Prepare to get slightly more paranoid.
By Nathan Collins
(Photo: Ian Waldie/Getty Images)
Hang out with computer security experts for more than a minute and you’ll notice a lot of them have a little piece of tape covering their laptops’ built-in cameras. A few of them might also have put an old headphone plug into their audio jacks. Both are decidedly low-tech solutions to the high-tech threat of hackers playing spy through computer hardware. Yet, even with those life hacks, there’s another simple way for hackers to spy on you: through your headphones.
The problem, Mordechai Guri and his colleagues at Ben-Gurion University’s Cyber Security Research Center point out, is one of both basic physics and a perhaps unfortunate choice on the part of the companies that design and manufacture computer audio hardware.
First, the physics. Although it may seem like speakers, headphones, and microphones are entirely different things, they’re all based on what’s called an audio transducer. When you apply an electrical signal, the transducer vibrates, creating sound waves, like a loudspeaker. At the same time, sound waves vibrate the transducer, creating electrical signals—in other words, a microphone. In that sense, it’s not just that it’s easy to turn a speaker into a microphone; from a mechanical point of view, a speaker is a microphone, and a microphone is a speaker.
Potential hackers do face one more hurdle. Turning the ones and zeros that make up a computer’s language into sound requires a digital-to-analog converter (DAC), and turning sound into ones and zeros requires an analog-to-digital converter (ADC). Both devices work only one way—so while you can turn a speaker into a microphone, you can’t turn a DAC into an ADC.
In their wisdom, however, digital audio hardware designers decided to build both DACs and ADCs into headphone jacks, built-in microphones, and more—and they made it easy to switch back and forth, which wasn’t an entirely unreasonable choice. For one thing, “jack re-tasking” makes it simple to repurpose a mic jack to replace a fried headphone jack. On the other hand, it means a hacker who can listen in on your microphone can also listen in through your headphones.
It remained to be seen, however, how well the idea of headphones as a listening device works. To find out, Guri and his team conducted a series of tests. While headphones don’t make particularly high-quality microphones, the results suggest standard headphones could make pretty good listening devices for the devoted spy.
Fortunately, there are some simple countermeasures including using a stereo amplifier in between headphone jacks and headphones—like DACs and ADCs, modern amplifiers don’t work in reverse. And since nearly all computer speakers have built-in amplifiers to boost volume, they’re not much of a problem, Guri and his colleagues write.