[southerndrift]

While still online, let people download a bunch of signed tokens from a server in exchange for their public key, which is linked to their phone number and thus identity and credit card.

Now when offline, they can create their tickets by linking a token to a fare and sign that with their private key.

At check-in, collect those tickets and when online again, charge the credit card.

[matthewmacleod]

Ah, so essentially require the inclusion of a pre-signed, server-generated token identifying the user in the ticket’s QR code and perform the actual charge when the ticket is used.

That’s much better, though it’s subject to some edge cases similar to the London Underground, caused by the potential for using invalid payment methods. Not bad though; probably low-enough potential for abuse to ignore.

[bschne]

Off the top of my head, if the backend holds either the payment details or an alias to them from a payment service provider, you could also implement a solution where the turnstile/conductor is connected to a network and the payment gets done on the backend once a ticket generated offline gets scanned/used