Film director Oliver Stone has said he "stayed off grid" during the making of his new biopic on Edward Snowden over concerns that hackers could target the film.
The double Oscar winner used encrypted messages to communicate with colleagues during the filming of Snowden, which tells the story of how the NSA whistleblower leaked details of mass government surveillance in 2013.
At the movie's premiere in Toronto, Oliver said: "The NSA is worldwide, the ability to intercept, to harm. Anything could happen. We've still not opened the film. It could get hacked tomorrow.
"We lived on our nerves. We stayed off the grid completely. We really tried to do everything in person, on paper, and encrypted as much we could. We encrypted all long distance communications."
Oliver, 69, said Snowden had brought to the world's attention "not only eavesdropping... cyber warfare and drone attacks too".
But he said it was up to the audience to decide if they wanted to brand Snowden a hero or a traitor.
"I think the movie is done realistically and factually and it allows you to decide," he added.
The film-maker also revealed that making Snowden had a "chilling effect" on his own use of social media and the internet.
"Even if we don't admit it, it's subconscious," he said.
"They can watch anything. It's a scary world and I didn't want to live like this and I don't. I believe in reform."
Oliver, who won best director Oscars for Platoon and Born On The Fourth Of July, also urged young people to limit their use of social media.
"As they get older they may realise the important of privacy," he said.
Snowden is to be screened at the London Film Festival on October 15.