
A proof-of-concept system for counter-surveillance against spy drones - breck
https://www.wired.com/story/a-clever-radio-trick-can-tell-if-a-drone-is-watching-you/
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personjerry
My tl;dr understanding: Drones send video back to operator. Video is typically
compressed so that it only updates part of the picture that have changed. Even
if the video is encrypted, the researchers are able to measure the bitrate.
Thus when the researchers make a significant change like putting a board
against a window, and seeing if the traffic increases, the researchers can
determine whether the drone is looking at that window.

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Sargos
I'm assuming this is predicated on the drone being absolutely still and that
nothing in the scene (leaves on trees) is moving? Is that even realistic?

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kmm
Shouldn't simple linear movement of a scene still compress really well?

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godelski
Yes. The compression works by analyzing changes in the screen. A simple
example of this would be if you are displaying data. Say you are taking real
time data and it is scrolling across the screen. Your display moves the data
to the left, drops off what moved out of the frame, and adds what is new.
Pictures are more complex, but the idea is the same. Only take what is new.

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anfilt
Not sure if I would call it clever. Crypto will always leak the amount of data
if you don't pad or ensure a constant data rate. Its also far from the first
time compression has lead to a side channel. For instance
[https://en.m.wikipedia.org/wiki/CRIME](https://en.m.wikipedia.org/wiki/CRIME)

-edit- Even durning world war you if you were listening on enemy radio commucations and they had not been decrypted you could tell if the enemy was planning somthing by noticing a large increase in radio communication. This a long known issue if you dont pad your data and send it at a constant stream. You may leak some information.

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kbhn
> Not sure if I would call it clever

What do you mean it's not clever? Sure, they applied a known weakness of non-
constant data rate encoded messages to drone monitoring, but that's never been
done before. What have you done that's so impressive that gives you the
confidence to scoff at the accomplishment of the talented engineers and
developers who created something brand new?

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anfilt
Perhaps, how I said that was a bit ambiguous/unclear. However, the article is
calling it clever. The research demonstrates a weakness with the communication
layer. Just because it applies to drones(insert buzz word) does not make it a
novel idea. I just find it funny that Wired is writing about it since it deals
with drones, but I have read multiple other papers on the mis-application of
compression and cryptography. Finding, flaws is important, I just feel like
the article was kinda over selling the concept.

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codewritinfool
Isn't this an expected outcome of variable-rate compression? Switch to a
constant-rate compression or pad with random data to reach that constant rate.

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zaksoup
What's really interesting to me here is the watermarking part of this. I
imagine a future where everybody has little IR pins on their shoulder and
phones scanning encrypted traffic for their watermark...

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gruez
it doesn't work if everybody's doing it. your pin's flashing will get drowned
out in the noise of everyone else's pin.

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FabHK
well, you might be able to recover it using something akin to CDMA.

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alistproducer2
>In other words, they can see what the drone sees, pulling out their
recognizable pattern from the radio signal, even without breaking the drone's
encrypted video.

The most interesting part of this is how encrypted traffic leaks enough info
to be reliably analyzed. Really cool stuff.

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DannyB2
The key insight is that the drone should store the video to local storage for
later (1) recovery or (2) buffered encrypted transmission to the mother ship
which would eliminate the leakage of clues about what subject is being spied
upon.

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cavanasm
The problem with that is the video feed is being actively used. If you want to
know where someone is going, you can't just make your drone fly around
randomly for hours recording video of the area you think they're in, you have
to visually acquire your target, and then follow them.

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jeanmichelx
Old technique with "spying drone" buzzword

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anfilt
My thought as well.

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icegreentea2
This defense is plausible for fixed frequency broadcast video feeds. Likely as
drone usage grows and becomes ever more sophisticated (say nothing of current
'nation state grade' systems), their video feeds will likely be spread
spectrum and highly directional (if nothing else, directional transmission
makes the drone's video feed far more power efficient), making intercept and
this type of analysis difficult.

