| Centralized ticket server. QR tickets consist of a counter and signature. Train companies have a login to the ticket server to let them mint tickets. Checkers, when offline just check the signature, and log to the central server later. When online, they ask the central server for all other ticket details and check those. Any ticket that is misused and isn't detected at the time is blacklisted, and that blacklist distributed for offline use. A blacklist of the 10 million most-misused tickets could perhaps be just 10 megabytes - easy to download/update every few hours over mobile internet. Now, anyone can wrongly use a ticket just once, as long as they are sure the checker is offline. And if they are mistaken and the checker is online, or they reuse a ticket, they will get caught. main benefit: tickets can be smaller (and therefore read far quicker). Tickets can also be cancelled easily. For example, train companies could send out promotional tickets which only charge you if used, and are cancelled otherwise. |