David Chaum: The pioneer of internet anonymity
David Chaum has been called the "godfather of anonymous communication" thanks to his pioneering work on systems to protect users against internet surveillance by giving them online anonymity.
He rarely gives interviews, but has spoken to BBC's Horizon team as part of its documentary, Inside the Dark Web.
In this clip, Dr Joss Wright of the Oxford Internet Institute introduces Mr Chaum by explaining the problem the American coder had set out to solve.
UK viewers can watch Horizon: Inside the Dark Web on BBC Two on Wednesday at 21:00 BST and on iPlayer afterwards. It will be broadcast on BBC World at a later date.