public function caFile($sslPath = NULL)
    {
        $sslPath = $sslPath ? $sslPath : DIRECTORY_SEPARATOR . '..' . DIRECTORY_SEPARATOR .
                   'ssl' . DIRECTORY_SEPARATOR;
        $caPath = __DIR__ . $sslPath . 'api_braintreegateway_com.ca.crt';

        if (!file_exists($caPath))
        {
            throw new Exception\SSLCaFileNotFound();
        }

        return $caPath;
    }