Date Added: Feb 2011
A bandwidth puzzle was recently proposed to defend against colluding adversaries in peer-to-peer networks. The colluding adversaries do not do actual work but claim to have uploaded contents for each other to gain free credits from the system. The bandwidth puzzle guarantees that if the adversaries can solve the puzzle, they must have spent substantial bandwidth, the size of which is comparable to the size of the contents they claim to have uploaded for each other. Therefore, the puzzle discourages the collusion.