Edward Snowden and hardware hacker Andrew “bunnie” Huang have built a prototype iPhone add-on called the Introspection Engine that will detect if the devices are secretly transmitting.

The tool is meant to help users like journalists and human rights activists—especially those in repressive regimes that have smartphone-hacking capabilities—verify that when their phones are in airplane mode, they’re truly not sending or receiving signals. Developed as open source hardware, the overlay sits in an iPhone’s battery case and displays when the phone is using its Wi-Fi, cellphone, Bluetooth, or GPS radio systems.

“Today, journalists, activists, and rights workers occupy a position of vulnerability,” Huang and Snowden write. They cite the case of reporter Marie Colvin, who, according to a lawsuit against the Syrian government filed in 2016, was deliberately targeted and killed by Syrian government artillery fire in 2012. Her location was discovered in part through the use of intercept devices that monitored satellite-dish and cellphone communications.

“A great portion of this vulnerability originates from the opacity of modern devices,” they write. “There are simply no tools available through which one can determine what is happening beneath the glass and icons, preventing the development of a natural understanding of dangerous device states.”

Their next step is to develop what they call a “Silent Phone”—a modified iPhone with its broadcasting systems disabled, something that they say is relatively easy to do with a few tweaks to the iPhone’s hardware. The devices would still be able to communicate through wired Ethernet connections, potentially sent through routers connected to the anonymizing Tor network, letting users safely connect their phones when needed without worrying about broadcast-based spying.

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“Given the relative simplicity, robustness, and elegance of the Silent Phone solution, we intend to pivot our efforts from validating the Introspection Engine to creating a set of Silent Phones and associated wired connectivity accessories for field use by journalists,” they write.