``{'iteration_count':131072}`` for scrypt.

.. warning::
    If you don't provide a passphrase, the private key will be
    exported in the clear!

.. note::
    When exporting a private key with password-protection and `PKCS#8`_
    (both ``DER`` and ``PEM`` formats), any extra parameters
    to ``export_key()`` will be passed to :mod:`Cryptodome.IO.PKCS8`.

.. _PEM:        http://www.ietf.org/rfc/rfc1421.txt
.. _`PEM encryption`: http://www.ietf.org/rfc/rfc1423.txt
.. _OpenSSH:    http://www.openssh.com/txt/rfc5656.txt
.. _RFC5480:    https://tools.ietf.org/html/rfc5480
.. _SEC1:       https://www.secg.org/sec1-v2.pdf

Returns:
    A multi-line string (for ``'PEM'`` and ``'OpenSSH'``) or
    ``bytes`` (for ``'DER'``, ``'SEC1'``, and ``'raw'``) with the encoded key.
Ú