PDQ

PDQ is Meta's open perceptual image hash, released in 2019 as an open alternative to PhotoDNA. It produces a 256 bit hash that stays similar for visually similar images, so you compare hashes by Hamming distance and set a threshold. Implementations exist in C++, Python, PHP, Java and Rust, and a FAISS based matcher handles millions of hashes.

It's the format GIFCT and many newer lists use, and because it's fully open you can inspect and run it anywhere.