Returns the x509 private key of the SP.
        :returns: SP private key
        :rtype: string or None

def get_sp_key(self):
        """
        Returns the x509 private key of the SP.
        :returns: SP private key
        :rtype: string or None
        """
        key = self.__sp.get('privateKey')
        key_file_name = self.__paths['cert'] + 'sp.key'

        if not key and exists(key_file_name):
            with open(key_file_name) as f:
                key = f.read()

        return key or None