  We consider a cooperative wireless network comprised of a source, a
destination and multiple relays operating in the presence of an eavesdropper,
which attempts to tap the source-destination transmission. We propose
multi-relay selection scheme for protecting the source against eavesdropping.
More specifically, multi-relay selection allows multiple relays to
simultaneously forward the source's transmission to the destination, differing
from the conventional single-relay selection where only the best relay is
chosen to assist the transmission from the source to destination. For the
purpose of comparison, we consider the classic direct transmission and
single-relay selection as benchmark schemes. We derive closed-form expressions
of the intercept probability and outage probability for the direct transmission
as well as for the single-relay and multi-relay selection schemes over Rayleigh
fading channels. It is demonstrated that as the outage requirement is relaxed,
the intercept performance of the three schemes improves and vice versa,
implying that there is a \emph{security versus reliability trade-off} (SRT). We
also show that both the single-relay and multi-relay selection schemes
outperform the direct transmission in terms of SRT, demonstrating the advantage
of the relay selection schemes for protecting the source's transmission against
the eavesdropping attacks. Finally, upon increasing the number of relays, the
SRTs of both the single-relay and multi-relay selection schemes improve
significantly and as expected, multi-relay selection outperforms single-relay
selection.
