A convenience method which pads and decrypts at once.

def Decrypt(self, data):
    """A convenience method which pads and decrypts at once."""
    decryptor = self.GetDecryptor()

    try:
      padded_data = decryptor.update(data) + decryptor.finalize()
      return self.UnPad(padded_data)
    except ValueError as e:
      raise CipherError(e)