Are any encrypted messaging apps fail-safe? Subjects of Mueller’s investigation are about to find out.

Special counsel Robert S. Mueller III's team is reportedly reviewing the encrypted messaging apps of witnesses in the Russia investigation. The team is looking at what experts say are some of the best apps at keeping messages private. Not all encrypted messaging apps disclose their user numbers, so it's hard to pinpoint just how prolific they have become. But the most popular among them, WhatsApp, claims 1.5 billion users around the world. Offering people what's known as end-to-end encryption, in which only the sender and the intended recipient can read a message, the apps are designed to help people communicate more securely. They are popular among activists, journalists, security professionals and government officials.