Write an SSH2-format private key file in a form that can be read by
        paramiko or openssh.  If no password is given, the key is written in
        a trivially-encoded format (base64) which is completely insecure.  If
        a password is given, DES-EDE3-CBC is used.

        :param str tag:
            ``"RSA"`` or ``"DSA"``, the tag used to mark the data block.
        :param filename: name of the file to write.
        :param str data: data blob that makes up the private key.
        :param str password: an optional password to use to encrypt the file.

        :raises: ``IOError`` -- if there was an error writing the file.

def _write_private_key_file(self, filename, key, format, password=None):
        """
        Write an SSH2-format private key file in a form that can be read by
        paramiko or openssh.  If no password is given, the key is written in
        a trivially-encoded format (base64) which is completely insecure.  If
        a password is given, DES-EDE3-CBC is used.

        :param str tag:
            ``"RSA"`` or ``"DSA"``, the tag used to mark the data block.
        :param filename: name of the file to write.
        :param str data: data blob that makes up the private key.
        :param str password: an optional password to use to encrypt the file.

        :raises: ``IOError`` -- if there was an error writing the file.
        """
        with open(filename, "w") as f:
            os.chmod(filename, o600)
            self._write_private_key(f, key, format, password=password)