WASHINGTON — (CNN) Edward Snowden has secretly been working on a project to stop smartphones from revealing the location of people in dangerous places.

Snowden said he envisions it as a protection for journalists and human rights workers, but it could be used by anyone who doesn’t want to be spied on.

On Thursday, the ex-NSA whistleblower unveiled his plans to build a revolutionary iPhone 6 case. It doesn’t block cell phone signals, but it does make sure the device stops transmitting data when you put it on “airplane mode.”

Most people think that “airplane mode” pulls a smartphone off the grid. It doesn’t. On the latest iPhones, for example, the device still receives GPS signals when it’s in “airplane mode.”

Governments can take advantage of that by hacking phones, then leaking out your location.

(That’s how the NSA can “turn on” your phone remotely.)

And governments do it regularly. It’s suspected that’s how the Syrian government tracked, targeted and killed American journalist Marie Colvin with a mortar barrage.

In a research paper, Snowden describes a sleeve that slips over the bottom of an iPhone, connects to the SIM card port, and watches for outbound signals.

An alarm would chime if it detects an unwanted signal, plus it provides extra battery juice.

“Front-line journalists are high-value targets, and their enemies will spare no expense to silence them,” Snowden wrote.

“Unfortunately, journalists can be betrayed by their own tools. Their smartphones, an essential tool for communicating with sources and the outside world — as well as for taking photos and authoring articles — are also the perfect tracking device.”

Snowden worked on this project with Andrew Huang, an American hacker living in Singapore.

Huang is an expert at reverse engineering.

To figure out how to build a device that limits iPhone signals, they had to become intimately familiar with the iPhone’s guts.

In their paper, they describe how they explored the Hua Qiang electronics markets in the Chinese city of Shenzhen.

Surrounded by iPhone repair stations, they found lots of spare parts and repair manuals — which included detailed blueprints of the iPhone 6.

The pair presented their findings at the Massachusetts Institute of Technology’s Media Lab on Thursday morning.

They expect to develop a prototype of this case in the next year. But don’t get your hopes up.

This is a tiny, experimental effort.

“The project is run largely through volunteer efforts on a shoestring budget,” the duo wrote.