def get_sp_key(self):
key = self.__sp.get('<STR_LIT>')<EOL>key_file_name = self.__paths['<STR_LIT>'] + '<STR_LIT>'<EOL>if not key and exists(key_file_name):<EOL><INDENT>with open(key_file_name) as f:<EOL><INDENT>key = f.read()<EOL><DEDENT><DEDENT>return key or None<EOL>
Returns the x509 private key of the SP.

:returns: SP private key
:rtype: string or None