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Abstract. Research in authentication mechanisms has led to the design and development of new schemes. The security provided by these procedures must be reviewed and analyzed before they can be widely used.In this paper, we analyze some weaknesses of the port knocking authentication method that makes it vulnerable to many attacks. We will present the NAT-Knocking attack, in which an unauthorized user can gain access to the protected server just by being in the same network than an authorized user. We will also discuss the DoS-Knocking attack, which could lead to service disruptions due to attackers "knocking" on many ports of the protected server. Finally, we will review further implementation issues.

In this paper we present an anonymous protocol for a mobile payment system based on a Kiosk Centric Case Mobile Scenario where the customer cannot communicate with the issuer due to absence of Internet access with her mobile device and the costs of implementing other mechanism of communication between both of them are high. Our protocol protects the real identity of the clients during the purchase and employs a digital signature scheme with message recovery using self-certified public keys that reduces the public space and the communication cost in comparison with the certificate-based signature schemes. Moreover, our proposed protocol requires low computational power that makes it suitable for mobile devices. As a result, our proposal illustrates how a portable device equipped with a short range link (such Bluetooth, Infrared or Wi-Fi) and low computational power should be enough to interact with a vendor machine in order to buy goods or services in a secure way.

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