Listening in: These graphs show measurements recorded by a smart phone as letters are typed on a nearby keyboard. The device can distinguish between “a” at the left of the keyboard and “l” on the right, as well as between two pairs: “pq” and “nm.”
A real-world attack would, of course, require a victim to habitually place a phone and keyboard on the same work surface. Vibrations inherent in the environment could also complicate matters. A tall building adds noise because it sways, and offices near a major road will be affected by traffic vibrations. The composition of the surface makes a big difference as well, says Traynor. Pine desktops conduct vibrations extremely well, as do glass ones, making them ideal surfaces for the attack. But a tiled kitchen counter is basically inscrutable.
To make the attack succeed, the dictionary would need to be tailored to the specific target. “The best-case scenario here, if you are an attacker, is to go after a very specific person,” says Traynor. “I think the attack is realistic in that case.”
As phone technology improves, attacks via the accelerometer could become more feasible. The researchers’ initial experiments used Apple’s iPhone 3GS, but the phone’s accelerometer lacked the necessary sensitivity. The researchers then moved to the iPhone 4, which uses a gyroscope to remove noise from the accelerometer data, and had much greater success.
While the attack technique is interesting, it’s unlikely to become a real threat for some time, says Charlie Miller, principal security consultant with Accuvant, a compliance and security research firm. “It’s cool because it is very James Bond-ish,” he says. “But it might easier to turn on the mike and listen to the target talk on the phone.”