Richard* had a long drive ahead of him. About an hour earlier, at 5:30 AM, his wife Lisa* had phoned.

"The house is filled up," she said in a calm but audibly tense voice. Richard, having just woken up and now trying to make sense of the call, thought there must have been another water leak in the basement.

Instead, his wife told him, the house was full of FBI agents and they wanted to talk to Richard.

"Okay, I'm on my way," Richard said. He threw on some clothes, grabbed his laptop and phone as requested by the FBI, and stepped out into the night. The interstate drive from Milwaukee, where he was working as a software engineer, back to his home in Indianapolis would take a good five hours, more than enough time to figure out what this was all about.

It was something to do with computers, Lisa had said. The only thing Richard thought may be linked to that was his Tor exit node.

The Tor network—originally a project fund?ed by the US Navy—is a collection of servers, some big, some smaller, spread across the world. When a user connects to the network, her internet traffic is randomly pinged between at least three of these servers, all the while covered in layers of encryption, making it near impossible for anyone monitoring the traffic to determine who is sending it or where it is going to.

It allows dissidents to communicate anonymously, citizens to bypass government censorship, and criminals to sell drugs or distribute child pornography. Tor also facilitates special sites called "hidden services," part of the so-called dark web. These allow the owners of websites and their users to remain largely anonymous.

The final set of servers that Tor uses in this process are called "exit nodes," because they are the points at which a user's traffic exits the Tor network and joins the normal web that we use everyday.

Rather than being run by one company, most of these exits are set up by volunteers, or "operators." A few organizations maintain the larger exits, a number of universities have their own, and individual activists run some too. Edward Snowden rep?ortedly had one.

Richard was one of these operators.

Richard's exit could have been implicated in just about anything

Although Richard, 57, assumed the call was related to his exit, he still didn't know what specifically the FBI was investigating as he started the drive home.

"A child porn ring had been busted? Or a hacking attack? Or a bomb threat called in? I had no idea what it was," Richard later told me over the phone.

When someone uses Tor, his IP address is that of the exit node he has been randomly assigned. This means that if someone emails a death threat, or sends a barrage of spam, it is the exit node's IP that appears when the authorities start investigating the digital fingerprints of the crime. Richard's exit could have been implicated in just about anything.

However, Kurt Opsahl, the deput?y general counsel of the Electronic Frontier Foundation (EFF), believes that running a Tor exit is legal, at least under US law.

But if an operator runs an exit from his or her home, and on their own internet connection, "they may be confused with being the source of the traffic, instead of an exit node of the traffic," Opsahl told me. To anyone looking at activity flowing from the exit—whether that's child abuse material, or an attempt to hack a website—it looks one and the same as the operator's own personal usage. This could lead to a raid on the operator's house, even though running an exit is arguably legal.

For this reason, and ?others listed on the Tor Project website, operators are strongly advised to only run their exits remotely, by renting out server space.

This is what Richard did. Through a St. Louis-based company, his Tor exit had been whirring away in a German data centre for 18 months. But it appears that wasn't enough to stop a raid on his house.