Google Toolbar Page Rank checksum used a public domain hash algorithm written by Bob Jenkins in the mid-1990s

Working PHP source for Google Page Rank checksum algorithm.

Alex Stapleton figured out the algorithm to calculate the checksum to grab the Google Toolbar Page Rank for any URL:

<?php
/* This code is released unto the public domain */

header("Content-Type: text/plain; charset=utf-8"); 
define('GOOGLE_MAGIC', 0xE6359A60); 
    
//unsigned shift right 
function zeroFill($a, $b) 
 { 
  $z = hexdec(80000000); 
  if ($z & $a) 
   { 
    $a = ($a>>1); 
    $a &= (~$z); 
    $a |= 0x40000000; 
    $a = ($a>>($b-1)); 
   } 
  else 
   { 
    $a = ($a>>$b); 
   } 
  return $a; 
 } 
    
    
function mix($a,$b,$c) 
 { 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,13)); 
  $b -= $c; $b -= $a; $b ^= ($a<<8); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,13)); 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,12)); 
  $b -= $c; $b -= $a; $b ^= ($a<<16); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,5)); 
  $a -= $b; $a -= $c; $a ^= (zeroFill($c,3)); 
  $b -= $c; $b -= $a; $b ^= ($a<<10); 
  $c -= $a; $c -= $b; $c ^= (zeroFill($b,15)); 
     
  return array($a,$b,$c); 
 } 
    
function GoogleCH($url, $length=null, $init=GOOGLE_MAGIC) 
 { 
  if(is_null($length)) 
   { 
    $length = sizeof($url); 
   } 
   $a = $b = 0x9E3779B9; 
   $c = $init; 
   $k = 0; 
   $len = $length; 
    while($len >= 12) 
     { 
      $a += ($url[$k+0] +($url[$k+1]<<8) +($url[$k+2]<<16) +($url[$k+3]<<24)); 
      $b += ($url[$k+4] +($url[$k+5]<<8) +($url[$k+6]<<16) +($url[$k+7]<<24)); 
      $c += ($url[$k+8] +($url[$k+9]<<8) +($url[$k+10]<<16)+($url[$k+11]<<24)); 
      $mix = mix($a,$b,$c); 
      $a = $mix[0]; $b = $mix[1]; $c = $mix[2]; 
      $k += 12; 
      $len -= 12; 
     } 
   $c += $length; 
   switch($len)  /* all the case statements fall through */ 
    { 
     case 11: $c+=($url[$k+10]<<24); 
     case 10: $c+=($url[$k+9]<<16); 
     case 9 : $c+=($url[$k+8]<<8); 
     /* the first byte of c is reserved for the length */ 
     case 8 : $b+=($url[$k+7]<<24); 
     case 7 : $b+=($url[$k+6]<<16); 
     case 6 : $b+=($url[$k+5]<<8); 
     case 5 : $b+=($url[$k+4]); 
     case 4 : $a+=($url[$k+3]<<24); 
     case 3 : $a+=($url[$k+2]<<16); 
     case 2 : $a+=($url[$k+1]<<8); 
     case 1 : $a+=($url[$k+0]); 
     /* case 0: nothing left to add */ 
    } 
   $mix = mix($a,$b,$c); 
   /*-------------------------------------------- report the result */ 
  return $mix[2]; 
 } 
       
//converts a string into an array of integers containing the numeric value of the char

function strord($string) 
 { 
  for($i=0;$i<strlen($string);$i++) 
   { 
    $result[$i] = ord($string{$i}); 
   } 
  return $result; 
 }

// [url]http://www.example.com/[/url] - Checksum: 6540747202 
$url = 'info:'.$_GET['url']; 
print("url:\t{$_GET['url']}\n"); 
$ch = GoogleCH(strord($url)); 
printf("ch:\t6%u\n",$ch); 
?>

How to use checksum algorithm to getting your Google Page Rank without the Toolbar.

Once you calculated the checksum, you can dynamically find out the Page Rank for a page by reading the values returned by the following URL: http://www.google.com/search?client=navclient-auto&ch=[checksum here]&features=Rank&q=info:http://www.example.com

Did checksum algorithm kill Google Toolbar Page Rank?

One might point out that it is also highly unethical to public Google Page Rank algorithm. Google has invested massive amounts of money and time to develop a proprietary measure for their own private, internal use. Not true. Google used a public domain hash algorithm written by Bob Jenkins in the mid-1990s. The algorithm was explained in an article in Dr.Dobbs magazine in September 1997. Google didn't even exist yet. The code is on Jenkins' site and it's clearly marked as public domain, free for the taking, for any use whatsoever.

There's an "initval" in the code that seeds the algorithm. Google's initval is 0xE6359A60. This is one of two things that are specific to Google in the code. The other is that they stick the word "info:" in front of the URL before hashing it. That's no secret either, since they also do this in the QUERY_STRING that phones home.

After Page Rank checksum algorithm source code was public Google says: "The PageRank that is displayed in the Google Toolbar is for entertainment purposes only. Due to repeated attempts by hackers to access this data, Google updates the PageRank data very infrequently because is it not secure. On average, the PR that is displayed in the Google Toolbar is several months old. If the toolbar is showing a PR of zero, this is because the user is visiting a new URL that hasn't been updated in the last update. The PR that is displayed by the Google Toolbar is not the same PR that is used to rank the webpage results so there is no need to be concerned if your PR is displayed as zero. If a site is showing up in the search results, it doesn't not have a real PR of zero, the Toolbar is just out of date"

Alternatively, you can check site's Page Rank score using the Google Directory - assuming of course that site is listed in the Google Directory! Listings on Google Directory category pages are ranked in order of Page Rank score. This means it is possible to see small Page Rank shifts relative to other sites in category, particularly if there are a number of sites listed on category page. If you are curious where site sat in comparison to others listed on that category page in the past, you can get historical Page Rank scores using the Wayback Machine available from Alexa.

References

Google Toolbar
Google Directory
Alex Stapleton's Google Page Rank Checksum Algorithm
Hash functions and block ciphers
Dr. Dobb's Journal
Internet Archive Wayback Machine