Change counter value for AES-CTR

[markberger]

In immutable/upload.py we have the following code:

def _hash_and_encrypt_plaintext(self, data, hash_only):
        assert isinstance(data, (tuple, list)), type(data)
        data = list(data)
        cryptdata = []
        # we use data.pop(0) instead of 'for chunk in data' to save
        # memory: each chunk is destroyed as soon as we're done with it.
        bytes_processed = 0
        while data:
            chunk = data.pop(0)
            self.log(" read_encrypted handling %dB-sized chunk" % len(chunk),
                     level=log.NOISY)
            bytes_processed += len(chunk)
            self._plaintext_hasher.update(chunk)
            self._update_segment_hash(chunk)
            # TODO: we have to encrypt the data (even if hash_only==True)
            # because pycryptopp's AES-CTR implementation doesn't offer a
            # way to change the counter value. Once pycryptopp acquires
            # this ability, change this to simply update the counter
            # before each call to (hash_only==False) _encryptor.process()
            ciphertext = self._encryptor.process(chunk)
            if hash_only:
                self.log("  skipping encryption", level=log.NOISY)
            else:
                cryptdata.append(ciphertext)
            del ciphertext
            del chunk
        self._ciphertext_bytes_read += bytes_processed
        if self._status:
            progress = float(self._ciphertext_bytes_read) / self._file_size
            self._status.set_progress(1, progress)
        return cryptdata

As part of #1382 I am removing the TODO and creating a ticket for this instead.