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Recovering redacted information from pixelated videos

positive.security

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Re: Recovering redacted information from pixelated videos

#41

Post by danuker »

Rygian wrote: post #23
RunSet wrote: post #4

Yet again mosaic blur's inferiority to opaque rectangle is demonstrated.

I recently made use of my right to access personal information from someone who had sent me unsolicited marketing material. The person who fulfilled my request sent me a PDF copy of their full customer list, where all entries had been blacked out except mine. As you may anticipate, that blacking out was simply a black box drawn on top of the actual data. It took me all of 3 seconds to select all, copy, then paste in…

Congrats! You now have a data leak to report to the GDPR authority of your state, in addition to the unsolicited marketing.

Re: Recovering redacted information from pixelated videos

#42

Post by josefx »

jerf wrote: post #14
mannykannot wrote: post #13

Earlier quoted context omitted.

I’m slightly curious as to how, in these cases, someone came to decide that blurring was the right way to do it. It seems unlikely that they never thought of simply blacking/ blanking the text, but if so, then blurring must have seemed preferable. My best guess is that, having seen blurring of faces (which is arguably OK when one merely wants to avoid casual attempts at identification, while retaining a ‘natural’ loo…

I think it leaves the visual interest behind. Black bars are both boring and jarring. It's aesthetically displeasing on many levels. Unfortunately, part of "leaving the visual interest behind" is precisely "pixels that depend on their real underlying values"....

You could use filler text like lorem ipsum to keep a natural appearance without exposing any information. Of course that is a bit more work than just dropping a blur effect on an existing document and exporting it as PDF.

Re: Recovering redacted information from pixelated videos

#43

Post by thrashh »

Rygian wrote: post #23
RunSet wrote: post #4

Yet again mosaic blur's inferiority to opaque rectangle is demonstrated.

I recently made use of my right to access personal information from someone who had sent me unsolicited marketing material. The person who fulfilled my request sent me a PDF copy of their full customer list, where all entries had been blacked out except mine. As you may anticipate, that blacking out was simply a black box drawn on top of the actual data. It took me all of 3 seconds to select all, copy, then paste in…

About 10 years ago or something, someone in the US government made this mistake and it was all over the news.

That’s when a redaction tool suddenly appeared in Adobe Acrobat.

Re: Recovering redacted information from pixelated videos

#45

Post by kauguste281 »

jabbany wrote: post #18
NikolaeVarius wrote: post #2

So much brain power only to be used to de-blur japanese p0rn

Hah, I know you're joking here but this is (was) a thing: https://www.reddit.com/r/programming/comments/9sc0qj/deepcre...

Deblurred porn (aka "Decensored") has left the realm of jokes a while ago, that stuff is already out in the wild on your average porn site.

Re: Recovering redacted information from pixelated videos

#46

Post by MauranKilom »

Grustaf wrote: post #37
MauranKilom wrote: post #31

Earlier quoted context omitted.

Sure, but the technique used was also very trivial. Just aligning and averaging all the video frames basically leaves a mosaic-pixel-sized blur on everything (assuming the camera movement is uncorrelated with the mosaic grid). You can get much further by applying deconvolutions and using more math. I've been meaning to put some time into this myself but never got it off the ground. I wonder if the author would be ope…

Yes, I've also always felt there must be ways to extract more data from a moving clip, precisely because of the effect he explains, but then it seems that just superimposing the images doesn't actually extract that information, at least not all of it. But I wonder how to actually do it, do you have concrete ideas for a simple algorithm?

If you can figure out, for each frame, which sets of (pre-aligned) pixels have been averaged, you can create a large system of equations that captures those relations and solve it to find the unblurred pixel values.

Depending on camera movement (and whether you might get "ground truth" information from pixels entering and leaving the areas near the borders) the system will be more or less well-conditioned. I'm going to try this for the data the author graciously provided and report back!

Re: Recovering redacted information from pixelated videos

#47

Post by twotwotwo »

You should be able to do better than just aligning and averaging frames. (Edit: looks like MauranKilom knows what they're talking about here, and expresses in their comment it clearer than I could.)

Imagine you were running averages on successive windows of a 1D array--when the average changes, that tells you the difference between the values that entered your window and the ones that just left. That's information about a sliver of data much smaller than the overall window. It's weirder with 2D and random-ish movement, but if your average (pixelation) filter is moving across text due to camera wobble or such, when the average goes up and down tells you something about where edges are in the content underneath.

I'm butchering the words because this isn't my thing, but this feels like it might be related to some actual signal-processing task (i.e. undoing some kind of signal-mangling that happens in the wild) which increases the chance that there's some good or at least well-studied solution.

The brute-force-ish approach for text reconstruction would also probably more effective if it checked against a few shifted-around blurred copies of the text, rather than just one.

Re: Recovering redacted information from pixelated videos

#49

Post by Scoundreller »

RunSet wrote: post #4

Yet again mosaic blur's inferiority to opaque rectangle is demonstrated.

Half of the fun of redacted rectangles is figuring out which letters/words can fit into that rectangle. Movable type makes things even easier.

I once read a gov document that opaque squared the pronouns, but it was clearly about a she/her!

Re: Recovering redacted information from pixelated videos

#50

Post by Someone »

odyssey7 wrote: post #44

Any relationship between optical character recognition and "deblurring" images into characters or words that the system recognizes?

OCR algorithms typically aren’t targeting heavily blurred text. It’s more about handling all the ways letters can look, including ligatures, determining paragraph breaks, detecting tables, ignoring staples and coffee stains, etc. than about correcting for bad scans/photos.
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