Photo DNA

Microsoft's Photo DNA is a media hashing system used for photos and videos. Apparently:

"an image can be resized, cropped or colours changed and our hashes will still find a match to the original image."

In 2009, Microsoft partnered with Dartmouth College to develop PhotoDNA, a technology that aids in finding and removing known images of child exploitation. PhotoDNA Cloud Service is free for non-profits & law enforcement.

It is used by over 200 organisations worldwide, including the Internet Watch Foundation's Hash List in association with ChildLine, and the National Center for Missing & Exploited Children (NCMEC).

Video

It was designed for photos but they developed a video version with IWF: https://news.microsoft.com/on-the-issues/2018/09/12/how-photodna-for-video-is-being-used-to-fight-online-child-exploitation/.

It works by taking key frames from the video, and generating hashes from them. Apparently:

“When people embed illegal videos in other videos or try to hide them in other ways, PhotoDNA for Video can still find it. It only takes a hash from a single frame to create a match.”