Security researchers in China have invented a clever way of activating voice recognition systems without speaking a word. By using high frequencies inaudible to humans but which register on electronic microphones, they were able to issue commands to every major “intelligent assistant” that were silent to every listener but the target device.
The team from Zhejiang University calls their technique DolphinAttack (PDF)[1], after the animals’ high-pitched communications. In order to understand how it works, let’s just have a quick physics lesson.
Here comes the science!
Microphones like those in most electronics use a tiny, thin membrane that vibrates in response to air pressure changes caused by sound waves. Since people generally can’t hear anything above 20 kilohertz, the microphone software generally discards any signal above that frequency, although technically it is still being detected — it’s called a low-pass filter.
A perfect microphone would vibrate at a known frequency at, and only at, certain input frequencies. But in the real world, the membrane is subject to harmonics — for example, a tone at 400 Hz will also elicit a response at 200 Hz and 800 Hz (I’m fudging the math here but this is the general idea. There are some great gifs illustrating this at Wikipedia[2]). This usually isn’t an issue, however, since harmonics are much weaker than the original vibration.
But say you wanted a microphone to register a tone at 100 Hz but for some reason didn’t want to emit that tone. If you generated a tone at 800 Hz that was powerful enough, it would create that 100 Hz tone with its harmonics, only on the microphone. Everyone else would just hear the original 800 Hz tone and...