When Hany Farid started working a few decades ago on analyzing digital images, he was thinking about how to detect forgeries and copies of still photos and probably hadn’t even heard of the word “jihad.” Fast forward to 2016. The Dartmouth professor’s research has now morphed into something called robust hashing and is being touted as an important new tool against online videos of violence, beheadings and anti-Western diatribes that are used to recruit terrorists and spread violent hate.