public function getWIF($private_key = null, $network = 'main', $public_key_format = 'compressed')
    {
        if (empty($private_key) === true) {
            return ($this->WIF_address != '') ? $this->WIF_address : '';
        } else {
            return $this->encodeWIF($private_key, $network, $public_key_format);
        }
    }