The National Security Agency spends billions to snoop on the world’s online activity. But how much would it cost for you or me to digitally spy on our friends? Just a tad shy of $600.

That’s what security researcher Brendan O’Connor determined after conducting a little experiment in online surveillance. He cobbled together a few common computer parts (a tiny CPU, WiFi sensors) and assembled them into ten wallet-sized computers which he connected to a command and control system. Each box cost $57.

With that, O’Connor could take a peek into the stream of data his iPad or iPhone sent to any public WiFi. Sitting at a cafe, for instance, he could remotely record every website he visited. When he left the WiFi behind, the black boxes could still record his movements, thanks to WiFi “pings” his devices sent out whenever they encountered another network. O’Connor treated the project like a whitehat hack, a way to show just how vulnerable society is. He called it “creepyDOL” because, well, it’s damn creepy.

“Actually it’s not hard,” he told the New York Times Bits Blog. “It’s terrifyingly easy.” And you could use it for any number of unscrupulous reasons.

You could spy on your ex-lover, by placing the sensor boxes near the places the person frequents, or your teenage child, or the residents of a particular neighborhood. You could keep tabs on people who gather at a certain house of worship or take part in a protest demonstration in a town square…. The boxes are small enough to be tucked under a cafe table or dropped from a hobby drone. They can be scattered around a city and go unnoticed.

O’Connor only spied on himself—for ethical reasons, and because doing otherwise would probably break the Computer Fraud and Abuse Act. But your average tech-savvy criminal, stalker, or scumbag isn’t likely to be beholden to those same ethics. So the next time you take a trip to the neighborhood coffee shop, you might want to take a peek inside the potted plots for any mysterious little black boxes.

H/T New York Times | Illustration by Jason Reed