  We study the Markov chain $x_{n+1}=ax_n+b_n$ on a finite field
$\mathbb{F}_p$, where $a \in \mathbb{F}_p$ is fixed and $b_n$ are independent
and identically distributed random variables in $\mathbb{F}_p$. Conditionally
on the Riemann hypothesis for all Dedekind zeta functions, we show that the
chain exhibits a cut-off phenomenon for most primes $p$ and most values of $a
\in \mathbb{F}_p$. We also obtain weaker, but unconditional, upper bounds for
the mixing time.
