Signal, a text-and-voice app that is a little over a decade old, is seen as the gold standard for end-to-end encrypted communications, according to mobile security experts. It has been widely adopted - not just by privacy-conscious dissidents but by officials, lawmakers, generals, and corporate leaders as well.

A journalist's account of being added to a group chat of national security officials coordinating plans for airstrikes has raised questions about how sensitive information is supposed to be handled.