Re: [tor-talk] Using SDR

[Sean Lynch <seanl@xxxxxxxxxxxx>]

On Fri, Feb 5, 2016 at 7:23 PM coderman <coderman@xxxxxxxxx> wrote:

> On 2/5/16, Sean Lynch <seanl@xxxxxxxxxxxx> wrote:
> > ... 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
>

Indeed. Having a spotter there is probably the best solution.


> > 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.
>

This is why you use an attenuator. I wouldn't think law enforcement DF
equipment would be fooled by such a thing, since for example FCC will often
be looking for people who are outputting too much power, which on the ham
bands is going to be multiple kilowatts (I think they've mostly given up on
CB except when it starts interfering with licensed users).


> > 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!
>

Perhaps, but I'm not about to suggest that anyone break the law.

> 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.
>

How do you train out of band? By modelling the environment? That's an
interesting thought, and I suspect Google Earth has enough data to be able
to do it in a lot of places. Are you aware of free or inexpensive software
packages for doing this?


> synthetic aperature millimeter wave vision systems are also pushing
> along this boundary, for cross-pollination of suitable phased sync'ed
> UWB MIMO signal processing.
>

Aren't you just talking phased array for something like this though? Or do
you mean using phase information from the receive antennas to reconstruct
the environment rather than using phasing at the transmit side to steer
your beam? That's a very interesting idea since it can give you a 360
degree view with no need to steer your beam, in the same way that some
blind humans can use clicks to get a picture of their entire environment.
(I use humans and not bats because I think bat sonar is pretty directional,
whereas human ears can localize sound quite precisely without any need to
turn one's head.)


> i could go on, if you're curious, but perhaps on another list? :)
>

This is definitely an area I'm interested in, so I'd love to hear more of
your ideas, as may Jeremy, so if it's beyond what is generally tolerated on
this list, private email would be fine, or if you have a list in mind I'd
be happy to subscribe if I'm not already.
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