China’s social-messaging giant Tencent has been monitoring political content posted by foreign accounts to train its censorship algorithms for domestic users, a new study has found.

The research by the University of Toronto’s Citizen Lab comes as foreign governments, particularly the US, have been questioning the role of China’s technology companies in their markets, fearing their citizens’ data could be compromised.

Residents of China have long known that private messages sent over WeChat, the country’s dominant messaging platform, could land them in trouble for speech deemed politically sensitive by the government. At least one man has been jailed for calling President Xi Jinping a “steamed bun”.

The app is almost unavoidable in China, where it has 1bn user accounts. Residents rely on it for everything from social messaging to reading news and making mobile payments, as well as hosting a platform for “mini-apps” that functions as a light version of Google’s and Apple’s app stores.

Citizen Lab has found that this surveillance of private messages is also applied to accounts registered to foreign mobile numbers, in order to build up its repository of sensitive files and thus better censor China-registered accounts.

The research shows how Tencent not only conducts censorship, but also informs and develops its own censorship strategies.

In addition, the company is likely to support the government’s political research. “If the Chinese government has any need to regulate public opinion, they will certainly use the database of politically sensitive content by WeChat” to learn from, said a Beijing-based professional who has worked closely with the government.

The professional added that WeChat’s database of sensitive content was “probably the most comprehensive and updated one in China”.

Citizen Lab’s researchers found that images or documents transmitted exclusively between foreign accounts are analysed by WeChat’s algorithms to see whether they might be politically sensitive. Once labelled as sensitive, such content — for example, a satirical cartoon — will be censored if sent by China-registered users. Foreign-registered users are not censored.

“The impact is in the violation of the privacy of the user. It’s just this idea that whatever you’re saying is being subjected to these analysis algorithms,” said Jeffrey Knockel, a Citizen Lab researcher.

There is so far no clear privacy policy referring to such content surveillance for foreign users by Tencent.

Tencent said: “With regard to the suggestion that we engage in content surveillance of international users, we can confirm that all content shared among international users of WeChat is private.”

In 2013, the latest date for which data is available, Tencent had 70m foreign-registered users. State media reported that in recent years, most of WeChat’s growth came from south-east Asia, Europe and the US, and that many overseas Chinese use WeChat to stay in touch with their relatives and friends in China.

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