  Most of the well-known fading distributions, if not all of them, could be
encompassed by Fox's $H$-function fading. Consequently, we investigate the
exact and asymptotic behavior of physical layer security (PLS) over Fox's
$H$-function fading wiretap channels. In particular, closed-form expressions
are derived for secrecy outage probability (SOP), probability of non-zero
secrecy capacity (PNZ), and average secrecy capacity (ASC). These expressions
are given in terms of either univariate or bivariate Fox's $H$-function. In
order to show the comprehensive effectiveness of our derivations, three metrics
are respectively listed over the following frequently used fading channels,
including Rayleigh, Weibull, Nakagami-$m$, $\alpha-\mu$, Fisher-Snedecor (F-S)
$\mathcal{F}$, and extended generalized-$\mathcal{K}$ (EGK). Our tractable
results are more straightforward and general, besides that, they are feasible
and applicable, especially the SOP, which was mostly limited to the lower bound
in literature due to the difficulty of achieving closed-from expressions. In
order to validate the accuracy of our analytical results, Monte-Carlo
simulations are subsequently performed for the special case $\alpha-\mu$ fading
channels. One can observe perfect agreements between the exact analytical and
simulation results, and highly accurate approximations between the exact and
asymptotic analytical results.
