4 ms·

In the manual review failure case, you're saying that the reviewer would decrypt the steganographical encryption, discover the hidden offensive content, and the

by tehnub 5y ago

In the manual review failure case, you're saying that the reviewer would decrypt the steganographical encryption, discover the hidden offensive content, and thereby decide that it's a true positive? And related to that, I wonder how steganographically hidden offensive content would affect a perceptual hash. Would it help or would it hurt the similarity measure if an attacker were trying to generate a benign-looking collision in the manner described.

GistNoesis 5y ago
My guess is that adding steganographically hidden content is almost free for the attacker as perceptual hash is resistant to small variation. There is a free parameter called gap in my code which help make sure that you can modify the image a little bit and still get the same hash. So probably the attacker can use the least significant bit of each pixel as a hidden channel. Take 8 pictures and you can hide any grey image ((or hide them in the same one if the original image is of high resolution (perceptual hash is almost like acting on thumbnails) ). The human eye won't be able to see anything special in the picture, but a forensic software will definitely notice that something's off in the least significant bits of your pictures and light some red flag. Then once the reviewer figure the steganographical scheme, he will decode the hidden picture, and after having a ha-ah moment he will be convinced 100% that this is offensive content and that it should be reported.
- chaboud 5y ago
  What you're highlighting is the lack of avalanche effect with perceptual hashes (on purpose). This makes them great for identifying transformed content (e.g. local TV channels via a cable box, ads on channels with slid in lower third tickets, etc) but horrible as an evidentiary basis. That said, there's no need to clickbait here. Perceptual hashes are going to be a hot topic for a while here.

by tehnub 5y ago

GistNoesis 5y ago

My guess is that adding steganographically hidden content is almost free for the attacker as perceptual hash is resistant to small variation. There is a free parameter called gap in my code which help make sure that you can modify the image a little bit and still get the same hash. So probably the attacker can use the least significant bit of each pixel as a hidden channel. Take 8 pictures and you can hide any grey image ((or hide them in the same one if the original image is of high resolution (perceptual hash is almost like acting on thumbnails) ). The human eye won't be able to see anything special in the picture, but a forensic software will definitely notice that something's off in the least significant bits of your pictures and light some red flag. Then once the reviewer figure the steganographical scheme, he will decode the hidden picture, and after having a ha-ah moment he will be convinced 100% that this is offensive content and that it should be reported.

chaboud 5y ago
What you're highlighting is the lack of avalanche effect with perceptual hashes (on purpose). This makes them great for identifying transformed content (e.g. local TV channels via a cable box, ads on channels with slid in lower third tickets, etc) but horrible as an evidentiary basis. That said, there's no need to clickbait here. Perceptual hashes are going to be a hot topic for a while here.