When U.S., European and Canadian law enforcement officials claimed success last year in largely obliterating militant group Islamic State's online propaganda network following a two-year operation, it was a public database of domain names that partly helped.

In an effort to crack down on websites, blogs, and Twitter accounts that relayed IS propaganda whenever there was an attack, authorities used the internet's WHOIS database to identify about 400 domains hosting the content and registered by IS supporters, resulting in a number of arrests.

But the same work would be much more difficult to do today, according to a European law enforcement official, due to Europe's strict new data privacy rules, the General Data Protection Regulation, which entered into force last May.