Reposting due to automod.

“Known as PDQ and TMK+PDQF, these technologies are part of a suite of tools we use at Facebook to detect harmful content, and there are other algorithms and implementations available to industry such as pHash, Microsoft’s PhotoDNA, aHash, and dHash. Our photo-matching algorithm, PDQ, owes much inspiration to pHash although was built from the ground up as a distinct algorithm with independent software implementation.”

I don’t know if all are open source, some might be proprietary.

Source: meta/fb blog post about how they fight CSAM.

There are hash functions that work on images that aren’t susceptible to basic alterations. They aren’t the ones you would use for say hashing passwords.