  Wireless networks operating at terahertz (THz) frequencies have been proposed
as a promising candidate to support the ever-increasing capacity demand, which
cannot be satisfied with existing radio-frequency (RF) technology. On the other
hand, it likely will serve as backbone infrastructure and could therefore be an
attractive target for eavesdropping attacks. Compared with regular RF spectrum,
wireless channels in the THz range could be less vulnerable to interceptions
because of their high beam directionality and small signal coverage. However, a
risk for eavesdropping can still exist due to the multipath effects caused by
unintended scattering. In this work, an eavesdropping risk for THz channel
passing atmospheric turbulences and producing compromising emissions is
investigated from a physical layer perspective. A model combining signal
attenuation due to turbulence, gaseous absorption and beam divergence, is
developed for prediction of deterministic and probabilistic signal leakages.
The secrecy capacity and outage probability of the THz channel are derived and
analyzed with respect to variations of the turbulence strength and other
channels characteristics. The dependence of the channel performance on the
eavesdropper's position is investigated with respect to the maximum safe data
transmission rate (MSR) and the signal leakage region. Design results for THz
channels are provided to minimize an eavesdropping risk at physical layer.
