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The challenging characteristics of sensor nodes, including the constrained resources, the ad-hoc nature of their deployment and the vulnerability of wireless media, pose a need for unique security solutions. The advantages of Public Key Cryptography (PKC) for sensor network security are widely acknowledged and include resilience, scalability and decentralized management. Recent work has indicated that PKC is feasible in the wireless sensor network (WSN) environment, paving the way for many new security services and opportunities. However, the computational effort involved in performing PKC operations remains substantial. From an energy consumption perspective, it is imperative that the processing and communication resources be utilized only when required. To that end, PKC implementations are more vulnerable to Denial of Service (DoS) attacks, when compared to traditional security methods that require less resources. In particular, if a malicious party attacks a sensor node by repetitive requests to establish a key, the resources of the attacked node can be exhausted quite rapidly. In this paper, we propose a novel RSA-based framework for combating DoS attacks in WSN by ensuring that the malicious party will exhaust its resources prior to exhausting those of its counterparts. Under the proposed approach, the mathematical operations performed by the malicious party require two or three orders of magnitude more resources than those required by the attacked party. We also present three methodologies for establishing an ephemeral key, in which the proposed DoS mitigation mechanism is an embedded component. Implementation results on the Intel Mote 2 platform substantiate the clear advantages of the proposed method.