4 ms·

Ok, so now all the harassers out there are just going to edit a few pixels on the images before uploading, so the hash isn't caught. All the same tricks use don

Ok, so now all the harassers out there are just going to edit a few pixels on the images before uploading, so the hash isn't caught. All the same tricks use don YouTube to avoid the copyright bot, will work here as well.

BinaryIdiot 9y ago
Yeah I'm curious how their hashing mechanism works. Like, is it a straight up sha hash of the image or something a little more sophisticated that wouldn't be fooled by minor edits? Though in the latter case can that even technically be considered hashing? I guess it all depends on how they're doing it which brings us back to my first question...
- evgen 9y ago
  It is called PhotoDNA and was developed by Microsoft. This is the same tech that is used to find child porn images even when people go to various lengths to prevent simple hashing from being able to determine photo similarity. It works surprisingly well at the task from what I have seen/heard.
  - b4lancesh33t 9y ago
    Photo dna is just one of a class of algorithms called perceptual image hashes (there are also perceptual audio and video hash algorithms). They by no means invented the technique, just one particular branded implementation of it.
- FRex 9y ago
  Hash by strictest definition is just a function that maps any input to fixed size output. There is a special subcategory of hashes that are locality sensitive and change a little when input changes a little, just like there is a special subcategory of crypto hashes. See: https://en.wikipedia.org/wiki/Locality-sensitive_hashing https://en.wikipedia.org/wiki/Locality-sensitive_hashing Technology for searching for similar images (I have no idea how it works interally) already exists and is very widely deployed. You can easily find an image similar (uncropped, cropped, with different filter, greyed, ungreyed, with a logo in corner, etc.) to your input image on https://tineye.com/ https://tineye.com/ and https://images.google.com https://images.google.com .
- 15155 9y ago
  They're (more than likely - almost certainly) using proprietary image fingerprinting techniques. If you had access to the original implementation, you could probably defeat them permanently with minimal alteration via reverse engineering. Which is why you don't have access to the original software, its specifications, or implementation details. (https://twitter.com/alexstamos/status/928646228472078336 https://twitter.com/alexstamos/status/928646228472078336)
baddox 9y ago
I’m not saying that people won’t be able to fool it, but I’m sure it would require way more than just changing a few pixels. I’ve seen YouTube videos that were clearly edited to circumvent the copyright bot, and the video transformations were so significant that the videos were essentially unwatchable.
giobox 9y ago
I have no knowledge of the implementation, but I think it's more than likely a safe bet that someone at Facebook probably thought of this and chose a slightly more sophisticated approach. The entire system would be completely pointless otherwise!
CGamesPlay 9y ago
This is likely the same image hashing they use to detect child pornography, and there's been a lot of work invested into making sure that those types of filters don't prevent the detection. You can very effectively require that the "real" image is visually entirely dissimilar. Now, obviously you can't prevent the actual data from being distributed. For example, you could base64 encode the image and send it as a series of messages instead. The motivation for revenge porn is to hurt the other person. One vector for this is to post it to all of your mutual friends so that the victim is shamed. This adds a barrier so that the mutual friends have to invest work into getting and distributing the porn, and won't accidentally see it flowing through their feed.
oh_sigh 9y ago
md5/sha isn't the only hashing method. There are perceptual hashes which are fairly resilient to simply editing few pixels, or rotating/resizing/cropping an image.

BinaryIdiot 9y ago

Yeah I'm curious how their hashing mechanism works. Like, is it a straight up sha hash of the image or something a little more sophisticated that wouldn't be fooled by minor edits? Though in the latter case can that even technically be considered hashing? I guess it all depends on how they're doing it which brings us back to my first question...

evgen 9y ago
It is called PhotoDNA and was developed by Microsoft. This is the same tech that is used to find child porn images even when people go to various lengths to prevent simple hashing from being able to determine photo similarity. It works surprisingly well at the task from what I have seen/heard.
- b4lancesh33t 9y ago
  Photo dna is just one of a class of algorithms called perceptual image hashes (there are also perceptual audio and video hash algorithms). They by no means invented the technique, just one particular branded implementation of it.
FRex 9y ago
Hash by strictest definition is just a function that maps any input to fixed size output. There is a special subcategory of hashes that are locality sensitive and change a little when input changes a little, just like there is a special subcategory of crypto hashes. See: https://en.wikipedia.org/wiki/Locality-sensitive_hashing https://en.wikipedia.org/wiki/Locality-sensitive_hashing Technology for searching for similar images (I have no idea how it works interally) already exists and is very widely deployed. You can easily find an image similar (uncropped, cropped, with different filter, greyed, ungreyed, with a logo in corner, etc.) to your input image on https://tineye.com/ https://tineye.com/ and https://images.google.com https://images.google.com .
15155 9y ago
They're (more than likely - almost certainly) using proprietary image fingerprinting techniques. If you had access to the original implementation, you could probably defeat them permanently with minimal alteration via reverse engineering. Which is why you don't have access to the original software, its specifications, or implementation details. (https://twitter.com/alexstamos/status/928646228472078336 https://twitter.com/alexstamos/status/928646228472078336)

baddox 9y ago

I’m not saying that people won’t be able to fool it, but I’m sure it would require way more than just changing a few pixels. I’ve seen YouTube videos that were clearly edited to circumvent the copyright bot, and the video transformations were so significant that the videos were essentially unwatchable.

giobox 9y ago

I have no knowledge of the implementation, but I think it's more than likely a safe bet that someone at Facebook probably thought of this and chose a slightly more sophisticated approach. The entire system would be completely pointless otherwise!

CGamesPlay 9y ago

This is likely the same image hashing they use to detect child pornography, and there's been a lot of work invested into making sure that those types of filters don't prevent the detection. You can very effectively require that the "real" image is visually entirely dissimilar. Now, obviously you can't prevent the actual data from being distributed. For example, you could base64 encode the image and send it as a series of messages instead. The motivation for revenge porn is to hurt the other person. One vector for this is to post it to all of your mutual friends so that the victim is shamed. This adds a barrier so that the mutual friends have to invest work into getting and distributing the porn, and won't accidentally see it flowing through their feed.

oh_sigh 9y ago

md5/sha isn't the only hashing method. There are perceptual hashes which are fairly resilient to simply editing few pixels, or rotating/resizing/cropping an image.