sub binmodexp {
    my ($base, $exp, $n) = @_;
    die "Negative exponent" if $exp<0;
    my $res = 1;
    my $mul = $base % $n;
    while ($exp) {
	if ($exp & 1) {
	    $res = ( $res * $mul ) % $n;
	}
	$exp>>=1;
	$mul = ($mul*$mul) % $n;
    }
    return $res;
}