  This paper proposes a new security cooperative protocol, for dual phase
amplify-and-forward large wireless sensor networks. In such a network, a
portion of the K relays can be potential eavesdroppers. The source agrees to
share with the destination a given channel state information (CSI) of a
source-trusted relay-destination link to encode the message. Then, in the first
hop, the source will use this CSI to map the right message to a certain sector
while transmitting fake messages to the other sectors using sectoral
transmission thanks to analog beamforming. In the second hop, the relays
retransmit their received signals to the destination, using the distributed
beamforming technique. We derived the secrecy outage probability and
demonstrated that the probability of receiving the right encoded information by
an untrustworthy relay is inversely proportional to the number of sectors. We
also showed that the aggressive behavior of the cooperating untrusted relays is
not effective compared to the case where each untrusted relay is trying to
intercept the transmitted message individually.
