Since wireless sensor networks (WSNs) are vulnerable to threats and attacks due to the nature of wireless communications between sensors and fusion center (FC), it is often necessary to secure transmissions from any possible eavesdropping. In this paper, we study the eavesdropping issue when sensors are over-deployed and the channels between sensors and the ally FC are modeled as time-varying Rayleigh fading channels. We propose a transmission scheme in which only the sensors of strong and weak channel gains report their decisions in a predetermined way based on type-based multiple access protocol. By taking advantage of random behaviors of wireless
channels in the form of the multiuser diversity and carefully design different roles of sensors of strong and weak channel gains to the ally FC, the proposed scheme can confuse the enemy FC in making a decision. Eventually, it guarantees the perfect secrecy promised by an information theoretic measure. We also analyze detection error probability and equivocation at the ally and enemy FCs, respectively.