Essential reading on a hard-to-study but dominant communication tool, encrypted messaging apps—WhatsApp, Signal, and their kin—showing their promise and dangers.

Pundits have long pointed to social media’s power to catalyze or end political movements—and to strengthen or weaken democracies. But this power is shifting from social media platforms (such as Facebook and YouTube) to lesser-studied encrypted messaging apps (such as WhatsApp and Signal). This urgent book explains how these technologies work and how they differ from their predecessors, outlining their strengths and vulnerabilities at a crucial stage in their evolution.

While many Facebook users have at least heard of propaganda and the algorithms used to filter and highlight content on that platform, they are more likely to trust messaging apps as sources of authentic and secure information. Paired with a growing distrust of traditional media, more people are joining large groups on these apps and using them as news sources, even when those groups include relative strangers. This makes these apps particularly vulnerable to political propaganda—and even government espionage.

Security studies scholar Inga Kristina Trauthig and propaganda scholar Samuel Woolley consider both the role of these apps in spreading disinformation and as tools for maintaining free speech and civic discourse in the face of censorship. The same closed nature that makes these apps seem trustworthy has made them particularly difficult to research. And so this qualitative study draws on some 180 interviews with propagandists, activists, influence campaigners, and other users in fourteen countries. The book’s findings and the authors’ recommendations are crucial for understanding the future of online communication and safeguarding democratic speech.