There's nothing terribly new in this report about shortwave "numbers" stations, but the accompanying video does contain some interesting stock footage as it tells the story behind these mystery broadcasters. 

Incredibly, with all the improvements in communications - satellites, the web, TV and more - using shortwave to transmit a series of mostly impenetrable numbers is still considered one of the safest ways to get an encrypted message to someone in the field - and it's said to be virtually uncrackable.

How Spies Communicate Using Shortwave Numbers Stations