Threema, a secure messaging service that has grown to become one of the most popular apps in Germany, is officially launching in the US.

Created in 2012, Threema prides itself on its end-to-end encryption and security features. It's a minnow next to big messaging apps like WhatsApp and Facebook Messenger. WhatsApp has 800 million monthly active users; in contrast, Threema has just 3.5 million.

But Threema's userbase has skyrocketed by almost 900% in the last year, and has never marketed itself on a global level. Plus, other apps are free — while Threema costs $1.99 to download.

And it's seen success in its own right. The app has sat at the top of the German paid download charts for the last two years; according to analytics company App Annie, it's outranking Minecraft and WeatherPro (it's also currently second in the Google Play charts). Threema is significantly bigger than its rival, popular chat app Wickr, which according to Google Play Store metrics has only seen between 100,000 and 500,000 downloads.

Encryption is a hot topic right now. Strong encryption refers to scrambling data and messages in such a way that it cannot be understood without the correct "key," even if the authorities have a court order. Over the last year or so, there has been a concerted effort by some of the biggest tech companies around to implement strong encryption into their products, brought on by whistleblower Edward Snowden's leaks about NSA mass surveillance.

Edward Snowden whistleblowing activities have sparked a massive global debate over government surveillance and privacy. REUTERS/Mark Blinch Brought out just prior to the Snowden news, the leaks caused an initial flurry of interest, boosting the user numbers to the hundreds of thousands. "But our real transformation into a business happened 9 months later," CEO Martin Blatter told Business Insider via email. "That was when Facebook purchased WhatsApp ... That deal created huge privacy concerns with people here in Europe. And hundreds of thousands of people came to us and downloaded Threema. Our user base literally doubled over night ... It was pretty wild."

It's understandable why Germany has taken so warmly to Threema. The country has had a particularly strong response to Snowden's revelations, which included a claim that the NSA bugged chancellor Angela Merkel's phone. German intelligence agency BND recent "drastically reduced its cooperation with the US National Security Agency in response to a growing fallout over their alleged joint surveillance of European officials and companies," the Guardian reported in May.

Threema is based in Switzerland — Blatter says the country has "some of the most user friendly privacy laws in the world" — and has 12 full-time staff. Following its German success, the company is now keen to push into new markets, and sees America, with its ongoing debates about surveillance and privacy, as the natural target. (Though previously available to download in the US, the company has never actively targeted the country before.) Part of this push involves a price cut — from $1.99 to $0.99 for a limited time.

Users don't need to provide a phone number or any other identifying details to use the app, and encryption keys are stored only on phones — making it impossible for Threema to read the contents of messages.

ISIS: Threema fans, apparently. AP As well as being Germany's most popular app, Threema has another, more dubious honour: It is recommended by ISIS. Documents put together by ISIS-affiliated internet users recommend using the app as a way to avoid detection by authorities. When I asked Blatter about this, he said it "plays into an utterly misleading narrative," pointing out that the app is also "used by investigative journalists in Germany who want to protect their sources and by the LGBT community in Lebanon and Iran, whose privacy we are helping to protect in order to protect their lives."

And in a way, it's testament to the success of Threema. Security experts have long acknowledged that encryption products will be misused by a minority — but the alternative, backdoors and weak protections, are ultimately far worse for users.

As security researcher the Grugq puts it: "If your secure communications platform isn't being used by terrorists and pedophiles, you're probably doing it wrong."