encoded with a
    simple `PKCS#1`_ structure (``RSAPrivateKey``). The key cannot be
    securely encrypted.

    With ``pkcs=8``, the private key is encoded with a `PKCS#8`_ structure
    (``PrivateKeyInfo``). PKCS#8 offers the best ways to securely
    encrypt the key.

    .. note::
        This parameter is ignored for a public key.
        For DER and PEM, the output is always an
        ASN.1 DER ``SubjectPublicKeyInfo`` structure.

  protection (string):
    (*For private keys only*)
    The encryption scheme to use for protecting the private key
    using the passphrase.

    You can only specify a value if ``pkcs=8``.
    For all possible protection schemes,
    refer to :ref:`the encryption parameters of PKCS#8<enc_params>`.
    The recommended value is
    ``'PBKDF2WithHMAC-SHA512AndAES256-CBC'``.

    If ``None`` (default), the behavior depends on :attr:`format`:

    - if ``format='PEM'``, the obsolete PEM encryption scheme is used.
      It is based on MD5 for key derivation, and 3DES for encryption.

    - if ``format='DER'``, the ``'PBKDF2WithHMAC-SHA1AndDES-EDE3-CBC'``
      scheme is used.

  prot_params (dict):
    (*For private keys only*)

    The parameters to use to derive the encryption key
    from the passphrase. ``'protection'`` must be also specified.
    For all possible values,
    refer to :ref:`the encryption parameters of PKCS#8<enc_params>`.
    The recommendation is to use ``{'iteration_count':21000}`` for PBKDF2,
    and ``{'iteration_count':131072}`` for scrypt.

  randfunc (callable):
    A function that provides random bytes. Only used for PEM encoding.
    The default is :func:`Cryptodome.Random.get_random_bytes`.

Returns:
  bytes: the encoded key

Raises:
  ValueError:when the format is unknown or when you try to encrypt a private
    key with *DER* format and PKCS#1.

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

.. _RFC1421:    http://www.ietf.org/rfc/rfc1421.txt
.. _RFC1423:    http://www.ietf.org/rfc/rfc1423.txt
.. _`PKCS#1`:   http://www.ietf.org/rfc/rfc3447.txt
.. _`PKCS#8`:   http://www.ietf.org/rfc/rfc5208.txt
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