Optimistic validation of electronic tickets

Electronic tickets, or e-tickets, give evidence that their holders have permission to enter a place of entertainment, use a means of transportation, or have access to some Internet services. E-tickets can be stored in desktop computers or personal digital assistants for future use. Before being used, e-tickets have to be validated to prevent duplication, and ensure authenticity and integrity. The paper discusses e-ticket validation in contexts in which users cannot be trusted and validation servers may fail by crashing. The paper considers formal definitions for the e-ticket problem and proposes an optimistic protocol for validation of e-tickets. The protocol is optimistic in the sense that its best performance is achieved when e-tickets are validated only once.