How sensitive are acoustic side-channels to compression with a narrowband codec?



  • Assume sensitive audio emissions from a mechanical keyboard. These audio emissions are often sufficient to reconstruct the actual key presses that generated the sound. If the audio is compressed using a narrowband audio codec such as G.711, how much of the information is destroyed?

    Put another way, can acoustic side-channel attacks ever be done using modern telephony?



  • According to Wikipedia:

    "... G.711 passes audio signals in the range of 300–3400 Hz and samples them at the rate of 8,000"

    Nyquist criteria limits the top frequency to be less than half the sample rate, or less than 4 KHz in this case. Further G.711 filtering apparently cuts this down to 3.4 KHz top.

    A quick impromptu experiment

    Laying the microphone of a USB headset next to my very clicky old Dell keyboard and recording the sound gives me an amplitude time space recording of: Keyboard recording

    Running a Fourier transform to frequency space yields: enter image description here

    It looks to me like all of the subtle key impulse differentiation lies in the 5-20 KHz region, which by definition cannot be passed by G.711

    This was a quick and dirty experiment, take it for what it's worth.



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