private function readPublicKey(Reader $reader)
    {
        $file = $reader->getFile() . '.pubkey';

        if (!file_exists($file)) {
            throw FileException::createUsingFormat(
                'The path "%s" is not a file or does not exist.',
                $file
            );
        }

        if (false === ($key = @file_get_contents($file))) {
            throw FileException::createUsingLastError();
        }

        return $key;
    }