[HN Gopher] Police Can Trace Cameras Thanks to Sensor Imperfecti...
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Police Can Trace Cameras Thanks to Sensor Imperfection
'Fingerprints'
Author : mikece
Score : 69 points
Date : 2022-07-01 14:59 UTC (8 hours ago)
(HTM) web link (petapixel.com)
(TXT) w3m dump (petapixel.com)
| [deleted]
| buildbot wrote:
| Any camera does a huge amount of processing before even giving
| you a raw file, in most cases. And any algorithm for data
| reduction like converting true raw sensor output to processed
| data, say JPG, is somewhat hash like. But not a cryptographic
| hash, more like a locality sensitive hash. Of course this can be
| exploited to identify cameras.
|
| Really old Sinar digital backs came with a calibration file
| unique to the CCD serial, newer cameras have it embedded in their
| firmware typically. You can also build these calibration files,
| but the way you apply it will still leave processing "marks" that
| someone will be able to pick up.
| vmoore wrote:
| This reminds me of an article a while back: `Facebook can track
| who you know using the dust on your camera`[0]
|
| [0]
| https://www.dailymail.co.uk/sciencetech/article-5262297/Face...
| deepnotderp wrote:
| Will the lack of existence of any stable fingerprint be a valid
| heuristic for the detection of a deepfake?
| vasco wrote:
| Should be straightforward to train a model to add these based
| on a few examples. Could last for a short bit though.
| lkxijlewlf wrote:
| Laser printers (color ones) are (were?) required to have some
| sort of built in imperfection because they were too good and
| could be used for counterfeiting.
|
| I wouldn't be surprised if this eventually is a requirement for
| cameras, you know, just because law enforcement wants it.
| sbierwagen wrote:
| https://en.wikipedia.org/wiki/Machine_Identification_Code
|
| There's also a pattern of circles on currency that color
| copiers and Photoshop will read and then refuse to work on:
| https://en.wikipedia.org/wiki/EURion_constellation
| wubbert wrote:
| About 15 years when I got my first digital camera, I noticed that
| there were always a few dots in the same spots that were bright
| white/red/green/blue no matter what the settings on the camera
| were or what the picture was of. I found out these were "hot
| pixels" and were caused by defects in the sensor. I'd always
| wondered if someone could create an algorithm to match photos to
| a specific camera based on hot pixels. It seems like this is
| exactly that.
| dontcare007 wrote:
| Seems like an opportunity for an anonymous service. Post
| calorie editing if photos to remove identifying fingerprints...
| noja wrote:
| You could do something similar with the lens too.
| Manuel_D wrote:
| Yeah, cameras calibrate out those pixels essentially by taking
| a photo of total darkness (taking an exposure while the shutter
| is closed) and discarding any pixels that still read bright. I
| noticed this happen a while back and the location of the dead
| pixel was visible if I took a photo of fabric. I could see
| where the pixel was being interpolated from surrounding pixels.
| Super subtle, I could only notice by knowing a-priori where the
| dead pixel was, but something that an algorithm could detect.
|
| I wonder how well this fingerprinting technique works when
| photos are resized or manipulated, though.
| trompetenaccoun wrote:
| For certain types of photography (for example low-light) it's
| important to remove them if you care about quality, I think
| software for getting rid of hot pixels came first. All you need
| is enough suitable shots from the same camera, so it's sort of
| trivial. Of course this can also be used as a sort of
| fingerprint, so it's true that if you want to share photos
| truly anonymously you need to get rid of those pixels in
| addition to the metadata.
| Arnt wrote:
| I have a feeling that someone like Samy Kamkar is already
| thinking about writing a convenient tool to replace the
| fingerprint of one camera with that of another on a photo.
|
| (Hm, the latest entry on samy.pl is two years old. Is he well?)
| danielfoster wrote:
| >Groningen is a city in the Netherlands, which is the biggest
| distributor of child sex abuse images in the world.
|
| How does a city of only 200,000 people earn this title?
| tomcam wrote:
| Well the Dutch are famous for their work ethic. Seriously, an
| assertion like that is easy to make but it could well be that
| they are better about keeping statistics and are more willing
| to publish them transparently.
| piva00 wrote:
| Same issue with the numbers on sexual abuse in Sweden, they
| are more often reported and the definition is broader than
| the common sense belief. Numbers appear ridiculous (to the
| point where the alt-right loves taunting Swedes with "rape
| capital of the world" and such) but it's just a matter of
| much better statistical collection, and more nuanced (and
| broader) interpretation of what configures sexual abuse.
| SoftTalker wrote:
| It's unclear whether "which" in that sentence is referring to
| Groningen or the Netherlands.
| [deleted]
| oropolo wrote:
| Which leads to the question: will cameras be "fingerprinted"
| before being sold?
| mistrial9 wrote:
| digital cameras have extensive internal IDs that are
| transferred into the image file -- this varies a lot by
| manufacturer and model
|
| _edit_ https://packages.debian.org/search?keywords=exif
| pessimizer wrote:
| The file, but not the image. Also easily removable, although
| companies have been clearly encouraged to make this difficult
| in mainstream software and to set maximal defaults. Probably
| doesn't take much encouragement, because the more metadata,
| the more automagic.
| dogma1138 wrote:
| It would be far easier to force phone and camera manufacturers
| to "embed" a fingerprint in the photo than to measure every
| sensor.
|
| Also these fingerprints in reality are very flaky and the
| higher the quality of the sensor the less of a fingerprint
| there is to work with.
