I'm happy to be able to announce that my student Sirus Shahini's paper "CHAOS: Exploiting Station Time Synchronization in 802.11 Networks" will appear at #NDSS '25! We're still working on the final copy, so no link yet, but the basic idea is this:
802.11 (#WiFi) beacon frames have a timestamp field that is at microsecond granularity; this is used for synchronizing various timing aspects of 802.11. Real access points are supposed to send beacon frames about every 100ms* but there's a fair amount of variation in how close they are to hitting this target each time, for a variety of reasons. As a result, there's a lot of noise in the observed values of the timestamps.
This means that in any area with WiFi, there are probably hundreds of beacon frames per second flying around, and they have very precise timestamps in them that are subject to a lot of noise. Sirus used this to build a covert channel that uses noise that *looks* like standard access point clock jitter, but can be used to broadcast data in public, secretly. Additionally taking advantage of the fact that this jitter causes beacon frames to be received in different orders, he uses ordering to boost the throughput of this secret channel to hundreds of bits per second.
One of the neat things about this is that while it is a type of a timing channel, the timing values are written directly into the beacon frames. So unlike most timing channels, you don't have to have a high degree of precision in your transmission or reception, and measurement error is not really a problem. Both the transmitter and receiver can just use any off the shelf WiFi card that provides raw access to beacon frames (which is many of them), and Srius did a lot of the work on #raspberrypi s.
It's very cool work, and I look forward to being able to post the full paper and his talk!
* Technically, 102,400us