[ajb]

This could perhaps be made easier to use if you had a UI like this: You phone pops up a message saying: "Hey, I notice you seem to be in the same room with Bob! We can increase security of Bob's messages to you my exchanging a fingerprint. Do this now? (Yes/No/Woah, Bob isn't here!)

If you click yes, you then exchange fingerprints using eg QR codes, and the authenticity of messages from Bob are retrospecively checked

Problem is, it's not obvious this can be done without compromising privacy of location.

[marcosdumay]

> Problem is, it's not obvious this can be done without compromising privacy of location.

That's a problem for Free Software running on local machines!