China has long had one of the most pervasive online censorship systems in the world.

The country's infamous "Great Firewall" blocks access to numerous Western websites like Facebook, YouTube, Twitter and The New York Times. But China also has laws that force Chinese companies and social networks to apply censorship within their own services.

See also: The 10 Countries With the Most Internet Freedom

On Sina Weibo, China's Twitter-like service that boasts almost 300 million users, hundreds of posts get censored every day. The social network has a sophisticated system that automatically censors certain keywords ("June 4 massacre," referring to the Tiananmen Square protests, is blocked), and a team of in-house censors manually monitors other accounts and messages to catch whatever escapes the automated system.

Censorship on Sina Weibo is not only effective, it's lightning quick. Researchers last year found that some posts, or "Weibos," would be deleted as quickly as five minutes after being posted.

A secretive group of online activists called GreatFire has been monitoring Chinese online censorship for three years. GreatFire's three collaborators track blocked websites and collect censored posts on Weibo, which they then publish on FreeWeibo.com.

Now, the GreatFire activists are launching an app that they believe will make the Great Firewall of China and its mighty censorship powers obsolete, thanks to a relatively new approach called "collateral freedom."

The Android app, also called FreeWeibo, allows users to read posts that are deleted from Sina Weibo, giving Chinese netizens a chance to see what their government censors, and what their fellow countrymen are really talking about.

The activists believe that the way they designed the app makes it impossible to be blocked — which they hope will show others an effective way to circumvent Chinese Internet restrictions, furthering their goal of ending online censorship in China.

"Since the founding of our organization, I don't think we've come as close to achieving that goal as we are about to with the release of [the FreeWeibo] Android app. Because it's really changing the rules of the game," says a GreatFire founder who goes by the pseudonym Charlie Smith, in an interview via encrypted phone with Mashable.

The group is confident FreeWeibo cannot be blocked because of the way they have built it: All of the app's content is hosted on the Amazon Web Services (AWS) S3 could-hosting platform.

What makes Amazon's cloud perfect for this project is that it's all encrypted, making the task of censoring content on it almost impossible. When a Chinese netizen visits content hosted on AWS, Chinese censors can see only that the user is visiting s3.amazonaws.com, but not the exact final address. China can't selectively block just some content on AWS without blocking the entire service.

And therein lies the key to the project: Thousands of businesses rely on Amazon's cloud in China. The government probably can't afford to block the service entirely just to kill FreeWeibo. It's a strategy researchers call "collateral freedom." It makes it impossible for governments to block access to a service because the collateral economic costs of doing so would be too crippling.

"That's incredibly powerful because then it's an all-or-nothing strategy," says Michael Carbone, the Manager of Tech Policy and Programs at the human rights organization Access, who specializes in Chinese digital rights. "Either the censors allow access to that website and that service entirely, or they have to block the whole thing. They can't do selective blocking of certain pages that have censored content."

The term was first mentioned in a paper published last year, which explained how hosting sensitive content on platforms that are widely used within China makes it much harder to be blocked. The first real-world test of this theory came in January 2013, when China blocked the popular coding site GitHub and had to unblock it just two days later after Chinese developers protested against the move.

The new FreeWeibo app is a perfect example of this strategy.

"What they're doing with collateral freedom with this Amazon Web Services thing is really potentially transformative because it shows people a way of really sharing information without having fear of having information restricted," Jason Q. Ng, the author of the book and website Blocked on Weibo, and a researcher at the University of Toronto, tells Mashable.

GreatFire's first tries at collateral freedom

GreatFire was founded in 2011 by Smith, Martin Johnson and Percy Alpha — all pseudonyms. None of the three has ever revealed his real name; Smith says they don't use their real names even among themselves out of fear of China's reprisal.

Their fears might be justified, given the most recent crackdowns on Chinese bloggers and dissidents.

Smith says the three activists have never met in person. Smith seems to have an American accent, but he says they have close ties to China. Smith declined to answer any questions about who they are, where they came from or where they are based.

This is not the first time GreatFire has tried the collateral freedom approach. In 2012, the group launched its FreeWeibo website, but China blocked it just five days later. Last year, GreatFire created a mirror site on AWS. Now the FreeWeibo site gets 600,000 visits a month. Based on the language settings of the visitors, Smith says 95% of them are from China.

In 2013, China censored Reuters' website, so GreatFire created a mirror site on Amazon, accessible on an s3.amazonaws.com address. That allowed readers inside China to visit the site even without using online circumvention tools like VPNs or Tor, which allow people to circumvent the firewall but can be hard to use for non-tech savvy users.

The downside of hosting a site on AWS is that a URL like https://s3.amazonaws.com/freeweibo./index.html is not as easy to remember as freeweibo.com.

For that reason, the GreatFire activists thought releasing a FreeWeibo app would be the next natural step. So last year the group launched a FreeWeibo app for iOS.

