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Client Puzzle Protocol (CPP) is a computer algorithm for use in Internet communication, whose goal is to make abuse of server resources infeasible. It is an implementation of a proof-of-work system (POW).
The idea of the CPP is to require all clients connecting to a server to correctly solve a mathematical puzzle before establishing a connection, if the server is under attack. After solving the puzzle, the client would return the solution to the server, which the server would quickly verify, or reject and drop the connection. The puzzle is made simple and easily solvable but requires at least a minimal amount of computation on the client side. Legitimate users would experience just a negligible computational cost, but abuse would be deterred: those clients that try to simultaneously establish a large numbers of connections would be unable to do so because of the computational cost (time delay). This method holds promise in fighting some types of spam as well as other attacks like Denial of Service.

Until 2009, Finney's system was the only RPOW system to have been implemented; it never saw economically significant use. In 2009, the Bitcoin network went online. Bitcoin is a proof-of-work cryptocurrency that, like Finney's RPOW, is also based on the Hashcash POW. But in Bitcoin double-spend protection is provided by a decentralized P2P protocol for tracking transfers of coins, rather than the hardware trusted computing function used by RPOW. Bitcoin has better trustworthiness because it is protected by computation; RPOW is protected by the private keys stored in the TPM hardware and manufacturers holding TPM private keys. Hackers who steal a TPM manufacturer key, or anyone capable of obtaining the key by examining the TPM chip itself, could subvert that assurance. Bitcoins are "mined" using the Hashcash proof-of-work function by individual nodes and verified by the decentralized P2P Bitcoin network.