  This paper contributes to the challenging field of security for wireless
sensor networks by introducing a key agreement scheme in which sensor nodes
create secure radio connections with their neighbours depending on the aid of
third parties. These third parties are responsible only for the pair-wise key
establishment among sensor nodes, so they do not observe the physical
phenomenon nor route data packets to other nodes. The proposed method is
explained here with respect to four important issues: how secret shares are
distributed, how local neighbours are discovered, how legitimate third parties
are verified, and how secure channels are established. Moreover, the
performance of the scheme is analyzed with regards to five metrics: local
connectivity, resistance to node capture, memory usage, communication overhead,
and computational burden. Our scheme not only secures the transmission channels
of nodes but also guarantees high local connectivity of the sensor network, low
usage of memory resources, perfect network resilience against node capture, and
complete prevention against impersonation attacks. As it is demonstrated in
this paper, using a number of third parties equals to 10% of the total number
of sensor nodes in the area of interest, the proposed method can achieve at
least 99.42% local connectivity with a very low usage of available storage
resources (less than 385 bits on each sensor node).
