code: def _new_master_key(self, key_id):
          
          _key_id = to_str(key_id)  # KMS client requires str, not bytes
          return KMSMasterKey(config=KMSMasterKeyConfig(key_id=key_id, client=self._client(_key_id)))
docs: Returns a KMSMasterKey for the specified key_id.
  
          :param bytes key_id: KMS CMK ID
          :returns: KMS Master Key based on key_id
          :rtype: aws_encryption_sdk.key_providers.kms.KMSMasterKey
          :raises InvalidKeyIdError: if key_id is not a valid KMS CMK ID to which this key provider has access