[Cryptography] "The Visual Microphone: Passive Recovery of Sound from Video"

[Jerry Leichter]

On Aug 5, 2014, at 12:40 AM, Christian Huitema <huitema at huitema.net> wrote:
>> Did you watch through to the end?  Starting at around 3 minutes in, they show 
>> how to use artifacts due to shutter motion to recover sound reasonably well using a standard camera - frequencies 5 times the frame rate of the camera.
> 
> Yes I saw that. What they call a standard camera is in fact a rolling
> shutter DLSR camera.... From the paper: "We took a video of a bag of candy (Fig. 10(a)) near a loudspeaker playing speech, and took a video from a viewpoint orthogonal to the loudspeaker-object axis, so that the motions of the bag due to the loudspeaker would be horizontal and fronto-parallel in the camera’s image plane. We used a Pentax K-01 with a 31mm lens....
A Pentax K-01 is available from Amazon for $399 new.  The appropriate lens will depend on circumstances, but suppose we want the same one they used.  They only specify "31mm" and there are multiple 31mm lenses available for it, but if we go specifically for a Pentax version, a new one at Amazon - a very nice F/1.8 bit of glass, 5-star rating - will set you back $1168.84 new.  On the optics side, we're talking $1500 or so for what's pretty standard (nowhere near high-end) "prosumer" gear, and we could find equivalent stuff for much less with a bit of shopping.

Their high-speed camera was a Phantom V10.  This is a rather different beast.  http://www.palhd.com/ will rent you one - for $1800/day.  I wasn't able to find anyone specifically selling it on-line; the closest was http://www.abelcine.com/store/Phantom-Flex-High-Speed-Digital-Camera/ which says:  "Price Range:  Phantom camera line from $50,000 to $150,000 depending on specific model and features."   Not many people could put that on their credit card, so perhaps it's not surprising that it's hard to find on-line sellers....  :-)

Of course the Phantom may be overkill for this application; there are probably cheaper high-speed cameras out there.  But you're not currently likely to find one for what most people would consider a reasonable price.

The spooks will, of course, have no problem with these prices - and will likely build their own equipment anyway.  But the rolling shutter trick - which one might be able to emulate using much cheaper equipment, perhaps a couple of web cams on a spinning board for a start - is well within the range of amateurs.  (The back-end processing is still prohibitive, but we all know what happens to the cost of processing.)

The point is that the Nyquist theorem doesn't protect you here.  It just makes the attacker's job a bit harder and more expensive.

                                                        -- Jerry