If there was some way to learn the identity of said terrorist simply by observing the origin and destination of an encrypted message, then perhaps. It would po

If there was some way to learn the identity of said terrorist simply by observing the origin and destination of an encrypted message, then perhaps. It would possible for them to coordinate and avoid this outcome. And there are still plenty of secure crypto systems out there so we wouldn't be able to decrypt their messages (unless quantum computers turn into an actual thing).