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anigbrowl
Constant bit rate compression isn't a cure-all. It's generally inferior to VBR
which is why the latter took off in the first place. CBR is easily degraded by
just increasing the amount of visual complexity. Leaves moving on trees in a
breeze is a typical problem case.

So if you're worried about being spied on, high-resolution mosaic patterns are
your friend. Mirror or high specularity pigments will enhance the effect. For
fixed installations a few discoballs and a laser will mess up someone's day.

Of course, if you want real security you'll triangulate on the drone signals
and jam, because if you really need to do so getting busted by the FCC is
probably the least of your worries.

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electrograv
The minimum bar for usable quality can be defined precisely, and an encoding
algorithm designed around it.

One obvious extreme example of this would be no compression at all, but it's
easy to imagine much better designs.

For example, imagine I define a spec where I require quality _no worse than a
lossless 500x500x24bit /frame video stream_. If we budget a bit rate higher
than is required for the naive raw transmission of those frames, we can first
encode the naive frames, then encode the deltas between them (upscaled to full
resolution) such that the reconstruction error is minimized while not
exceeding any maximum tolerated error per pixel.

With this approach, you are not crippled when facing an adversarial pattern
generator of some kind, yet when transmitting regular natural scenes, you can
benefit from significant quality increase.

As you suggest though, it's true that generating the most complex patterns
possible will reduce any drone's video quality as much the codec's design
permits -- however accomplishing this seems much more expensive for relatively
little benefit versus other counter-drone approaches.

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misterbowfinger
Original paper:
[https://arxiv.org/pdf/1801.03074.pdf](https://arxiv.org/pdf/1801.03074.pdf)

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jscheel
Couldn't you defeat this detection by moving the drone or camera in a pre-
programmed pattern that you could then reverse to stabilize the video later?

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nostromo
It'd be easy enough to prevent this by not compressing the video feed before
it's transmitted.

That's not feasible for Netflix to do, of course, but it'd be trivial for the
NSA or other spy agencies.

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vernie
A more practical solution would be to use constant bitrate compression or to
randomize bitrate on the sender side.

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a3n
> All of that may seem like an elaborate setup to catch a spy drone in the
> act, when it could far more easily be spotted with a decent pair of
> binoculars. But Nassi argues that the technique works at ranges where it's
> difficult to spot a drone in the sky at all, not to mention determine
> precisely where its camera is pointed.

This technique could also be used in legal proceedings to establish that
someone or some org was spying illegally.

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callumprentice
I expect image detection in a video stream is pretty advanced these days but
I'd imagine you'd need to rotate the image being recorded 3D space to match
the plane of the window so you can compare them both? If you know the position
of the drone relative to the window, that's doable. Maybe the algorithms are
clever enough to account for that these days.

Interesting article - thanks for sharing.

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m4x
Their technique doesn't actually do any image analysis at all, since the video
stream from the drone is encrypted and they can't view it.

Rather, they monitor the bitrate of the video stream and exploit the fact that
static/unchanging scenes compress well and have a low bitrate while scenes
which are changing have a higher bitrate. They then deliberately change the
scene (by blacking out a window) and see whether the bitrate of the video
stream increases.

If there's a strong correlation between blacking out the window and changes in
bitrate, they can conclude the drone is observing their window.

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callumprentice
Got it - thank you.

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stretchwithme
And that is why drones will start landing on tree branches and the poles of
street lamps. And why they'll start using cell networks.

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gruez
easy countermeasure: CBR video compression

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tehsauce
Pretty clever! Definitely possible to beat by changing the way video is
compressed/transmitted, but perhaps not without a performance loss

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kang
The important lesson being don't secure the signal, hide it in the noise.

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exabrial
I thought this was supposed to be prevented by your cipher mode?

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irishcoffee
"A clever radio trick can tell you nothing" \- works on drones for a living.

