private function makeKey($block, $valContext)
    {
        $pwarray = str_repeat("\0", 64);

        for ($i = 0; $i < 5; $i++) {
            $pwarray[$i] = $valContext[$i];
        }
        
        $pwarray[5] = chr($block & 0xff);
        $pwarray[6] = chr(($block >> 8) & 0xff);
        $pwarray[7] = chr(($block >> 16) & 0xff);
        $pwarray[8] = chr(($block >> 24) & 0xff);

        $pwarray[9] = "\x80";
        $pwarray[56] = "\x48";

        $md5 = new PHPExcel_Reader_Excel5_MD5();
        $md5->add($pwarray);

        $s = $md5->getContext();
        return new PHPExcel_Reader_Excel5_RC4($s);
    }