The big data revolution is here, with masses more personal information available. And for authoritarian governments, this information is another weapon to use against their people. Gordon Corera discovers how agencies like the Stasi always longed for such technological power, and explores what might now be possible for politicians armed with masses of data about everyone, and the means to analyse it. How have Western companies been caught up in this world, and what can be done in response? Big data promises huge benefits in many areas - but can its darker side be resisted?

Producer: Chris Bowlby

Editor: Richard Vadon.