IWF: https://www.iwf.org.uk/our-technology/our-services/image-hash-list/

This is an interesting problem because we must first obtain CSAM in order to purposefully train a CSAM detection model, so the first step is illegal territory, hence the strict license for this particular academic research... This makes it hard to open source, unlike any previous only NSFW detection.

See also: https://nsfwjs.com/