It is billed as an easy and secure way to chat by video or text message with friends and family, even in a country that has restricted popular messaging services like WhatsApp and Skype.

But the service, ToTok, is actually a spying tool, according to American officials familiar with a classified intelligence assessment and a New York Times investigation into the app and its developers. It is used by the government of the United Arab Emirates to try to track every conversation, movement, relationship, appointment, sound and image of those who install it on their phones.

ToTok, introduced only months ago, was downloaded millions of times from the Apple and Google app stores by users throughout the Middle East, Europe, Asia, Africa and North America. While the majority of its users are in the Emirates, ToTok surged to become one of the most downloaded social apps in the United States last week, according to app rankings and App Annie, a research firm.

ToTok amounts to the latest escalation in a digital arms race among wealthy authoritarian governments, interviews with current and former American foreign officials and a forensic investigation showed. The governments are pursuing more effective and convenient methods to spy on foreign adversaries, criminal and terrorist networks, journalists and critics — efforts that have ensnared people all over the world in their surveillance nets.

Persian Gulf nations like Saudi Arabia, the Emirates and Qatar previously turned to private firms — including Israeli and American contractors — to hack rivals and, increasingly, their own citizens. The development of ToTok, experts said, showed that the governments can cut out the intermediary to spy directly on their targets, who voluntarily, if unwittingly, hand over their information.

A technical analysis and interviews with computer security experts showed that the firm behind ToTok, Breej Holding, is most likely a front company affiliated with DarkMatter, an Abu Dhabi-based cyberintelligence and hacking firm where Emirati intelligence officials, former National Security Agency employees and former Israeli military intelligence operatives work. DarkMatter is under F.B.I. investigation, according to former employees and law enforcement officials, for possible cybercrimes. The American intelligence assessment and the technical analysis also linked ToTok to Pax AI, an Abu Dhabi-based data mining firm that appears to be tied to DarkMatter.

Pax AI’s headquarters operate from the same Abu Dhabi building as the Emirates’ signals intelligence agency, which until recently was where DarkMatter was based.

The U.A.E. is one of America’s closest allies in the Middle East, seen by the Trump administration as a bulwark against Iran and a close counterterrorism partner.

Its ruling family promotes the country as an example of a modern, moderate Arab nation, but it has also been at the forefront of using surveillance technology to crack down on internal dissent — including hacking Western journalists, emptying the banking accounts of critics, and holding human rights activists in prolonged solitary confinement over Facebook posts.

The government blocks specific functions of apps like WhatsApp and Skype, a reality that has made ToTok particularly appealing in the country. Huawei, the Chinese telecom giant, recently promoted ToTok in advertisements.

Spokesmen for the C.I.A. and the Emirati government declined to comment. Calls to a phone number for Breej Holding rang unanswered, and Pax employees did not respond to emails and messages. An F.B.I. spokeswoman said that “while the F.B.I. does not comment on specific apps, we always want to make users aware of the potential risks and vulnerabilities that these mechanisms can pose.”

When The Times initially contacted Apple and Google representatives with questions about ToTok’s connection to the Emirati government, they said they would investigate. On Thursday, Google removed the app from its Play store after determining ToTok violated unspecified policies. Apple removed ToTok from its App Store on Friday and was still researching the app, a spokesman said. ToTok users who already downloaded the app will still be able to use it until they remove it from their phones.

It was unclear when American intelligence services first determined that ToTok was a tool of Emirati intelligence, but one person familiar with the assessment said that American officials have warned some allies about its dangers. It is not clear whether American officials have confronted their counterparts in the Emirati government about the app. One digital security expert in the Middle East, speaking on the condition of anonymity to discuss powerful hacking tools, said that senior Emirati officials told him that ToTok was indeed an app developed to track its users in the Emirates and beyond.

ToTok appears to have been relatively easy to develop, according to a forensic analysis performed for The Times by Patrick Wardle, a former National Security Agency hacker who works as a private security researcher. It appears to be a copy of a Chinese messaging app offering free video calls, YeeCall, slightly customised for English and Arabic audiences.

ToTok is a cleverly designed tool for mass surveillance, according to the technical analysis and interviews, in that it functions much like the myriad other Apple and Android apps that track users’ location and contacts.

