Russia's top court ruled Tuesday that the popular Telegram messaging app can be forced to provide user data to Russian security services.
It's the latest blow to the app, which encrypts user communications and has faced pressure from Russian and other authorities in the past.
The Russian Supreme Court threw out an appeal by Telegram protesting demands from the Federal Security Service intelligence agency, or FSB, to provide access to user data.
Telegram argued that the FSB violated consumer rights when it demanded that social networks provide authorities with encryption keys and chat histories.
China blocked Telegram in 2015, and authorities in several countries say Telegram is the message service of choice among Islamic State extremists.
The FSB said the app was used by a suicide bomber who killed 15 people in St. Petersburg last year. French authorities said it was used by IS radicals who killed a priest in Normandy. And Spanish police said this week the man suspected of plotting an IS-inspired attack in Colombia sent suspicious messages on Telegram.
Yet the app is also popular among ordinary people around the world — from Russian election observers to French President Emmanuel Macron's staff. Telegram has claimed it has more than 100 million users.
Following Tuesday's court ruling, Russia's communications regulator ordered Telegram to provide the encryption keys within 15 days. The app could eventually be blocked in Russia if it doesn't comply.
Telegram founder Pavel Durov tweeted: "Threats to block Telegram unless it gives up private data of its users won't bear fruit. Telegram will stand for freedom and privacy."
A ban may be impossible to enforce anyway. When Telegram faced threats of shutdown in the past, it developed an updated version of the app that allows users to bypass an eventual ban by using a virtual private network, or VPN.
Russia adopted counter-terrorism legislation in 2016 obliging communications companies to store call logs and data for months, and to store it in local servers instead of abroad.
The FSB is not asking to see user messages directly, but encryption keys — technical details that would allow security agents to access electronic messages shared through the app if authorities have reason to suspect a threat.
Privacy defenders warn that could give authorities broad range to crack down on dissent.
Telegram, based in Britain, was under threat of closure in Russia last year. Authorities backed off when Durov agreed to register the company in Russia.
Communications Minister Nikolai Nikiforov said Tuesday there's no reason Telegram should be exempt from rules applied to other messaging apps, according to Russian news agencies.