message: Raise NotImplementedError() on metaclass encrypt method for consistency.
diff: @@ -48,7 +48,7 @@ class Aead(object):
  Raises:
  tink.TinkError if the encryption fails.
  """
  - pass
  + raise NotImplementedError()
  @abc.abstractmethod
  def decrypt(self, ciphertext: bytes, associated_data: bytes) -> bytes:
  