Google PageRank checksum PHP algorithm revealed

In a blog entry, the Google PageRank checksum algorithm was revealed.

The Google PageRank functionality in Google Toolbar works by querying Google’s server for information on the PageRank of a specific page. This might seem easy enough to implement in your own program/website, but the problem is that the toolbar calculates a checksum on the page URL before querying the server, and the server only responds if the checksum is correct. Fortunately the checksum algorithm was reverse engineered from Google Toolbar 7. I was provided the hand decompiled version of the algorithm in C from a friend. Then I went ahead and rewrote it in PHP for web development usage. You can find both versions below.

As an example, the query URL for the page ‘http://en.wikipedia.org/wiki/Cypherpunk’ is http://toolbarqueries.google.com/tbr?client=navclient-auto&features=Rank&q=info:http://en.wikipedia.org/wiki/Cypherpunk&ch=783735859783

Any other query with a checksum other than 783735859783 will result in a ‘403 forbidden’ response.
Enjoy.

PHP CODE:

<?php

/* the floating point hacks are due

   to PHP's bugs when handling

   large unsigned integers */

functionfch($csm)

{

    if($csm< 0)

        $csm+= 4294967296.0;

    $a= (int)fmod($csm, 10);

    $t= 1;

    $b= (int)($csm/ 10);

    while($b) {

        $c= $b% 10;

        if($t)

            $c= (int)($c/ 5) + ($c* 2) % 10;

        $b= (int)($b/ 10);

        $a+= $c;

        $t^= 1;

    }

    

    $a= 10 - $a% 10;

    if($a== 10)

        returnord('0');

    

    if($t)

        returnord('0') + (int)(($a& 1 ? $a+ 9 : $a) / 2);

    returnord('0') + $a;

}

functionchecksum($str)

{

    if(strlen($str) == 0)

        return0x1000;

        

    /* the floating point hacks are due to PHP's bugs when handling integers */

    $a= 5381.0;

    for($i= 0; $i< strlen($str); $i++)

        $a= fmod($a+ ($a* 32) + ord($str[$i]), 4294967296.0);

    if($a> 2147483647.0)

        $a-= 4294967296.0;

    $a= (int)$a;

    $b= 0.0;

    for($i= 0; $i< strlen($str); $i++)

        $b= fmod(($b* 64) + ($b* 65536) - $b+ ord($str[$i]), 4294967296.0);

    if($b> 2147483647.0)

        $b-= 4294967296.0;

    $b= (int)$b;

    

    $a= (($a>> 6) & 0x3ffffc0) | (($a>> 2) & 0x3f);

    $c= (($a>> 4) & 0x3ffc00) | ($a& 0x3ff);

    $d= (($c>> 4) & 0x3c000) | ($c& 0x3fff);

    $c= ((($d& 0x3c0) << 4) | ($d& 0x3c)) << 2;

    $a= $b& 0x0f0f;

    $e= $b& 0x0f0f0000;

    $b= (($d& 0xffffc000) << 4) | ($d& 0x3c00);

    return($b<< 10) | $c| $a| $e;

}

$PR_SERVER= "http://toolbarqueries.google.com/tbr?client=navclient-auto&features=Rank&q=info:";

$page= "http://en.wikipedia.org/wiki/Cypherpunk";

$csm= checksum($page);

printf($PR_SERVER. "%s&ch=7%c%u\n", $page, fch($csm), $csm);

?>

The author of the PHP code is Dmitry Molotov.