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Peer-to-Peer by Andy Oram

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Micropayment schemes

Accountability measures based on micropayments require that each party offer something of value in an exchange. Consider Alice and Bob, both servers in a peer-to-peer system that involves file sharing or publishing. Alice may be inserting a document into the system and want Bob to store it for her. Alternatively, Alice may want Bob to anonymously forward some email or real-time Internet protocol message for her. In either case, Alice seeks some resource commodity—storage and bandwidth, respectively—from Bob. In exchange, Bob asks for a micropayment from Alice to protect his resources from overuse.

There are two main flavors of micropayments schemes. Schemes of the first type do not offer Bob any real redeemable value; their goal is simply to slow Alice down when she requests resources from Bob. She pays with a proof of work (POW), showing that she performed some computationally difficult problem. These payments are called nonfungible , because Bob cannot turn around and use them to pay someone else. With the second type of scheme, fungible micropayments, Bob receives a payment that holds some intrinsic or redeemable value. The second type of payment is commonly known as digital cash. Both of these schemes may be used to protect against resource allocation attacks.

POWs can prevent communication denial of service attacks. Bob may require someone who wishes to connect to submit a POW before he allocates any non-trivial resources to communication. In a more sophisticated ...

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