isis uses to spread its propaganda and recruit new members. it's also a way that isis can secretly wire money or carry on private text messaging. >> i'm going to say hello and send. so it's just like a conversation. >> since the attacks in paris, telegram has taken action, blocking hundreds of public isis-related channels. it's a step. but private encrypted messaging between members has not been affected. for u.s. intelligence officials encryption is of grave concern. >> stop watching us. >> but to privacy activists it is a necessary reaction to the kind of mass surveillance programs former nsa staffer edward snowden revealed. >> end mass surveillance. >> director james clapper says that snowden's leaks have irreparably harmed intelligence gathering.