this post was submitted on 28 Aug 2023
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I just developed and published a script to clear your pict-rs object storage from potential CSAM. (github.com)
submitted 1 year ago* (last edited 1 year ago) by [email protected] to c/selfhosted
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I noticed a bit of panic around here lately and as I have had to continuously fight against pedos for the past year, I have developed tools to help me detect and prevent this content.

As luck would have it, we recently published one of our anti-csam checker tool as a python library that anyone can use. So I thought I could use this to help lemmy admins feel a bit more safe.

The tool can either go through all your images via your object storage and delete all CSAM, or it canrun continuously and scan and delete all new images as well. Suggested option is to run it using --all once, and then run it as a daemon and leave it running.

Better options would be to be able to retrieve exact images uploaded via lemmy/pict-rs api but we're not there quite yet.

Let me know if you have any issue or improvements.

EDIT: Just to clarify, you should run this on your desktop PC with a GPU, not on your lemmy server!

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[–] [email protected] 18 points 1 year ago* (last edited 1 year ago) (2 children)

match the hashes

It's more than just basic hash matching because it has to catch content even if it's been resized, cropped, reduced in quality (lower JPEG quality with more artifacts), colour balance change, etc.

[–] [email protected] 12 points 1 year ago (1 children)

Well, we have hashing algorithms that do exactly that, like phash for example.

[–] [email protected] 2 points 1 year ago (1 children)

Definitely. A lot of the good algorithms used by big services are proprietary though, unfortunately.

[–] [email protected] 1 points 1 year ago (2 children)

Can you point me to some of them? I'm quite interested in visual hashing.

[–] [email protected] 4 points 1 year ago* (last edited 1 year ago)

Microsoft's PhotoDNA is probably the most well-known. Every major service that has user-generated content uses it. Last I checked, it wasn't open-source. It was built for detecting CSAM, but it's really just a general-purpose similarity hashing algorithm.

Meta has some algorithms that are open-source: https://about.fb.com/news/2019/08/open-source-photo-video-matching/

Google has CSAI Match for hash-matching of videos and Google Content Safety API for classification of new content, but both are proprietary.

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago)

There's better approaches than hashing. For comparing images I am calculating "distance" in tensors between them. This can match even when compression artifacts are involved or the images are slightly altered.

[–] [email protected] 3 points 1 year ago

Ah, of course - that's unfortunate, but thanks for the pointer.