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Delay tolerant networks (DTNs) provide a promising solution to support delay tolerant applications in areas where end-to-end network connectivity is not available. In DTNs, the intermediate nodes on a communication path are expected to store, carry and forward the in-transit messages (bundles) in an opportunistic way, which is also named as opportunistic data forwarding. Opportunistic data forwarding depends on the hypothesis that each individual node is ready to forward packets for others. This, however, might be easily violated due to the existence of selfish nodes or even malicious ones, who may be reluctant to serve as the bundle relays to save their precious wireless resources. To address this problem, we propose a secure credit based incentive scheme to stimulate bundle forwarding cooperation among DTNs nodes. The proposed scheme can be implemented in a fully distributed way to thwart various attacks without relying on any tamper-proof hardware. In addition, we introduce several efficiency optimization techniques to improve the overall efficiency by exploiting the unique characteristics of DTNs. Extensive simulations confirm the efficacy and efficiency of the proposed scheme.