code: def _pad(self, data):
          
          length = 16 - (len(data) % 16)
          data += chr(length)*length
          return data
docs: Pad value with bytes so it's a multiple of 16
          See: http://stackoverflow.com/questions/14179784/python-encrypting-with-pycrypto-aes
          :param data:
          :return data: