Signal, a popular messaging app, has come into the spotlight after reports that several senior Trump administration officials had used the tool to conduct war planning — inadvertently including a journalist in one message group and the wife, brother and personal lawyer of Secretary of Defense Pete Hegseth in another.

The app, which was started in 2014 and has hundreds of millions of users, is popular among journalists, activists, privacy experts and politicians — anyone who wants to secure his or her communications with encryption.

But the app’s use by government officials resulted in multiple intelligence breaches that took place outside the secure government channels that would normally be used for classified and highly sensitive war planning. The incidents have raised questions about Signal’s security and why government officials were using it. (Federal officials are generally not allowed to install Signal on their government-issued devices.)

Here’s what to know.

What is Signal used for?

Signal is an encrypted messaging application that is used to communicate securely. It encrypts messages from end to end, meaning that what a user says is encrypted on the user’s device and isn’t decrypted until it reaches the recipient. This method protects the message from being intercepted and read by anyone, including internet service providers, hackers or Signal itself, while it is in transit.

Users can also set Signal messages to disappear after a certain length of time. Users who want their messages to disappear can turn on the feature in the settings for each of their individual chats.

Who owns Signal?

Signal is owned by an independent nonprofit in the United States called the Signal Foundation. It is funded by donations from its users and by grants.