The app became relatively successful, but after just one week, China blocked the app's download link. GreatFire initially created a workaround by pushing for periodical updates (Apple creates a different download link for every new update). At that point, the activists were confident nothing could stop FreeWeibo on iOS.

"When you are able to get around the Chinese censorship authorities, that's pretty cool. We thought: OK, we're there," Smith says.

But then, something unexpected happened. The app was completely removed from the App Store. FreeWeibo on iOS was effectively dead.

"The nightmare that you hope would never happen, actually happened," Smith says. "And that was it."

Smith says the Apple app review team sent an email saying the app had been removed because it included "content that is illegal in China." He showed a copy of the message to Mashable.

After that disappointment, in November 2013, Smith, Johnson and Alpha started working on the Android app. Now, almost six months later, the app is ready. The group released it two weeks ago, without publicity. It has been downloaded 2,000 times so far, according to Smith.

Can this really work?

GreatFire, of course, is not the only group fighting censorship in China. Since the early 2000s, when Chinese-born software engineer Bill Xia created Freegate, one of the first VPNs targeted specifically at China, many similar circumvention programs have been released, including Psiphon and Ultrasurf.

VPNs have long been one of the most effective ways to get around the Great Firewall and access blocked sites, but at the end of last year, some VPN providers started reporting major disruptions in China.

The new crackdown on VPNs is just the latest in a long cat-and-mouse game between Chinese authorities and Internet freedom activists, says Carbone, the specialist at Access.

FreeWeibo doesn't provide the same circumvention capabilities of VPNs, and it's targeted to just one service, Sina Weibo. But given that the censorship of Western sites has pushed many Chinese to stay within the firewall and mostly use local Internet services, the FreeWeibo app could make a real difference, Carbone says.

Giving citizens the opportunity to read blocked Weibos may not be revolutionary, but the app does have the potential to reach many people.

According to the latest estimates, almost 300 million people in China have an Android phone, and Sina Weibo is extremely popular, with 280 million registered users.

Despite its popularity, Sina Weibo is crippled by censorship. The company itself, in its recent IPO filing, admitted that China's censorship requirements are an obstacle to the company's growth.

"Failure to [censor] may subject us to liabilities and penalties and may even result in the temporary blockage or complete shutdown of our online operations," the company wrote in its regulatory filing with the SEC.

FreeWeibo gets around the censorship thanks to software that automatically scrapes Sina Weibo, downloading and storing posts continuously. The software then checks to see which posts have disappeared, and saves those. Smith says they don't catch all the posts, but they get the vast majority. FreeWeibo now hosts more than 370,000 censored posts, according to Johnson.

The big hope for GreatFire is that FreeWeibo on Android won't be as easy to block as its iOS counterpart. Android apps in China are distributed through multiple, decentralized markets — none controlled by Google — so it's going to be harder for China to get FreeWeibo removed. GreatFire is also relying on various distribution channels: the group's website, its AWS mirror and the Google Play Store for those outside of China.

The single point of failure is potentially Amazon. Even if China can't block FreeWeibo's app itself, it could perhaps persuade Amazon to remove it or shut down GreatFire's AWS account.

When GreatFire used Amazon to host the mirror of the Reuters website, Amazon kept quiet, declining media requests for comment and basically playing the role of an innocent bystander. But it's unclear what it will do if FreeWeibo becomes a huge hit, especially now that Amazon is moving into China, where it launched a localized version of Amazon Web Services in December of last year.

Amazon could theoretically agree to censor some content on AWS as part of the deal to open AWS China — although it's hard to see how any type of deal like that would affect accounts hosted on the American version of AWS, like GreatFire's accounts.

David Robinson, one of the authors of the collateral freedom report, says that at this point it's not known "exactly what censorship laws, if any, Amazon may have agreed to abide by in its China-based service."

Amazon did not respond to Mashable's request for comment.

The nightmare scenario, Carbone says, is that China might force all local businesses to move to Amazon's Chinese version of AWS (hosted at amazonaws.cn), where the government can presumably exert more control. At that point China could even afford to block the regular AWS domain (amazonaws.com) entirely.

Experts like Ng and Robinson think that's a possibility, however remote.

"Practically speaking it would be incredibly difficult to push everyone to a localized service," says Harlan Yu, another co-author of the collateral freedom paper.

Smith, the GreatFire collaborator, says they're ready for something like this: They would then rely on other cloud services providers, but none of those would probably be as big as Amazon.

It's also hard to know whether the FreeWeibo app's model can be replicated. Smith says they hope organizations like The New York Times, which is currently blocked in China, could put out an app with content hosted on AWS and get around censorship.

But it won't be that easy. It's one thing to host a simple app on AWS; it's another to host the mirror of an entire news site that gets constantly updated.

"That's actually a pretty big engineering challenge," Yu says. "It's not as simple as hosting an Android app."

Whatever happens, Smith, Alpha and Johnson say they will continue the fight.

"Our goal is to end online censorship in China. That's the endgame for us," Smith says. "And we won't rest until we have achieved that goal."