|
| The fingerprints are also dependent on specific operating
| conditions which can change with firmware and operating
| parameters (e.g. digital zoom / cropping) as well as
| environmental conditions such as light levels and even
| temperature.
| gonesilent wrote:
| camera manufacturers already put unique QR like barcode on
| the sensors for lot tracking and such.
| thfuran wrote:
| Slapping a barcode on a sensor is a lot less involved than
| using it to take some careful measurements and then storing
| and retaining the results.
| zorlack wrote:
| ...and also leads would-be anonymous image-posters to increase
| the noise-floor of their photographs.
| ancientworldnow wrote:
| It makes more sense to just denoise.
| dogma1138 wrote:
| Most phones do that for you anyhow. Unless there are
| serious defects in the sensor that would probably mean it
| would fail QA even for bargain bin phones the amount of
| "AI" post processing that phones do these days is probably
| sufficient to erase any sensor fingerprint.
|
| Even with DSLRs and RAW files you often don't get a RAW
| output from the sensor all of them do their own "color
| science" magic and other alterations like denoising too
| even on the rawest of the RAW settings.
|
| RAW files today just mean that the files are uncompressed
| or the least compressed since there might be some
| compression/downsampling happening at readout anyhow and
| that you get a ton of metadata that can be used by a photo
| editing app to better work with the image.
| zorlack wrote:
| Or to just dither the hell out of your image.
|
| Luminance sensitivity information could probably be most-
| easily detected in the dark areas of an image. So just
| crush those areas.
|
| I suspect the more you think about this problem the more
| the answer becomes: Compress your image in order to remove
| information density.
|
| Still, a determined adversary might find this information
| discernable over a long enough series of images.
| toss1 wrote:
| I wonder if this would still work with cameras like the S22 Ultra
| which has iirc, ~108MP sensors and uses pixel binning to improve
| the pic and outputs ~12MP photos. Seems the binning algorithm
| would obscure many of the 'sins' of the individual pixels and
| blend the output to something more like what is generically
| expected from a photo. Or, could the algorithm actually wind up
| amplifying some of the flaws, perhaps once they got past a
| certain size?
| formerly_proven wrote:
| There are usually blobby patterns to the overall non-
| uniformity, with tiling for larger (full-frame and up) sensors
| where the different parts of the sensors are stitched during
| lithography and commonly vertical lines or groups of lines for
| differences between the column amplifiers and ADCs (CMOS image
| sensors usually have one PGA and ADC for every column). You can
| look at this by searching for dark frames astrophotography
| people post.
| stillbourne wrote:
| This seems like something pretty easy to spoof.
| conductor wrote:
| In some cameras you can use "pixel mapping" to "fix" the sensor's
| imperfectoins, see https://www.fujix-forum.com/threads/pixel-
| mapping.73391/.
| buildbot wrote:
| As well as dark/flat file calibration - it can get pretty
| complex: https://nightskypix.com/calibration-frames/
| https://www.astropy.org/ccd-reduction-and-photometry-guide/v...
| PaulHoule wrote:
| Doesn't surprise me at all.
| kornhole wrote:
| After learning about this, all kiddie pornographers will learn to
| put their photos through postprocessing to mask any signatures.
| Activists photographing corporate or government crimes may not
| take the same precautions.
| cabirum wrote:
| To extract the noise from sensor imperfections, wouldn't one need
| unaltered images in raw format?
|
| Automatic denoise and image compression ought to erase all but
| most obvious defects.
| dboreham wrote:
| Guessing that with several compressed images you can recover
| the original sensor noise pattern.
| ashwagary wrote:
| Call me cynical but I've noticed an uptick in phony police
| capability stories like these that I expect make parallel
| construction easier.
| 1-6 wrote:
| It's possibly difficult with one or two images but with
| several, I'm pretty sure you can apply some sort pattern
| matching and statistically hit a certainty threshold. The
| question then becomes, can you use this tech as forensic
| evidence in a court?
| FrenchyJiby wrote:
| A lifetime ago, I summarized the techniques of sensor detection
| for a stackoverflow answer, see [1].
|
| The generic answer is every step of the acquisition/storage
| process can create artifacts worth analyzing statistically:
| lens defects, color filter array patterns, imaging sensor
| noise, color filter array interpolation technique, JPEG
| quantization coefficient varying per camera/manufacturers...
|
| [1]: https://stackoverflow.com/a/33674103/4576325
| dr_zoidberg wrote:
| From reading the article, it looks like it talsk about
| PRNU[0] without really saying it's PRNU. For a while this
| technique was known (the earliest papers about it are from
| the early 2000s) but it was computationally complex at the
| moment, and the domain of a few very knowledgeable persons. A
| couple[1] of years ago, MobilEdit finally included a decent
| UI and implementation of the technique into their
| software[2], which has made it available. However, I'm unsure
| if the majority of those who use it truly understand what the
| technique does and what conclusions you can draw from it.
|
| Oh and great summary on SO FrenchyJiby! It's a good intro to
| those that aren't aware of what can and can't be done in
| image analysis :)
|
| [0] https://forensicnerd.wordpress.com/techniques/image-
| forensic...
|
| [1] https://xkcd.com/1070/
|
| [2] https://www.mobiledit.com/camera-ballistics-features
| jdavis703 wrote:
| Yes, but it sounds like forensic researchers envision this tool
| being the equivalent to bullet striation. So in other words,
| law enforcement finds a suspect, captures their camera,
| photographs RAW images, and then compares the sensor
| imperfections in the seized camera to known illicit materials.
|
| Of course this breaks down with used sales and camera rentals,
| especially since those aren't tracked as intensively as
| firearms sales.
| twoxproblematic wrote:
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