Many peer-assisted content-distribution systems reward a peer based on the amount of data that this peer serves to others. However, validating that a peer did so is, to our knowledge, an open problem; e.g., a group of colluding attackers can earn rewards by claiming to have served content to one another, when they have not. We propose a bandwidth puzzle mechanism to make contribution-aware peer-assisted content distribution robust to such collusion. Our construction both ties solving the puzzle to possession of content and, by issuing puzzle challenges simultaneously to all parties claiming to have the same content, prevents one content-holder from solving many others' puzzles. We bound (in the random oracle model) the adversaries' ability to defeat our puzzle scheme. We also describe our integration of bandwidth puzzles into a fully operational media streaming system, and demonstrate the attack resilience offered by bandwidth puzzles through simulations of both streaming and file-sharing systems.

30 pages

*Department of Computer Science, University of North Carolina, Chapel Hill, NC
**Computer Science Department, Florida State University, Tallahassee, FL

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