Comment by PaulHoule

A system like that could work in an entirely disconnected mode where the "ticket" device has a cryptographic token whose signature can be checked at the door without either side having internet access. The weakness of that system is that you can't "revoke" or sell tickets. Such revocation would be possible though if either the ticket or the validator device is internet connected.

I saw the New York Red Bulls play not long ago and had to use Ticketmaster's system for the first time. I travel with a tablet, not a smartphone, and I was expecting trouble. Turns out the only trouble I had was that they didn't want to let me in with a tablet but they did when I explained my ticket was on my tablet. It did require an internet connection but Red Bull Arena has great WiFi so that was no problem.