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September 16, 2005

Keyboard Click-and-Clack Reveals Passwords

Courtesy of TechWeb News

Attackers armed with electronic equipment that costs less than $10 can sniff out what's typed on keyboards simply by recording keystroke sounds, a trio of researchers said in a soon-to-be-published paper.

In the paper, Doug Tygar, a professor of computer science at the University of California, Berkeley, and two PhD students, the husband-wife team of Feng Zhou and Li Zhuang, outlined how they came up with software and used off-the-shelf tools to record keystroke sounds, then turn them into a transcript that's accurate 96 percent of the time.

At the least, said the researchers, password security should be beefed up to take into account possible audio-based attacks like the one they described. "The practice of relying only on typed passwords or even long passphrases should be reexamined," wrote Tygar, Zhou, and Zhuang.

Their research is based on the fact that each key makes a slightly different sound when struck, thanks to the angle at which it's pressed and its location above the keyboard supporting plate.

Once the different sounds had been recorded, Tygar and his associates separated them into classes, then mapped them to the most likely keystrokes based on the English language's constraints, including the limited number of key combinations to make words and the limited number of words because of its grammar. Finally, they used spelling and grammar checking software to refine the transcriptions.

"The key insight in our work is that the typed text is often not random," said Tygar.

Other research into recording keyboard sounds -- most recently by a pair of IBM researchers who published a paper in 2004 -- was far more limited in that it required a text copy of the characters typed and needed to train a neural network for each typist. Tygar's method makes it possible, he claimed, to identify what's typed by anyone, on virtually any kind of keyboard with just 10 minutes of recorded sounds. Once that's done, a run-of-the-mill Pentium PC can decode the text in real time, as it's typed on the bugged keyboard.

The snoop wouldn't even need to be nearby, said Tygar, but could use a wireless microphone in the user's area or snatch sounds with a parabolic microphone. Such devices are commonly available, and are often sold online. eBay, for instance, had more than a dozen listed as of Friday, some as low as $5 (and touted as "perfect for bird watching").

Although the spying technique could be applied to grabbing swaths of text, Tygar was most concerned about the risk of password hijacks. Even supposedly "random" passwords -- those not in the dictionary or user names -- could be recorded and guessed with disturbing ease.

With 20 "guesses," a five-character password could be obtained 90 percent of the time. Eight- and ten-character passwords were hardly more secure; they were found 77 and 69 percent of the time, respectively. By upping the number of guesses to 75, Tygar broke 10-character passwords 80 percent of the time.

Some intuitive defensive measures work, others don't, added Tygar. Increasing the background noise decreased the recognition rate of their software, he admitted. "Attacks will be less successful when, say, the user is playing music while typing."

But other tactics, such as turning to a so-called "quiet" keyboard, may not stymie attacks. In their experiments, the researchers found that keyboards with less of the click-and-clack were ineffective against attack.

His recommendation? Beef up password security with two-factor authentication that combine passwords with one-time-password tokens or smart cards, or with biometric recognition, like fingerprint readers.

"Keyboard emanation attacks are more serious than previous work suggested," said Tygar.

The paper is slated to be appear for presentation at the ACM Conference on Computer and Communications Security next month in Alexandria, Va., but a pre-preview is available for downloading in PDF format from Tygar's Web site.

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