public function setSecretKey($secretKey)
    {
        if (!is_string($secretKey)) {
            $this->secretKeyError = 'invalid type';

            return false;
        }

        if (empty($secretKey)) {
            $this->secretKeyError = 'empty string';

            return false;
        }

        if (strlen($secretKey) > 64) {
            $this->secretKeyError = 'secret key is length is too big';

            return false;
        }

        if (preg_match("/ /i", $secretKey)) {
            $this->secretKeyError = 'invalid format; white spaces not allowed';

            return false;
        }

        $this->secretKey = $secretKey;

        return true;
    }