  In this paper, we consider an active eavesdropping scenario in a cooperative
system consisting of a source, a destination, and an active eavesdropper with
multiple decode-and-forward relays. Considering an existing assumption in which
an eavesdropper is also a part of network, a proactive relay selection by the
eavesdropper is proposed. The best relay which maximizes the eavesdropping rate
is selected by the eavesdropper. A relay selection scheme is also proposed to
improve the secrecy of the system by minimizing the eavesdropping rate.
Performances of these schemes are compared with two passive eavesdropping
scenarios in which the eavesdropper performs selection and maximal ratio
combining on the relayed links. A realistic channel model with independent
non-identical links between nodes and direct links from the source to both the
destination and eavesdropper are assumed. Closed-form expressions for the
secrecy outage probability (SOP) of these schemes in Rayleigh fading channel
are obtained. It is shown that the relay selection by the proactive
eavesdropper is most detrimental to the system as not only the SOP increases
with the increase in the number of relays, but its diversity also remains
unchanged.
