Edward Snowden wants you to know at all times whether the NSA is keeping tabs on your iPhone.

Along with Andrew Huang, his coauthor and fellow hacker, Snowden presented his research on phone “hardware introspection” at MIT, which aims to give users the ability to see whether their phone is sending out secret signals to an intelligence agency.

“This work aims to give journalists the tools to know when their smart phones are tracking or disclosing their location when the devices are supposed to be in airplane mode,” the pair wrote in their technical paper.

Snowden, an ex-NSA contractor living in exile in Moscow, and Huang, a prominent hacker who has reverse-engineered the Xbox and other hardware, believe that their solution can protect journalists and activists from being betrayed by their smartphones.

In their paper, they mention Marie Colvin, a journalist who was killed by Syrian forces in 2012. A 2016 lawsuit against the Syrian government alleges that her cellphone signals were intercepted and she was deliberately targeted.

Your phone is lying to you

In their paper, Snowden and Huang make it clear that what you see on your phone’s screen is not always true.

If you turn off Bluetooth or cellular service, then the phone’s radios and other electronics can still be made to send signals, especially if they are compromised by a sophisticated intelligence agency or hackers. Even airplane mode isn’t a defense, since the current version of Apple’s iOS still keeps the GPS active while in that state.

“Trusting a phone that has been hacked to go into airplane mode is like trusting a drunk person to judge if they are sober enough to drive,” they write.

So instead of trusting the phone’s software to tell the user if something is fishy, the pair proposes something else: A device that plugs into the hardware and constantly scans to see whether it is transmitting.

Though they think that their research can be applied to just about any phone, for now they are building a device for the 4.7-inch Apple iPhone 6.

The ‘case’ for countering digital surveillance

After a lengthy technical analysis of exactly what your iPhone is sending out and when, the pair proposes the creation of what looks like a battery case.

And actually, it will be.

The add-on hardware will give the iPhone some added juice, while also giving a digital readout of whether a user’s device is “dark” — not transmitting when it’s not supposed to — or whether a GPS, cellular, Bluetooth, or Wi-Fi signal has betrayed them.

But it’s not actually built yet. Right now, there’s just research and a technical analysis of their findings, but Snowden and Huang plan to create a prototype of the device this year.

“As the project is run largely through volunteer efforts on a shoestring budget, it will proceed at a pace reflecting the practical limitations of donated time,” they wrote. “If the prototype proves successful, [then the Freedom of the Press Foundation] may move to seek the necessary funding to develop and maintain a supply chain. This would enable the FPF to deploy modified iPhone 6 devices for field service among journalists in high-risk situations.”