BEIJING - The day before China's Communist Party published one of its most important policy statements in a decade, a copy of the reform plans was already circulating on Chinese social media.
The unprecedented Nov. 14 leak fuelled China's biggest stock market rally in two months as it spread on microblogs and passed from smartphone to smartphone on WeChat, a three-year-old social messaging app developed by Tencent Holdings Ltd.
WeChat, or Weixin in Chinese, meaning "micromessage", leapt from 121 million global monthly active users at the end of September 2012 to 272 million in just a year. It has quickly become the news source of choice for savvy mobile users in China, where a small army of censors scrub the country's Internet of politically sensitive news and "harmful" speech. "For me WeChat is an essential tool," said Hu Jia, a Beijing-based dissident.
Unlike popular microblogging services such as Sina Corp's Sina Weibo, where messages can reach millions of people in minutes, WeChat allows users to communicate in small, private circles of friends, and send text and voice messages for free - a big part of its success.
"Weibo is like a public square, and Weixin is like your sitting room," said Min Jiang, an associate professor studying China's Internet at the University of North Carolina at Charlotte.
WeChat limits the size of ordinary chatrooms to 40 people, and public pages, which users can subscribe to, can only post one message a day.
That does not mean communications cannot be monitored or censored, but it does give users a way to avoid running afoul of the government's new rules, that hold users accountable for "online rumours" read by 5,000 people or re-posted 500 times.
It may also be good news for China's rulers, because messages do not spread as rapidly as on Weibo's open platform.
"WeChat is less of a potential threat to the authorities than Weibo is," said one of the founders of the anti-censorship site GreatFire.org, who goes by the pseudonym Martin Johnson.
"People mostly use WeChat to exchange messages with people they already know. I still think the censors pay more attention to cleaning up Weibo. Weibo messages have the potential to reach millions of people very fast."
Weibo has been particularly singled-out in the ongoing crackdown on "rumour-mongering" by China's stability-obsessed government, which views public protest as a threat to its authority.
But WeChat has not escaped the government's attention, and its explosive growth means it is attracting more scrutiny than ever from the authorities.
"Online communications and national security has already become a conspicuous problem standing before us," said President Xi Jinping during a speech in November, in which he mentioned WeChat by name.
Social media operators in China are required to help censor content on behalf of the government, which trains their employees for the purpose.
Hu, the dissident, says the police have quoted him back messages he has sent through WeChat.
"Even though you know it's not safe... you have no choice, you need to use it, because all your friends are using it too much," he said. "I can still share many things related to human rights and politics."
In January, users were blocked from sending messages containing the characters for "nanfang zhoumo", Chinese for "Southern Weekly", a newspaper that was in open revolt against press control in Guangdong province.
Tencent's self-censorship, which has been shown to block sensitive messages sent outside China as well, may also affect its efforts to push WeChat outside China. The platform already has more than 100 million users outside China, and the company has signed football star Lionel Messi to promote it overseas.
"A big issue for Tencent would be convincing Americans and Europeans that they're not operating under the same self-censorship principles outside China as in China," said Doug Young, the Shanghai-based author of The Party Line. "Image-wise that could hurt them in their global expansion."
A spokesman for Tencent declined to comment on its censorship of WeChat because of the sensitivity of the issue.
"A conspicuous problem"
WeChat's relative newness means researchers are still studying how the app's censorship and monitoring work.
"If controls are present on the server side it makes it much more difficult to verify," said Masashi Crete-Nishihata, research manager of Citizen Lab, a research centre at the University of Toronto. "And so far we suspect that's implemented on the server side."
Citizen Lab has already been able to show that Line, another smartphone messaging app created by Line Corp, a Japanese subsidiary of South Korea's Naver Corp, blocks a regularly updated list of banned phrases in China. Work they did with the University of New Mexico found that the use of censored words in the old Chinese version of Skype could also trigger remote surveillance.
Microsoft Corp switched its Chinese partner for Skype in November and the programme is now believed to be more secure.
Weibo, the microblog, uses a computer system to scan each post before publication so sensitive ones can be flagged for censors employed by Sina, who decide whether to delete them.
"Just knowing how the censoring apparatus works, my personal guess would be they're going to use the same mechanisms," said Gary King, a Harvard professor who has researched how social media platforms are censored in China, but not WeChat.
Tencent's 15-year-old instant messaging service, QQ, is subject to active monitoring and censorship, according to Citizen Lab.
Speaking at a conference in London in November, Google Inc Chairman Eric Schmidt recalled a meeting with President Xi and Premier Li Keqiang, just weeks after China passed its new regulations on "online rumours".
"The most interesting thing about talking to the government, from the president all the way to the governors, is that they are obsessed with the Internet," said Schmidt, without elaborating on their conversations.
Authorities have signalled that they plan to increase their control of social media, including WeChat, and further "manage" China's Internet. "If anything happens and it becomes explosive, everybody knows that Weixin will be the next target," said Jiang, of the University of North Carolina at Charlotte.