This work presents a new secure system for deploying opportunistic communications for emergency management after any disaster in which network infrastructures have become collapsed or unavailable. This proposal uses different wireless technologies such as Bluetooth Low Energy, Wi-Fi Direct and LTE Direct to enable device-to-device communications between users through their smartphones. In this way, the system allows the deployment of decentralized communications in fragmented scenarios, what is essential for disaster relief applications. The general procedure of the proposed system is based on the use of chat rooms with possibility of real-time streaming. Two different types of chat rooms are defined: public chat rooms with open access for all registered and authenticated users, and private chat rooms that can be used by different authorized groups of people. The permissions required to use different chat room are established thanks to the use of a decentralized authentication scheme based on public keys and certificate graphs, where different trust levels can be applied. The proposed scheme includes also a decision protocol to choose the most appropriate communication technology in every moment, and a power saving protocol that minimizes the impact of the system on the device's battery consumption. The first results of field experiments made with a beta application developed for Android smartphones are promising.
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