Millions of Californians have gotten a push notification on their phones asking them to opt in to get warnings from their phones if they have been in close proximity to someone who later tests positive for the coronavirus, all while keeping everyone involved anonymous. We wanted to know more about how exposure notifications work and exactly how well privacy is protected. Gennie Gebhart with the Electronic Frontier Foundation, a digital civil liberties nonprofit, explains how the system works and what information is exchanged.