Caspar Bowden, who played a central role in defining and defending digital privacy for two decades, has died from cancer. Appropriately enough for someone who devoted himself to defending people's right to a personal sphere, little about his own private life has been reported in the media. That stands in stark contrast to his work, which is widely known and respected.

In 1998, Bowden co-founded and became the first director of the Foundation for Information Policy Research (FIPR), one of the leading Internet policy think-tanks in the UK. In 2000, he won a "Winston" award for his efforts to protect personal privacy—given alongside the Big Brother awards for those who had done most to threaten it. Bowden's award was for "building an anti-RIP movement," a reference to the UK's Regulation of Investigatory Powers Act—better known as "RIPA"—passed that year. RIPA is now widely recognised as deeply flawed, but this was obvious to Bowden 15 years ago.

Between 2002 and 2011, he worked for Microsoft, giving advice on privacy to Microsoft's national technology officers in 40 countries. However, he soon came to see Microsoft as a threat to privacy. As he told The Guardian in 2013, "I don't trust Microsoft now."

As a result, Bowden shifted to using entirely open-source software and stopped using a mobile phone, well before Snowden revealed they were vulnerable to surveillance. In an important 2012 report co-written by Bowden, the advocate pointed out that the NSA was permitted by US law to spy on everyone outside the US. After Snowden confirmed these fears in June 2013, Bowden wrote another detailed report for the European Parliament in which he analysed the impact of NSA surveillance on European citizens’ rights.

From then until his death, Bowden was a tireless defender of such rights. He was on the board of directors of Tor, and he spoke frequently at conferences and before government committees warning of the gravity of the threat the newly-revealed bulk surveillance represented to fundamental liberties. His death is a great loss to the privacy and digital rights worlds where he was such an major and much-loved figure, as can be gauged by the outpouring of sorrow on Twitter as word of his passing spread.