qaeda and isis have learned how to disappear from u.s. electronic surveillance completely. >> is it true that terrorists adversaries can go completely dark? >> absolutely. there is part of the world that is dark to us. in other words, we can't see. >> the time to go dark is what the fbi terms as seeing people's communication because of encryption and of course they are not happy about it. >> an online world hidden by widely available encryption, technology that makes messages impossible to monitor. >> so that simple tool wins versus the whole apparatus and infrastructure of the nsa? >> well, so that's maybe a little bit of an overstatement. what it does is it makes the job more difficult. >> are you letting on there -- and this is something that is fascinating to me -- that there is a way around encryption for the nsa? >> so that's a really difficult question to answer. sometimes there is. >> that gives a little bit of comfort, right, to imagine -- >> it shouldn't give you too