"The New York Times reported on Saturday that Facebook Messenger, which is not encrypted, accounted for nearly two-thirds of reports last year of online child sexual abuse imagery. On Wednesday, the Justice Department said that Facebook as a whole was responsible for 90 percent of the reports. Law enforcement agencies say encryption is a major obstacle in child sex abuse, terrorism and other investigations. 'There are really good reasons to have end-to-end encryption, but we have to acknowledge it comes with trade-offs,' said Hany Farid, a professor at the University of California, Berkeley, who helped develop technology in 2009 for detecting online child abuse imagery. He suggested scanning for abuse content by making a fingerprint of an image before the message was encrypted, and then comparing the fingerprint with a database of known illegal material. 'I don’t think there’s a technical barrier here,' Dr. Farid said. 'They’re doing this because they want to avoid liability.'"