Network Denial-of-Service (DoS) attacks, which exhaust server resources and network bandwidth, can cause the target servers to be unable to provide proper services to the legitimate users and in some cases render the target systems inoperable and/or the target networks inaccessible. DoS attacks have now become a serious and common security threat to the Internet community. Public Key Infrastructure (PKI) has long been incorporated in various authentication protocols to facilitate verifying the identities of the communicating parties. The use of PKI has, however, an inherent problem as it involves expensive computational operations such as modular exponentiation. An improper deployment of the publickey operations in a protocol could create an opportunity for DoS attackers to exhaust the server's resources. This paper presents a public-key based authentication and key establishment protocol coupled with a sophisticated client puzzle, which together provide a versatile solution for possible DoS attacks and various other common attacks during an authentication process. Besides authentication, the protocol also supports a joint establishment of a session key by both the client and the server, which protects the session communications after the mutual authentication. The proposed protocol has been validated using a formal logic theory and has been shown, through security analysis, to be able to resist, besides DoS attacks, various other common attacks.

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Network denial-of-service attacks, which exhaust the server resources, have become a serious security threat to the Internet. Public Key Infrastructure (PKI) has long been introduced in various authentication protocols to verify the identities ofthe communicating parties. Although the use ofPKI can present difficulty to the denial-of-service attackers, the underlying problem has not been resolved completely, because the use ofpublic-key infrastructure involves computationally expensive operations such as modular exponentiation. An improper deployment ofthe public-key operations in a protocol allows the attacker to exhaust the server's resources. This paper presents a public-key based authentication protocol integrated with a sophisticated client puzzle, which together provides a good solution for network denial-of-service attacks, and various other common attacks. The basic strategy to protect against denial of service is to impose an adjustable cost on the attacker while it launches the attacks. The proposed client puzzle protocol can also be integrated with other network protocols to protect against denial-of-service attacks.