The whistleblower who exposed the totality of the America’s mass domestic surveillance program suggested that users should look to free and open source software to preserve their privacy.

On Saturday, NSA whistleblower spoke to the Free Software Foundation’s LibrePlanet2016 conference regarding free software, cybersecurity, and privacy. He joined the event through video conference from Russia. During the event, Snowden explained that he was able to release the secrets of America’s mass surveillance projects more securely by using free software.

The whistleblower praised Debian, Tails, and Tor observing that what he did in 2013 never could have happened without the help of free software. He also advocates that activists and journalists switch to free and open source software to secure their privacy against the omnipresent US surveillance state.

“I didn’t use Microsoft machines when I was in my operational phase, because I couldn’t trust them,” explained Snowden. He went on to say that he couldn’t be sure that there wasn’t a backdoor coded into the Microsoft products that would enable government surveillance.

In making this dour assessment of Microsoft, he noted that of the current tech giants, the company has been the most cooperative with the US government.

He cited Microsoft founder Bill Gates calling on Apple to install a backdoor into the iPhone at the request of a federal court which opens millions of end users up to the danger of a growing mass surveillance program.

Snowden praised Apple’s fight with the FBI calling it an example of a corporation standing up for its users. Still, the whistleblower cautioned citizens against relying on multinational companies for transmissions that are intended to be secure.