Researchers of Berkley University showed that a simple audio recording of the PC keyboard clicks can betray the text one has entered.
The researchers were able to take several 10-minute sound recordings of users typing at a keyboard, feed
the audio into a computer, and use an algorithm to recover up to 96 percent of the characters entered.
"It's a form of acoustical spying that should raise red flags among computer security and privacy experts,"
said Doug Tygar, UC Berkeley professor of computer science and information management and principal
investigator of the study. "If we were able to figure this out, it's likely that people with less honorable
intentions can - or have - as well."
The results of their findings will be presented Nov. 10 at the 12th Association for Computing Machinery
Conference on Computer and Communications Security in Alexandria, Va.
What makes the technique feasible is that each keystroke makes a relatively distinct sound, however subtle,
when hit. Typical users type about 300 characters per minute, leaving enough time for a computer to isolate
the sounds of individual keystrokes and categorize the letters based upon the statistical characteristics
of English text. For example, the letters "th" will occur together more frequently than "tj," and the word
"yet" is far more common than "yrg."
Using statistical learning theory, the computer can categorize the sounds of each key as it's struck and
develop a good first guess with an accuracy of 60 percent for characters, and 20 percent for words. Then,
spelling and grammar checks can be used to refine the results, which are claimed to increase the character
accuracy to 70 percent and the word accuracy to 50 percent.
But that's not the end. The recording is then played back repeatedly in a feedback loop to "train" the
computer to increase its accuracy until no significant improvement is seen. In the UC Berkeley experiments,
three feedback cycles were often enough to obtain recovery rates of 88 percent for words and 96 percent for
characters.
The findings highlight a security hole that could be exploited and should be investigated, the researchers
said.