A Toronto-born app has become a go-to tool for millions of Iranians looking to break through Tehran’s repressive internet censorship.

Now just over a decade old, firewall circumvention app Psiphon is arguably the most easy-to-use and effective way to bypass state internet control. In recent weeks, it has become a popular way for protesters in Iran to break through the regime's "filternet," blocking entire websites and apps for the country’s 80 million citizens and sometimes cutting off internet altogether.

Since protests cropped up across Iran at the end of 2017, Psiphon reports that its usage has skyrocketed.

“Over the last week, Psiphon saw unprecedented app downloads and usage across our network from all platforms in Iran,” Irv Simpson, who works in development for Psiphon, told Motherboard.

Simpson broke down the numbers: On an average day, the app, available for Windows, Android, and iOS, is downloaded some 35,000-40,000 times. From New Year’s Eve until January 3 of this year, the app was hitting 700,000 per day.

That uptick has come with a five-fold increase in the amount of data going over the Psiphon servers.

The vast majority of this increased activity has come from Iran. Psiphon has tabulated a tenfold increase for mobile usage from the Middle Eastern country over the same four-day period.

Psiphon won’t know all the details of just how many users were actively using the app until it audits the data later on, but Simpson estimates that—at its peak—usage hit eight to 10 million users in Iran.

The huge spike in usage comes as Tehran tries to take out other apps, like encrypted messaging platform Telegram. The Centre for Human Rights in Iran cited one Iranian’s tweet saying that Psiphon was the only app working amid the crackdown.

Much of the filtering has been aimed at preventing protesters from organizing demonstrations, choking off reporting from those protests that do crop up, and limiting the flow of news from the outside world.

The mismash of internet filtering can be largely traced back to the patchwork of Iranian internet service providers, who do the heavy lifting at the behest of the state.

It also seems as though the state itself is at odds with how aggressively it is looking to censor speech online. Conflicting messages have emerged from the government regarding whether restrictions on apps like Instagram and Telegram will continue, with President Hassan Rouhani—of the more moderate wing of the Iranian government—indicating the restrictions will ease, and the more conservative hardliners within the regime suggesting they will continue.

For the popular encrypted messaging app, for example, it's unclear whether the state will continue to block its use or whether, as the hardliners propose, they will create their own platform they can more readily control. "It’s unclear what the official line of the Rouhani administration is,” said Mahsa Alimardani, who works with British human rights organization Article 19 and studies at the Oxford Internet Institute. She told me the range of censorship has been “diverse,” going on-and-off for specific apps and, sometimes, the internet more broadly.

Psiphon is different than just a regular VPN. That is, it is not a tool designed to mask or hide a user’s traffic, per se. Instead, its role is to hide the user altogether, as it allows the device to slip past internet filtering through one of the company’s many servers worldwide, giving it an advantage compared to other VPNs or censorship-circumvention software. Alimardani said Psiphon’s reputation has been that it is “good and responsive with getting new software out to replace the versions that get blocked.”

The app might be useful in circumventing efforts to limit access to specific websites or apps, but if Tehran decides to take aim at internet connections or mobile data as a whole, there’s not much Psiphon can do. Simpson says they’ve been targeted by Tehran before, during the previous round of elections, and are likely being affected by outages in specific parts of the country as the protests unfold.

Members of the Iranian public have proven themselves very resourceful in finding new and reliable technology to help them beat the filternet.

“There are some really stable circumvention tools that Iranians have been telling me about,” Alimardani said. She’s not releasing the names of those apps, “as the strength of this tech sometimes lies in not being publicized.”

Psiphon was developed inside the Citizen Lab, a part of the Munk School of Global Affairs at the University of Toronto. The lab, and the school, has spent years devising various ways to improve civic engagement in Iran, especially online, with some financial support from the Canadian government. (Citizen Lab did not receive government funding for Psiphon's creation.)

Correction: This piece originally referred to Iran's "Halal internet," the colloquial name for its national internet network, which disconnects from foreign traffic. It has been updated to include the term "filternet," which more accurately describes the censorship that occurs online. In addition, the piece has been updated to reflect that no government funding went towards Psiphon's creation.