What is Signal? The messaging app where Trump officials accidentally shared war plans with a journalist

The security of messaging app Signal was compromised by human error when top US officials accidentally added a journalist to their group chat.

But what is Signal - and did Donald Trump's team break the rules by using it?

Read more: 'Pathetic European free-loading': What was in chat

Signal is a secure messaging app that is end-to-end encrypted, meaning conversations on the platform can't be accessed by anyone except the people in the chat. Even Signal itself cannot see what is said.

Although end-to-end encryption is now fairly common, with WhatsApp, Facebook Messenger and Telegram all offering it, Signal was ahead of its time and began offering secure messaging in 2013.

The encryption technology it uses, called the Signal Protocol, is now considered the industry standard.

It aims to "end-to-end encrypt everything that we possibly can and to store as little [user data] as possible", according to Signal Foundation president Meredith Whittaker and developer Joshua Lund.

The non-profit's ethos of customer privacy has made it a go-to messaging platform for secure conversations.

Signal is also open-source, which means its users can check that the platform is actually doing what it says it is doing.

It is also unable to hand over information to law enforcement authorities, which some other encrypted messaging platforms do, because it can't break its own encryption.

"Signal protects all of this with strong encryption so even if Signal is threatened or coerced, Signal can't hand over that kind of data," Ms Whittaker said recently.

That became a sensitive topic for the Home Office in February after it was reported the UK government tried to force Apple to create a backdoor to its end-to-end encryption.

However, despite its focus on data privacy, Signal is still a commercial app and is not classified by US officials.

According to US media, some officials were allowed to download Signal on their White House-issued phones under Joe Biden's administration but were told never to share classified information on it.

Messaging apps "are NOT authorised to access, transmit, process non-public DoD information", Pentagon rules state.

When Trump officials accidentally included the editor of The Atlantic in a group chat where they discussed conducting airstrikes on Yemen's Iran-backed Houthis, it was not immediately clear if information on the military operation was classified.

However, information like this often is and would usually be kept secure to protect service members and operational security.

Brian Hughes, spokesman for the National Security Council, confirmed the veracity of the Signal group.

"This appears to be an authentic message chain, and we are reviewing how an inadvertent number was added to the chain," he said.

(c) Sky News 2025: What is Signal? The messaging app where Trump officials accidentally shared war plans with a journal