I haven't fully figured out the solution (if I had, I'd be writing it), but I suspect it will ultimately involve the widespread availability of smartphones and

I haven't fully figured out the solution (if I had, I'd be writing it), but I suspect it will ultimately involve the widespread availability of smartphones and laptops with the capacity to record and transmit video. For the time being, the human face and voice are a fairly robust medium for embedding data in a way that is resistant to MITM.