On 2/5/16, Sean Lynch
... Radio is being used right now to provide anonymity, but it's being used[1] to hide endpoints similar to the duct-taped payphone trick depicted in Hackers, in order to avoid attacks like the one used to capture Ross Ulbricht without giving him a chance to wipe his computer (they snuck up behind him and pinned his arms, but they would have just rushed him had that not been possible). If you use a device like the ProxyHam and you sit somewhere where you can see it, there's a reasonable chance you'd spot someone who's trying to find you, giving you a chance to hit your panic button and escape.
this assumes you're keeping it under constant supervision, of course :P
The older, lower-tech version of this trick is to use a high-gain antenna like the Cantenna or a Yagi to use a public wifi AP from a stealthy, defensible location. The problem with this is that this presents no challenge to RDF (radio direction finding) equipment designed for WiFi. That's the big advantage of the ProxyHam, since whoever is looking for you probably won't know in advance what frequency you're using. And solving that problem in a general way requires MUCH more expensive gear than just locating WiFi clients.
one of my favorite tricks, but rather rude in spectrum, is setting high power amplifier to maximum. DF tends to see this signal arriving from all around... *grin* this introduces it's own trade-offs, of course.
If you're concerned about someone coming after you with much more capable RDF equipment, you are now talking about LPI (low probability of intercept) communications. This could be something as simple as a longwave infrared optical link, which won't show up on regular CCD or CMOS camera even without an IR filter.
free space optics rides again!! :P
With SDR you might use some form of non-sinusoidal spread spectrum with extremely high processing gain so you'll be well below the noise floor and your carrier won't be recoverable using autocorrelation techniques. Some UWB techniques fall into this category, though because "legal" UWB is required to be confined to a narrower bandwidth, it's probably detectable.
right, UWB is the solution here with privacy and authenticity at the physical layer (not above MAC, or other deferred placement in stack)
It MAY be possible to use SDR to achieve LPI while still remaining within
if you're building LPI, you don't give a fuck about the FCC (compliance). by definition, if they've found you, you fucked up!
Actually, that gives me an idea: MIMO precoding[2] (versus spatial multiplexing, which is useless for your purposes). MIMO precoding devolves to beam-forming in the absence of reflectors like buildings, but in an urban environment, you get a complex combination of signal paths,
MIMO precoding requires a "training" phase where they discover one another by transmitting some easily "locked-onto" signal so that each receiver can find the other transmitter independently.
it is now possible for a professional's budget to accodomate the SDR equipment necessary to do this type of phase sync'ed active beam forming MIMO transmission, and not all methods require the training phase. in fact, omission of this (by out of band training, in a sense) in a method of "keying" phased delivery of UWB MIMO in a way more likely to achieve LPI. synthetic aperature millimeter wave vision systems are also pushing along this boundary, for cross-pollination of suitable phased sync'ed UWB MIMO signal processing. i could go on, if you're curious, but perhaps on another list? :) best regards,