Signal is a free app that cybersecurity experts consider to be one of the most secure messaging services because of its end-to-end encryption. Simply put, text messages or calls are seen only by you, the sender and whoever is in your Signal group chat.

The Signal app on a smartphone. Matthias Balk/Picture Alliance— Correspondent The Trump administration is facing heavy blowback for using Signal, a messaging app, to discuss sensitive military plans.

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