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A popular encrypted messaging app will allow Hong Kong users to cloak their telephone numbers from Chinese authorities.Jae C. Hong/The Associated Press

Telegram, a popular encrypted messaging app, will allow users to cloak their telephone numbers to safeguard Hong Kong protesters against monitoring by Chinese authorities, according to a person with direct knowledge of the effort.

The update to Telegram, planned for release over the next few days, will allow protesters to block Chinese authorities from discovering their identities in the app’s large group chats.

Thousands of Hong Kong protesters take their cues from more than 100 groups on Telegram. Protesters use encrypted apps like Telegram to mobilize swiftly through multiple group chats, with less risk of police infiltration, Reuters found in a report published earlier this month.

The groups are used to post everything from news on upcoming protests to tips on dousing tear gas canisters fired by the police to the identities of suspected undercover police and the access codes to buildings in Hong Kong where protesters can hide.

Protesters have grown concerned that Chinese authorities could use the movement’s reliance on Telegram to monitor and arrest organizers. Telegram chat groups used to organize public protests are often accessible to anyone and participants use pseudonyms.

But a feature in Telegram’s design may have allowed Chinese authorities to learn the real identities of users, according to a group of Hong Kong engineers who posted their finding on a online forum earlier this month.

Telegram allows users to search for other users by uploading phone numbers. This function allows a new user to quickly learn whether those in her phone’s contact book are already using the app, the group said.

Protesters believe Chinese security officials have exploited the function by uploading large quantities of phone numbers.

The app automatically matches phone numbers with the user names in the group. Chinese authorities then only need to request the owners of the phone numbers from the local telecom service in order to learn the users’ true identities.

Telegram has detected evidence that Chinese authorities may have uploaded numbers to identify protesters, said a person with direct knowledge of the situation. But it is unclear whether Chinese authorities have successfully used this tactic to locate protesters.

Government authorities in Hong Kong did not respond to a request for comment.

The fix Telegram is working on would allow users to disable matching by phone number. That option represents a balance between making it easy for users to find their contacts and the privacy needs of those who rely on the app for protection against state security agents.

Telegram hopes to help protect Hong Kong protesters with the update, the source said. But wide adoption of the optional security setting would make the app far harder to use for the vast majority of its more than 200 consumers, who rely on uploading phone contacts to identify friends and family members on the app, the source said.

The move by Telegram comes as Hong Kong police arrested a number of prominent activists and three lawmakers on Friday. Nearly 900 people have been arrested since the demonstrations began three months ago.

With protesters and authorities at an impasse and Hong Kong facing its first recession in a decade, speculation has grown that the city government may impose emergency law, giving it extra powers over detentions, censorship and curfews.

The government would consider using “all laws” to prevent violence, Hong Kong leader Carrie Lam, who has become a lightning rod for protesters’ anger, said this week.

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