The encrypted-messaging service Signal is the application of choice for dissenters around the world. The app has been downloaded by more than 100 million users and boasts high-profile endorsements from NSA leaker Edward Snowden and serial entrepreneur Elon Musk. Signal has created the perception that its users, including political dissidents, can communicate with one another without fear of government interception or persecution. But the insider history of Signal raises questions about the app’s origins and its relationship with government—in particular, with the American intelligence apparatus. Such a relationship would be troubling, given how much we have learned, in recent years,...