War on terror

Technology and the fight against terrorism

Violent extremists like ISIS meet in the open on Facebook or Twitter. Then they take the conversation private, using technology called encryption to encode their messages.

It's called "going dark." And it's one of the most alarming concerns facing police and counter-terrorism officials worldwide right now. They worry they can't prevent the next terrorist attack -- an anxiety stoked in a big way by the November 13 attacks on Paris that claimed 130 lives.

"It's clear that our terrorist adversaries have figured out and learned what kinds of communication we've been in the past able to intercept. And they now understand that if they ... find other ways to communicate that they can shield their communications from us," National Counterterrorism Center Director Nicholas Rasmussen told CNN's Jim Sciutto in the CNN Special Report, "Targeting Terror: Inside the Intelligence War," which airs Tuesday at 9 p.m. ET.