Establishing secure links between pairs of directly connected sensor nodes is an important primitive for building secure wireless sensor networks. This paper systematically identifies two important security requirements of pairwise key setup in wireless sensor networks, namely opaqueness and inoculation. Transitory master key schemes, such as the LEAP protocol, can satisfy both requirements if the master key has not been compromised. However, if the master key is compromised, every key in the network is exposed to an adversary. To prevent the master key from becoming a single point failure of the whole system, we propose a new opaque transitory master key (OTMK) scheme for pairwise key setup in sensor networks. In OTMK, even if the master key is compromised, an adversary can only exploit a small number of keys nearby the compromised node, while other keys in the network remain safe. To further investigate key establishment schemes, we experimented with a way to compromise a sensor node, and tested our key establishment time in a real sensor network environment.
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