Short-haul microwave links for low latency and high bandwidth, yes, but at shortwave frequencies you are literally talking about BITS per second. TFA says "Think dialup speeds" but this is lower than even dialup!
They certainly aren't receiving any meaningful amount of data (which I presume you need for HFT) with that shortwave receiver.
"High frequency trading has led to widespread efforts to reduce information propagation delays between physically distant exchanges. Using relativistically correct millisecond-resolution tick data, we document a 3-millisecond decrease in one-way communication time between the Chicago and New York areas that has occurred from April 27th, 2010 to August 17th, 2012. We attribute the first segment of this decline to the introduction of a latency-optimized fiber optic connection in late 2010. A second phase of latency decrease can be attributed to line-of-sight microwave networks, operating primarily in the 6-11 GHz region of the spectrum, licensed during 2011 and 2012. Using publicly available information, we estimate these networks’ latencies and bandwidths. We estimate the total infrastructure and 5-year operations costs associated with these latency improvements to exceed $500 million."
Given the low bandwidth I assume they can't use very sophisticated authentication mechanisms
and that's just what a few seconds of thinking came up with.
(Actually I see how that's possible, A XOR B, upticks are 1, etc.)
If the pad is coming via a side-channel then the speed increase is limited by that side-channel, you lose all your gains by waiting for the pad, then lose more by decrypting and verifying.
That aside my first thought was someone could jam the signal by blasting the receiver with noise; I wonder how they mitigate that.
The actual encryption of each bit just require an XOR on each end of the link. That's about half a nanosecond on a Xeon or one clock cycle on your FPGA.
A jammer wants to be close to the receiving antenna so he can use less power. Yet he wants to be far away so he's less likely to be caught. Willfully interfering with any any radio service is illegal in any regulatory domain AFAIK.
Jamming a licensed radio link vital to rich people who routinely deal with the government doesn't sound like a great idea to me.
Either way, the likelihood of loss between TX and RX means that you can't advance the cipher as bits arrive. I'm thinking you'd have to advance based on GPS-sync'd clocks. Given that each bit is at least 10's of microseconds, GPS sync is plenty good.