We consider the problem of defending against Sybil attacks using computational puzzles. A fundamental difficulty in such defenses is enforcing that puzzle solutions not be reused by attackers over time. We propose a fully decentralized scheme to enforce this by continually distributing locally generated challenges that are then incorporated into the puzzle solutions. Our approach consists of an all-to-all broadcast of challenges, with a combining function to ensure this can be done efficiently. The combining function generates certificates that can be used to prove that each node's challenge was delivered to and used by each other node, therefore proving the freshness of each puzzle. We show how our distribution and verification mechanisms can be implemented on top of the the Chord  overlay.
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