D-Link gws file decoder

gwsdecoder.php

#/usb/bin/php
<?php
//author rgranadino aug 22 2012
//tested on a D-Link DIR-615 B2 v2.25 firmware.
//this is a php translation of:
//     http://www.shulerent.com/2009/08/21/cracking-the-d-link-settings-file/

$file = $argv[1];
if (!is_readable($file)) {
  echo "Cannot read file: $file\n";
  exit(1);
}
$handle = fopen($file, 'r');
$size = filesize($file);
$out = array();
for ($i=0; $i <$size; $i++) {
  $char = ord(fread($handle,1)) - ($i%256);
  if ($char < 0) { 
    $char += 256;
  } 
  $out[] = chr($char);
}
fclose($handle);
echo implode('', $out);