    def _expand(self, key_material):
        output = [b""]
        counter = 1

        while (self._algorithm.digest_size // 8) * len(output) < self._length:
            h = hmac.HMAC(key_material, self._algorithm, backend=self._backend)
            h.update(output[-1])
            h.update(self._info)
            h.update(six.int2byte(counter))
            output.append(h.finalize())
            counter += 1

        return b"".join(output)[:self._length]
The product receives input or data, but it does
        not validate or incorrectly validates that the input has the
        properties that are required to process the data safely and
        correctly.