Thu, 22 Jan 2026 15:42:43 -0600Friendly Finder

In the film One Battle After Another the characters use devices to identify people they can trust.  I'll try not to give too much away, but the basic idea is that if your device plays a certain tune, you can trust the person you're talking to.

I thought this was a clever idea and wondered if it was based on a real thing.  So far I've had no luck finding such a thing.

I put the question to the Fediverse and someone pointed out the similarity to the COVID infection tracking apps from the pandemic.  This reminded me that I had already designed a device like this and got me thinking maybe it could be "refactored" to serve this new purpose.

Sadly, I'm unable to find a repository for that project.  It might have been mixed-into some other general-purpose repo, or worse some random "projects" folder that wasn't checked into source control.

That's OK, because I remember the basics of the hardware (it was based on a particular ESP32 dev board) and chances are the firmware was probably obsolete by now anyway.  

So how might this work?  If I start from the ESP32, it seems logical that the radio communication be Bluetooth (probably BLE).  I don't remember exactly how BLE works but I seem to recall that a device can scan for other devices and get a list of MACs (or some other physical address), so this might be a basis of trust, if these addresses are stored in a "trusty" list on each parties device.

This probably isn't enough to establish trust since these things can likely be faked, but it might serve as a first step in establishing that (and also avoid having conversations with every toaster and dishwasher in the room).  

So if the address is on your list, maybe you exchange a message with a key, and the other does the same, and if they match a "lock" on the other's side, we start to think this person might be OK.

But I think there needs to be something more, something that isn't entirely contained in the device lest someone simply steal the device and appear trusted.

In the film I think there was a verbal password exchange.  This is probably sufficient, but I feel like it's kind of risky because it relies on human memory, which at least in my case has limits, especially in times of stress.