'Cryptographically sign a blob (of bytes).
Implements abstract method
:meth:`oauth2client.client.AssertionCredentials.sign_blob`.
Args:
blob: bytes, Message to be signed.
Returns:
tuple, A pair of the private key ID used to sign the blob and
the signed contents.'

def sign_blob(self, blob):

    return app_identity.sign_blob(blob)
