Amid an unprecedented cyberattack on telecommunications companies such as AT&T and Verizon, US officials have recommended that Americans use encrypted messaging apps to ensure their communications remain hidden from foreign hackers.

The hacking campaign, nicknamed “Salt Typhoon” by Microsoft, is one of the largest intelligence compromises in U.S. history and has not yet been fully resolved. Officials declined in a news briefing Tuesday to set a timeline for declaring the country’s telecommunications systems free of intrusions. Officials had told NBC News that China had hacked AT&T, Verizon and Lumen Technologies to spy on customers.

A spokesperson for the Chinese Embassy in Washington denied that the country was behind the hacking campaign, telling NBC News in an email that “China firmly opposes and combats all types of cyber attacks.”

In the call Tuesday, two officials — a senior FBI official who asked not to be named and Jeff Greene, deputy director for cybersecurity at the Cybersecurity and Infrastructure Security Agency — recommended that Americans who want to minimize risk use encrypted messaging. Apps China intercepts your communications.

“Our suggestion that we’ve been telling people internally is not new here: Encryption is your friend, whether it’s text messaging or whether you have the ability to use encrypted voice communications. Even if the adversary were able to intercept the data, it would be impossible to encrypt it if it were encrypted,” Greene said.

The FBI official said: “People who want to further protect their mobile device communications would benefit from using a cell phone that automatically receives timely operating system updates, responsibly managed encryption, and phishing-resistant” multi-factor authentication for e-mail. Mail, social media and collaboration tool accounts.

The scope of the telecommunications compromise was so significant, Greene said, that it was “impossible” for authorities to “predict a time frame for when the full evacuation will occur.”

The hackers generally accessed three types of information, the FBI official said.

One type was call records, or metadata that shows the numbers that phones called and when. The hackers focused on records in the Washington, D.C. area, and the FBI does not plan to alert people whose phone metadata was accessed.

The second type was live phone calls to specific targets. The FBI official declined to say how many warnings he had sent to the targets of that campaign; Presidential candidates Donald Trump and Kamala Harris, as well as the office of Senate Majority Leader Chuck Schumer, D-N.Y., told NBC News in October that the FBI said they had been targeted.

The third is systems that telecommunications companies use in compliance with the Communications Assistance for Law Enforcement Act (CALEA), which allows law enforcement and intelligence agencies with a court order to track people’s communications. CALEA systems may contain secret court orders from the Foreign Intelligence Surveillance Court, which processes some U.S. Secret Service court orders. The FBI official declined to say whether classified material was accessed.

Data protection advocates have long advocated the use of end-to-end encrypted apps. Signal and WhatsApp automatically implement end-to-end encryption for both calls and messages. Google Messages and iMessage can also end-to-end encrypt calls and text messages.

The FBI and other federal law enforcement agencies have a complicated relationship with encryption technology and have historically advocated against full end-to-end encryption, which prevents law enforcement from accessing digital materials even with warrants. However, the FBI has also supported forms of encryption that allow law enforcement access under certain circumstances.

Although the hacking campaign was first made public in the run-up to the election, the US believes it was not an attempt to influence the results, the FBI official said, but rather a massive but traditional espionage operation by China, to collect information about US politics and government.

“We view this as a cyber espionage campaign not dissimilar to other approaches. “Certainly the way they went about it was very, very specific to the telecommunications companies and ISPs, but it fits into the realm of cyber espionage,” the FBI official said.

In a statement to NBC News, Rep. Ron Wyden, D-Ore., one of the Senate’s fiercest privacy advocates, criticized America’s reliance on CALEA because such sensitive information remains unencrypted.

“Whether it’s AT&T, Verizon or Microsoft and Google, when those companies inevitably get hacked, China and other adversaries can steal those communications,” he said.

CORRECTION (December 4, 2024, 5:30 p.m. ET): An earlier version of this article misstated what the acronym CALEA stood for. It’s the Communications Assistance for Law Enforcement Act, not the Commission on Accreditation for Law Enforcement Agencies.