Updated with GNU Radio flowgraph picture and .grc file link.
I and my HackRF software-defined radio spent some time at a big software conference recently. Because I can't resist gently sticking my nose where it doesn't belong, I decided to investigate the wireless microphones being used on the conference stages.
If you're not familiar with a software-defined radio, it's pretty disruptive technology. A software radio is a radio that doesn't know anything about AM, FM, Bluetooth, ZigBee, cellular – any of that. Instead, it just digitizes or transmits raw signal data. It relies on a computer running software that knows about different wireless technologies. And because the bulk of the work is now done in software instead of being set in stone (in hardware), the software can be updated to speak virtually any wireless technology – past, present, or future. You don't need to buy a new, different radio, just use different software. This vast flexibility also makes software-defined radio a great technology for investigating wireless security vulnerabilities.
So back to the conference. I wanted to learn more about the security vulnerabilities these wireless microphones might pose. Here's the process I followed:
Step 1: Discover what kinds of microphones were used. At this conference, the sound engineers were set up at the back of the room and would frequently leave their equipment unattended, sitting on a table in plain view. I was able to walk up and copy down the FCC equipment authorization IDs off the back panels of the microphone receivers. Armed with those IDs, I went to the FCC OET Web site and looked up internal photographs, test reports, and user manuals for the receivers. I discovered that the microphones operated in the 524 to 542 MHz range. I also learned that these microphones used simple frequency modulation (FM). This should be easy!
Step 2: With the HackRF and GNU Radio, I did a survey of the 524 to 542 MHz frequency range, looking for signals that had the tell-tale spectrum symmetry of an FM transmission. I found several candidates.
Step 3: I worked up a quick GNU Radio Companion flow graph of an FM demodulator, based on several FM demodulator examples I found on the Internet. As I tuned to each candidate frequency, the spectrum analysis of the demodulated signal turned from white noise to a much lower noise floor with bursts at the rate of typical conversation. A quick check on my laptop's audio output confirmed I had access to the audio directly from the microphone!
What are the security implications of these wireless microphones, used in this situation? A few things come to mind, in increasing levels of evil-ness:
- I could record the audio directly from the microphones. And if several microphones are in use, but with carriers all within HackRF's 20 MHz bandwidth, I could record all the microphones at the conference with a single radio.
- I could overwhelm the microphone signals with my own transmissions, replacing the audio the audience hears with my own audio.
- With a good high-gain, directional antenna, I could monitor presentations at conferences, from outside the perimeter of the building. I could listen to conferences I am not invited to, and purloin information I am not otherwise privy to.
How are these problems addressed? The most obvious answer, aside from using wired microphones, is to move to a digital modulation, digitize the audio, and encrypt the audio data stream. There are microphones on the market that address this – though I suspect they are expensive and power-hungry, much like digital mobile phones were when they were introduced as the replacement for analog "AMPS" phones. But until convention center customers express concern over the security of their conference content, venues will continue to use their cheaper FM systems, and digital systems will continue to be a niche product and expensive. Big, public conferences may not care too much about this vulnerability, but private groups and organizations may feel otherwise.