def serialize_private_key_to_pem(private_key, passphrase_bytes=None):
    """Serialize private key to PEM.

    Args:
      private_key:
      passphrase_bytes:

    Returns:
      bytes: PEM encoded private key

    """
    return private_key.private_bytes(
        encoding=cryptography.hazmat.primitives.serialization.Encoding.PEM,
        format=cryptography.hazmat.primitives.serialization.PrivateFormat.TraditionalOpenSSL,
        encryption_algorithm=cryptography.hazmat.primitives.serialization.BestAvailableEncryption(
            passphrase_bytes
        )
        if passphrase_bytes is not None
        else cryptography.hazmat.primitives.serialization.NoEncryption(),
    )