Edward Snowden, the ex-NSA contractor famous (or infamous) for revealing the surveillance activities of US governmental agencies, has revealed that he’s been designing an iPhone attachment meant to alert users if their devices are being monitored.

A mockup of the case, image courtesy of Wired

According to Wired, the attachment–which looks like a battery case with a monochromatic screen on the back–is meant to plug into an iPhone’s SIM card slot to warn if the phone’s radios are operating. This means that the case is theoretically a far better way to ensure your phone is completely off than simply turning it off or putting it into airplane mode.

As Snowden has said before, both airplane mode and turning your phone off aren’t failsafe ways of keeping your phone private. Hacker groups, both governmental and non-governmental, have found ways to exploit phones even when they’re in either of these states. Snowden’s co-designer, the hardware hacker Andrew Huang, explained to Wired that foreign governments have pretty advanced capabilities for exploiting phones, meaning journalists especially are at risk of surveillance when covering events overseas.

The case, which is only in the design stage right now, monitors the signals coming out of an iPhone 6’s GPS, bluetooth, Wi-Fi, and cellular receivers and sends an alert if the receivers turn on when the phone is meant to be off. In theory, the case could kill the transmission, too.

In addition to assisting journalists with coverage, the case will also help catch the US government surveilling private phones, Snowden claims. “All we have to do is get one or two or three big cases where we catch someone red-handed and suddenly the targeting policies at these intelligence agencies will start to change,” he told Wired.

The duo hopes to eventually create a supply chain for making these modified iPhones in China. China has a burgeoning iPhone modification industry and the skill (and, presumably, lax enough intellectual property laws) necessary to manufacture them and provide them to journalists.

Check out the story at Wired for more information on the attachment and Snowden and Huang’s entire paper on the project.