US prosecutors have dropped a case against a man accused of using a child porn site because the government refused to divulge how it revealed his identity. The Playpen site was located on the Tor network which is used to anonymise web-browsing activity. The FBI found a way around this to reveal the users' real IP addresses and led to 200 prosecutions. But it refused to reveal to the court how it managed the feat. The site was located on the Tor network which many people use to browse the web anonymously. It conceals their location and identity by routing...