  This paper studies the problem of detecting a potential malicious relay node
by a source node that relies on the relay to forward information to other
nodes. The channel model of two source nodes simultaneously sending symbols to
a relay is considered. The relay is contracted to forward the symbols that it
receives back to the sources in the amplify-and-forward manner. However there
is a chance that the relay may send altered symbols back to the sources. Each
source attempts to individually detect such malicious acts of the relay by
comparing the empirical distribution of the symbols that it receives from the
relay conditioned on its own transmitted symbols with known stochastic
characteristics of the channel. It is shown that maliciousness of the relay can
be asymptotically detected with sufficient channel observations if and only if
the channel satisfies a non-manipulable condition, which can be easily checked.
As a result, the non-manipulable condition provides us a clear-cut criterion to
determine the detectability of the aforementioned class of symbol manipulation
attacks potentially conducted by the relay.
