I do wonder how close you'd have to go to actually demodulate real Gigabit Ethernet over the air. Given the 8 simultaneous data streams (4 pairs times two directions), I imagine you'd need at least 8 antennas to get anywhere, probably arranged in very close proximity to the cable to pick up on the spatial differences between the pairs. Then you'd have to use MIMO demodulation techniques. At that point you might as well just tap the cable.
100BASE-TX would be a lot easier, since that just uses a single pair in each direction.
FWIW, this isn't a side channel, at least not the way he's presenting it. It's a covert channel. That's different; side channels leak (significant) information from uncooperating sources. Covert channels require a cooperating source. There's a huge difference. Covert channels are largely academic and almost never relevant in real life. This isn't like research on things like extracting RSA keys from CPU EMI emitted during OpenSSL operations, which is a real side channel and much more valuable research.
100BASE-TX would be a lot easier, since that just uses a single pair in each direction.
FWIW, this isn't a side channel, at least not the way he's presenting it. It's a covert channel. That's different; side channels leak (significant) information from uncooperating sources. Covert channels require a cooperating source. There's a huge difference. Covert channels are largely academic and almost never relevant in real life. This isn't like research on things like extracting RSA keys from CPU EMI emitted during OpenSSL operations, which is a real side channel and much more valuable research.