side-channel attacks
When it comes to protecting voice calls, security professionals typically focus on the security of the data carrying the audio — above all on encryption. Sooner or later though, the voice data stops being a stream of bits and becomes an analog signal that goes straight to the speakers. You might think that if someone is using headphones, nobody can listen in. But headphones are electronic devices, and their components have physical characteristics that can give away closely guarded secrets without any cyberattack at all. In a recent paper, researchers from two Hong Kong universities came up with InjectEave — an attack that exploits these properties to intercept the already-decoded analog audio signal.
How the InjectEave attack works
The electronic components in headphones convert data into analog form and amplify the signal to drive the speakers. This process generates weak electromagnetic emissions. Capturing those emissions and extracting information from them is a field unto itself, with many experts working in it. The main issue those experts face lies in the fact that stray emissions from headphones are too faint to intercept and analyze in practice.
The authors of InjectEave propose adding a radio transmitter that beams a signal at a specific frequency toward the target device. Those weak emissions from the headphones’ analog components modulate the transmitter’s signal. The altered signal can be intercepted from several meters away — provided the receiving antenna is sensitive enough. The attacker then analyzes the changes in the signal to reconstruct the original audio.
Technical details of the InjectEave attack
The main obstacle to intercepting electromagnetic emissions directly — without an external transmitter — is the relatively low signal frequency. For example, the audio amplifier in headphones operates at frequencies from 20Hz to 20kHz. By contrast, electromagnetic emissions are easiest to intercept at frequencies measured in megahertz. The researchers were able to find a transmitter frequency that would also “amplify” the low-frequency signal of interest.
The researchers ran a series of experiments. They used antennas to transmit the “amplifying” signal and receive the result, and a spectrum analyzer to capture and process the data. The authors have posted the raw recordings of the intercepted signals on a dedicated microsite, so anyone can check how well the attack works. In most cases, getting a usable result didn’t even require complex processing or recognition. Conversations picked up from wireless headphones are clearly intelligible despite the elevated noise level.
In total, the authors tested three models of wireless headphones and two models of basic wired headsets. The effective eavesdropping distance varied from one to six meters depending on the model. With a wired telephone, they were able to demonstrate two-way interception. They could hear not only the sound from the speakers but also the potential victim’s speech, which the headset’s built-in microphone and analog-to-digital converter had captured.
Advantages and limitations of the method
Any attack that requires getting very close to the target is challenging by definition. The need to tune the frequency of the “amplifying” radio signal for each specific device makes the attack even harder. In their experiments, the researchers had to use frequencies from one to eight megahertz.
Despite these caveats, the main achievement of this attack is its being able to target the quirks of the analog components in an electronic device. These components inherently produce stray emissions, and there’s almost no way to reduce them.
Similar surveillance methods that intercept a digital signal can be stopped by adding a layer of encryption. When it comes to the analog side of a device, protecting the signal itself is much harder. This research is another reminder of how complex and unpredictable even very simple, familiar devices can be. Sometimes even the strongest protection for digital data is powerless once that information is converted to analog form.
