Comment 21 for bug 1799746

The code incorrectly assumes that base64 is byte encoded so something like:

        elif content_encoding.lower() == 'base64':
            if not isinstance(unencrypted, six.binary_type):
                b64payload = unencrypted.encode('utf-8')
            else:
                b64payload = unencrypted

does the trick in terms of ensuring binary encoding of the data in this code path.