Back at my home in Hertfordshire I took a few precautions. I worked offline. I stored each draft chapter in a TrueCrypt folder, a virtual encrypted disk accessible only via a long, complicated password. When I conducted interviews I left my mobile behind. Having seen Snowden’s documents, I knew something of the NSA’s and GCHQ’s extraordinary capabilities. As of April 2013, the US spy agency had 117,675 active surveillance targets. Was I perhaps now one of them?

By September the book was going well – 30,000 words done. A Christmas deadline loomed. I was writing a chapter on the NSA’s close, and largely hidden, relationship with Silicon Valley. I wrote that Snowden’s revelations had damaged US tech companies and their bottom line. Something odd happened. The paragraph I had just written began to self-delete. The cursor moved rapidly from the left, gobbling text. I watched my words vanish. When I tried to close my OpenOffice file the keyboard began flashing and bleeping.