  Research in underwater communication is rapidly becoming attractive due to
its various modern applications. An efficient mechanism to secure such
communication is via physical layer security. In this paper, we propose a novel
physical layer authentication (PLA) mechanism in underwater acoustic
communication networks where we exploit the position/location of the
transmitter nodes to achieve authentication. We perform transmitter position
estimation from the received signals at reference nodes deployed at fixed
positions in a predefined underwater region. We use time of arrival (ToA)
estimation and derive the distribution of inherent uncertainty in the
estimation. Next, we perform binary hypothesis testing on the estimated
position to decide whether the transmitter node is legitimate or malicious. We
then provide closed-form expressions of false alarm rate and missed detection
rate resulted from binary hypothesis testing. We validate our proposal via
simulation results, which demonstrate errors' behavior against the link
quality, malicious node location, and receiver operating characteristic (ROC)
curves. We also compare our results with the performance of previously proposed
fingerprint mechanisms for PLA in underwater acoustic communication networks,
for which we show a clear advantage of using the position as a fingerprint in
PLA.
