Peer-to-peer systems, including ours, are potentially vulnerable to free-rider attacks. A free-rider attack is one where an attacker, called a free rider, benefits from the system without contributing their fair share. The classic example of a free rider is a person who watches the US Public Television System (PBS) without donating any money. (PBS is supported largely by viewer donations.) Systems vulnerable to free-rider attacks either run at reduced capacity or collapse entirely because, as more and more users free ride, the costs of the system weigh more and more heavily on the remaining honest users, encouraging them to either quit or free ride themselves.