It’s somewhere in the FAQ at https://stopncii.org/faq/, but It’s PDQ for photos (https://github.com/facebook/ThreatExchange/tree/main/pdq) and MD5 for videos. PDQ is resistant to some modifications (it focuses on the ones that come from regular usage, such as changing the format from gif to jpg, or a filter changing colors or brightness), but it’s not as resistant to modifications as you could get by training dedicated classifiers or other approaches that you might do with the original media or by storing more context, which StopNCII chose not to do.
