How Canada's spy agency hunts extremists through file-sharing sites

A newly released top-secret presentation by Canada's electronic spy agency shows how it sifts through millions of files uploaded online every day around the world in an attempt to find those intending to commit extremist acts. The Communications Security Establishment document provides extensive details about this mass surveillance, which taps into the massive databases kept by Canada's "Five Eyes" spying partners and is conducted under the code name Levitation. Here's how Levitation works.