developed using microphones, which sample vibrations far more frequently than accelerometers. However, nearly all phone operating systems ask a user's permission before granting applications access to the built-in microphone, which limits the utility of a keylogger.

Apps don't currently ask for users' permission for access to accelerometers and gyroscopes, which raises the remote possibility of iPhones or other accelerometer-equipped devices spying on keyboard inputs without users being the wiser.

"The way we see this attack working is that you, the phone's owner, would request or be asked to download an innocuous-looking application, which doesn't ask you for the use of any suspicious phone sensors," said Henry Carter, one of the project's researchers. "Then the keyboard-detection malware is turned on, and the next time you place your phone next to the keyboard and start typing, it starts listening."

The keylogger software works by detecting key pairs -- detecting individual key presses turned out to be too difficult and unreliable -- and by comparing paired accelerometer events against a built-in dictionary, the software can decipher keypresses with startling accuracy. Our own Mike Rose has coined "thump phreaking" to refer to this spying technique (after Van Eck phreaking, which uses CRT or LCD emissions to reconstruct the screen image) and it's as apt a term as any for what this software does.

It must be mentioned that this is only a proof of concept and not an actual attack that's out in the wild. The researchers themselves admit that this keylogger was difficult to build, and it's easily defeated by something as simple as moving your iPhone more than three inches away from the keyboard. That having been said, the technique is very James Bondian, and I wouldn't be at all surprised if something similar to this turns up in a forthcoming spy thriller or Batman movie.