Google PageRank checksum algorithm

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.

C Version (original):

	/* ~ überskill */
	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>
	char fch(uint csm)
	{
	int a, b, c, t;
	a = csm % 10;
	t = 1;
	b = csm / 10;
	while(b) {
	c = b % 10;
	if(t)
	c = c / 5 + (c * 2) % 10;
	b /= 10;
	a += c;
	t ^= 1;
	}
	a = 10 - a % 10;
	if(a == 10)
	return '0';
	if(t)
	return '0' + (a & 1 ? a + 9 : a) / 2;
	return '0' + a;
	}
	uint checksum(char *str)
	{
	int a, b, c, d, e;
	char *p;
	if(*str == '\0')
	return 0x1000;
	p = str;
	a = 5381;
	while(*p)
	a += (a << 5) + *p++;
	p = str;
	b = 0;
	while(*p)
	b = (b << 6) + (b << 16) - b + *p++;
	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;
	}
	#define PR_SERVER "http://toolbarqueries.google.com/tbr?client=navclient-auto&features=Rank&q=info:"
	int main(int argc, char *argv[])
	{
	uint csm;
	if(argc < 2) {
	fprintf(stderr, "usage: %s link\n",
	argv[0] ? argv[0] : "gchecksum");
	return EXIT_FAILURE;
	}
	csm = checksum(argv[1]);
	printf(PR_SERVER "%s&ch=7%c%u\n", argv[1], fch(csm), csm);
	return EXIT_SUCCESS;
	}

view raw pr_checksum.c hosted with ❤ by GitHub

PHP Version:

	<?php
	/* ~ überskill */
	/* the floating point hacks are due
	to PHP's bugs when handling
	large unsigned integers */
	function fch($csm)
	{
	if($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)
	return ord('0');
	if($t)
	return ord('0') + (int)(($a & 1 ? $a + 9 : $a) / 2);
	return ord('0') + $a;
	}
	function checksum($str)
	{
	if(strlen($str) == 0)
	return 0x1000;
	$a = 5381.0;
	for($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);
	?>

view raw pr_checksum.php hosted with ❤ by GitHub

~ Dmitry