'Cryptographically sign a blob (of bytes).
This method is provided to support a common interface, but
the actual key used for a Google Compute Engine service account
is not available, so it can\'t be used to sign content.
Args:
blob: bytes, Message to be signed.
Raises:
NotImplementedError, always.'

def sign_blob(self, blob):

    raise NotImplementedError('Compute    Engine    service    accounts    cannot    sign    blobs')
