Body movements covertly tracked via smartphone using pulses in music
Researchers at the University of Washington have demonstrated how it is possible to transform a smart device into a surveillance tool that can collect information about the body position and movements of the user, as well as other people in the device's immediate vicinity, by remotely hijacking smart devices to play music embedded with repeating pulses to track position, body movements and activity.
The team from the University of Washington's Paul G. Allen School of Computer Science & Engineering showed how it is possible to collect the detailed data on personal activity using CovertBand, software code they created to turn smart devices into active sonar systems. CovertBand can utilise built-in microphones and speakers in a smart device and can be controlled remotely.
CovertBand utilises the principles of active sonar to gather this information. Active sonar systems, such as on submarines, determine the position of objects by sending out an acoustic pulse. Those sound waves bounce off objects in their path, and the deflected waves can be picked up by a receiver to determine the object’s position, distance and shape.
Through the speaker of a smartphone or other device, CovertBand sends out a repeating pulse of sound waves in the 18 to 20 kHz range. Much like sonar on a submarine, these sound waves are reflected when they encounter objects in their path. CovertBand uses the device’s built-in microphones as a receiver to pick up these reflected sound waves. The smart device then transmits this information to the attacker, who could be a few feet away or halfway across the globe.
“To our knowledge, this is the first time anyone has demonstrated that it is possible to convert smart commodity devices – like smartphones and smart TVs – into active sonar systems using music,” said senior author Shyam Gollakota, an associate professor of computer science and engineering. “The physical information CovertBand can gather – even through walls – is sufficiently detailed for an attacker to know what the user is doing, as well as other people nearby.”
Co-lead author Rajalakshmi Nandakumar, a UW doctoral student in computer science and engineering and co-lead author, said: “Most of today’s smart devices including smart TVs, Google Home, Amazon Echo and smartphones come with built-in microphones and speaker systems, which lets us use them to play music, record video and audio tracks, have phone conversations or participate in videoconferencing.
“But that also means that these devices have the basic components in place to make them vulnerable to attack in this manner.”
The team tested CovertBand’s effectiveness using a smartphone hooked up to either a portable speaker or a standard flat-screen TV. In both cases, CovertBand’s data could be used to decipher repetitive movements such as arm-pumping, walking or pelvic tilts to a range of up to six metres from the smartphone, with a positional error of only 8-18cm. Researchers also discovered that, with the portable speaker, CovertBand’s pulses can transmit through thin, interior walls, although the range drops to 2-3m.
“Other surveillance approaches require specialised hardware, from the ‘classic’ hidden camera to an ultrasound-like device that must be placed on the wall of a neighbouring room,” said Alex Takakuwa, co-lead author and a doctoral student in computer science and engineering. “CovertBand shows for the first time that through-barrier surveillance is possible using no hardware beyond what smart devices already have.”
Currently, CovertBand can automatically identify and infer repetitive motions. More detailed inferences require manual analyses of data – or additional tools.
“Our initial goal was to demonstrate that it is possible to use passive acoustics to gather even basic – but still highly sensitive – information using CovertBand,” said Gollakota. “But if you have enough data from CovertBand, you could run it through machine-learning algorithms to help classify more movements for faster identification.”
The 18 to 20 kHz repeating pulses employed by CovertBand are on the low range of what many people can hear accurately, though children, younger adults and even pets might be able to hear it well, said Nandakumar. To increase the range of surveillance and work through walls, the authors increased the volume of these repeating pulses, which made them audible.
To mask the sound, they “covered” Covertband’s pulses by playing songs or other audio clips over them. Some songs work better than others, particularly compositions with repetitive, percussive beats. When they played the CovertBand pulses beneath 20 popular songs – including Lenny Kravitz’s ‘American Woman’ and Michael Jackson’s ‘Bad’ – listeners could identify the “hacked” version of the song 58 per cent of the time, just slightly above the 50 per cent accuracy expected by guessing randomly.
“Since Covertband enables through-the-wall surveillance, anyone can play music on their smart devices to track people through walls,” said Takakuwa. “This is concerning because if a neighbour is playing music, it could either be a benign act or an act of surveillance to determine if anyone is in the adjacent apartment, track their movements or infer their activities.”
The researchers said that soundproofing a room would prevent attacks through walls. Emitting a jamming signal at the same 18 to 20 kHz frequency range would also prevent hacked devices or attackers in the next room from gathering information. Currently, those are impractical defences for most people. Soundproofed rooms have no windows, for example, and jamming signals would have to be sent the moment an attack is detected.
Another potential – though partial – defence could be to allow users to deactivate the speakers or microphones on their smart devices. Such a move would go against industry trends for some of these devices.
“In many cases, when the device is on, then its speakers and microphones are also on,” said Nandakumar.
The team hopes that knowledge of what is possible will help develop awareness of privacy dangers and prompt scientists to develop practical countermeasures.
“We always want to stay one step ahead of the bad guys – of attackers who are trying to collect this information about users,” said co-author Tadayoshi Kohno, a UW professor of computer science and engineering. “We’re providing education about what is possible and what capabilities the general public might not know about, so that people can be aware and can build defences against this.”