On the surface, ToTok tracks users’ location by offering an accurate weather forecast. It hunts for new contacts any time a user opens the app, under the pretense that it is helping connect with their friends, much like how Instagram flags Facebook friends. It has access to users’ microphones, cameras, calendar and other phone data. Even its name is an apparent play on the popular Chinese app TikTok.

Though billed as “fast and secure,” ToTok makes no claim of end-to-end encryption, like WhatsApp, Signal or Skype. The only hint that the app discloses user data is buried in the privacy policy: “We may share your personal data with group companies.”

So instead of paying hackers to gain access to a target’s phone — the going rate is up to$2.5million for a hacking tool that can remotely access Android phones, according to recent price lists — ToTok gave the Emirati government a way to persuade millions of users to hand over their most personal information for free.

“There is a beauty in this approach,” said Mr. Wardle, now a security researcher at Jamf, a software company. “You don’t need to hack people to spy on them if you can get people to willingly download this app to their phone. By uploading contacts, video chats, location, what more intelligence do you need?”

In an intelligence-gathering operation, Mr. Wardle said, ToTok would be Phase 1. Much like the National Security Agency’s bulk metadata collection program — which was quietly shut down this year — ToTok allows intelligence analysts to analyze users’ calls and contacts in search of patterns, though its collection is far more invasive. It is unclear whether ToTok allows the Emiratis to record video or audio calls of its users.

Each day, billions of people freely forgo privacy for the convenience of using apps on their phones. The Privacy Project by the Times’s Opinion section published an investigation last week revealing how app makers and third parties track the minute-by-minute movements of mobile phone users.

Private companies collected that data for targeted marketing. In ToTok’s case — according to current and former officials and digital crumbs the developers left behind — much of the information is funneled to intelligence analysts working on behalf of the Emirati state.

In recent months, semiofficial state publications began promoting ToTok as the free app long sought by Emiratis. This month, users of a messaging service in the Emirates requiring paid subscriptions, Botim, received an alert telling users to switch to ToTok — which it called a “free, fast and secure” messaging app. Accompanying the message was a link to install it.

The marketing seems to have paid off.

In reviews, Emiratis expressed gratitude to ToTok’s developers for finally bringing them a free messaging app. “Blessings! Your app is the best App so far that has enable me and my family to stay connected!!!” one wrote. “Kudos,” another wrote. “Finally, an app that works in the UAE!” ToTok’s popularity extended beyond the Emirates. According to recent Google Play rankings, it was among the top 50 free apps in Saudi Arabia, Britain, India, Sweden and other countries. Some analysts said it was particularly popular in the Middle East because — at least on the surface — it was unaffiliated with a large, powerful nation.

Though the app is a tool for the Emirati government, the exact relationship between the firms behind it is murky. Pax employees are made up of European, Asian and Emirati data scientists, and the company is run by Andrew Jackson, an Irish data scientist who previously worked at Palantir, a Silicon Valley firm that works with the Pentagon and American spy agencies.

Its affiliate company, DarkMatter, is in effect an arm of the Emirati government. Its operations have included hacking government ministries in Iran, Qatar and Turkey; executives of FIFA, the world soccer organization; journalists and dissidents.

Last month, the Emirati government announced that DarkMatter would combine with two dozen other companies to create a defense conglomerate focused on repelling cyberattacks.

The F.B.I. is investigating American employees of DarkMatter for possible cybercrimes, according to people familiar with the investigation. The inquiry intensified after former National Security Agency hackers working for the company grew concerned about its activities and contacted the bureau. Reuters first reported the program they worked on, Project Raven.

At Pax, data scientists openly brag about their work on LinkedIn. One who listed his title as “data science team lead” said he had created a “message intelligence platform” that reads billions of messages to answer four questions: “who you are, what you do, how do you think, and what is your relationship with others.”

“With the answers to these four questions, we know everything about one person,” wrote the data scientist, Jingyan Wang.

Other Pax employees describe their experience creating tools that can search government data sets for faces from billions of video feeds and pinpoint Arabic dialects from transcribed video messages.

None mention an affiliation with ToTok.

Mark Mazzetti reported from Washington, Nicole Perlroth from San Francisco and Ronen Bergman from Tel Aviv. Adam Goldman contributed reporting from Washington, and Ben Hubbard from Beirut, Lebanon. (NYTimes)