Anonymizing network provides web services to the users and also hide the client’s IP address from the server. All data is wrapped with several layer of encryption. The success of this network, hackers can easily deface popular web site. If the users misbehave, blocking particular IP addresses is difficult. Nymble system is a credential system in which servers can blacklist misbehaving users in anonymizing networks, without compromising their anonymity. This system could block the IP address. However it is possible for a user to attack from another IP. To limit the number of credentials obtained by a single individual by raising the cost of acquiring credentials, we include client puzzles as a resource for obtaining credential where users are required to perform a certain amount of computation. This paper utilizes game theory to propose a number of puzzle-based defenses against flooding attacks.
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