WASHINGTON — Attorney General William P. Barr is set to press Facebook on Friday to create a so-called back door to its end-to-end encryption on WhatsApp and other messaging platforms, which would give investigators access to now-secret communications, including from terrorists and other criminals as well as whistle-blowers, journalists and others.
Mr. Barr and his British and Australian counterparts were set to send a joint letter to Mark Zuckerberg, the Facebook chief executive, arguing that law enforcement needs a way to break encryption to fight terrorism, international organized crime and child exploitations, according to a copy of the letter reviewed by The New York Times that is dated Oct. 4.
With 1.5 billion users, Facebook’s WhatsApp is perhaps the most commonly used encrypted communications platform in the world. Privacy advocates and tech company officials have said that creating a back door will effectively destroy the secrecy of such platforms.
The Justice Department and its counterparts in Australia and Britain have pushed for back doors to other tech platforms but are focusing on Facebook because Mr. Zuckerberg has said that he intends to add end-to-end encryption to all of the company’s platforms, a government official said. BuzzFeed News first reported on Mr. Barr’s letter.
Facebook respects the role of law enforcement but believes people have a right to conduct private conversations online, said Andy Stone, a company spokesman.
“End-to-end encryption already protects the messages of over a billion people every day,” Mr. Stone said. “We strongly oppose government attempts to build back doors because they would undermine the privacy and security of people everywhere.”
The Justice Department has long pushed technology companies to help the government gain access to information on electronic devices. The conflict last came to a head in 2016, when investigators obtained a court order that required Apple to help the F.B.I. unlock an iPhone recovered after the mass shooting in San Bernardino, Calif., in December 2015.
The F.B.I. ultimately unlocked that phone without the help of Apple, easing tensions for a time with the tech companies.
Mr. Barr, who took office in February, has embraced the Justice Department’s push. In a speech in July, he called on tech companies to stop using advanced encryption that keeps out law enforcement officials.
Mr. Barr was to deliver remarks on Friday at a Justice Department summit on how encryption has stymied the government’s ability to access information, a problem that local and federal law enforcement agencies have coined “going dark.”
The summit, which Facebook representatives will also attend, will focus on the impact of encryption on child exploitation cases. Christopher A. Wray, the F.B.I. director, is also speaking and has pushed technology companies to cooperate more with law enforcement investigations.
Encryption not only created a platform for criminals to swap child pornography, but it has also impeded law enforcement officials’ searches for children being used to create pornographic images and videos of sexual assault, the government official said.
The letter highlighted an instance when a man was imprisoned on a conviction of sexually abusing a child starting at age 11 because prosecutors presented as evidence message logs between the man and the child that Facebook had provided.
Tech company officials have said that strong encryption is necessary to protect legitimate users of their platforms, including journalists and government critics.
The encryption on WhatsApp blocks Facebook from accessing the information its users send on the platform, similar to the system used by the Signal messaging service, considered to be one of the most thorough at protecting users’ privacy. Facebook has said it will expand its encryption system to Messenger and its other platforms.
Only platforms that use an encryption system that the company itself cannot break can be protected from hackers, technology company officers said. They said that the back door sought by the Justice Department will fundamentally weaken all encryption.
Even as technology companies move to more robust encryption, Facebook and others have devised methods to detect and remove child exploitative imagery, including one that relies on photo-matching technology. Facebook and WhatsApp are working with other major tech companies to share both information and photo-matching technology, relying on machine learning to ban groups suspected of trafficking in child pornography.
WhatsApp also regularly submits information to law enforcement officials when necessary and bans accounts suspected of or associated with illicit material.
But even with such measures, Facebook can hardly monitor the billions of pieces of content flowing through its encrypted systems every day, which is by design. Mr. Zuckerberg has predicted that such systems will grow increasingly popular as the internet evolves.
“The future of communication will increasingly shift to private, encrypted services where people can be confident what they say to each other stays secure and their messages and content won’t stick around forever,” Mr. Zuckerberg said in a statement this year announcing the changes. “This is the future I hope we will help bring about.”
But government officials disagreed. One said that “back door” was an inaccurate term because it implies a weakness in the encryption technology that hackers and others could also exploit.
In the letter to Facebook, Mr. Barr and the British and Australian officials said law enforcement must be able to unlock encryption systems to access information to “safeguard the public, investigate crimes and prevent future criminal activity.”
“Companies should not deliberately design their systems to preclude any form of access to content even for preventing or investigating the most serious crimes,” they wrote.
In the letter, the governments said they would seek to access Facebook and WhatsApp content only when public safety was at risk. They also said they recognize a right to privacy but that Facebook should be able to provide access if a judge has issued a warrant.
British and Australian officials have also pushed even longer for back doors, stretching back years.
Mr. Barr’s work with Australia and Britain has separately come under scrutiny as he has pressed for both countries’ cooperation with the Justice Department’s review of the origins of the investigation into Russian meddling in the 2016 United States election.
Julian E. Barnes and Katie Benner reported from Washington, and Mike Isaac from San Francisco.