'Generate the assertion that will be used in the request.'

def _generate_assertion(self):

    now = int(time.time())
    payload = {'aud': self.token_uri, 'scope': self.scope, 'iat': now, 'exp': (now + SignedJwtAssertionCredentials.MAX_TOKEN_LIFETIME_SECS), 'iss': self.service_account_name}
    payload.update(self.kwargs)
    logger.debug(str(payload))
    private_key = base64.b64decode(self.private_key)
    return crypt.make_signed_jwt(crypt.Signer.from_string(private_key, self.private_key_password), payload)
