  Let $q>2$ be a prime power and $f={\tt x}^{q-2}+t{\tt x}^{q^2-q-1}$, where
$t\in\Bbb F_q^*$. It was recently conjectured that $f$ is a permutation
polynomial of $\Bbb F_{q^2}$ if and only if one of the following holds: (i)
$t=1$, $q\equiv 1\pmod 4$; (ii) $t=-3$, $q\equiv \pm1\pmod{12}$; (iii) $t=3$,
$q\equiv -1\pmod 6$. We confirm this conjecture in the present paper.
