# MD5 Games 1, Web, 50pts

## Problem

10 years has passed since MD5 was broken, yet it is still frequently used in web applications, particularly PHP powered applications (maybe because there's a function after it?). Break it again to prove the point!

## Solution

We get simple form, where we can put our string.


Also, we are able to see PHP source of the challenge:

```php

Find a string that has a MD5 digest equal to itself!

<form>
<label>Answer: </label>
<input type='text' name='md5' />
<input type='submit' />
</form>

Source Code
```

The only way we can get the flag is to pass string, which MD5 hash is exact the same as the string itself. Because such string does not exists (or I don't know one) we have to find another solution.

The only way to solve this challenge is to exploit PHP type juggling (as ```$md5``` is compared with ```md5($md5)``` with ```==``` instead of strict comparision operator ```===```).

The easiest way to do this is to provide a number starting with ```0e```, which MD5 hash begins with ```0e``` as well and contains only numbers.

Thats because such comparision will return ```true```:

```php
// test.php
{}".format(s, hashed_s)
print "[+] in {} iterations".format(iters)
exit(0)

if iters % 1000000 == 0:
print "[+] current value: {} {} iterations, continue...".format(s, iters)

breakit()

```

When I run it, after couple of minutes and __almost 216 millions__ of iterations, I get the string which gave me the flag:

```
[+] current value: 0e196999999 197000000 iterations, continue...
[+] current value: 0e197999999 198000000 iterations, continue...
[+] current value: 0e198999999 199000000 iterations, continue...
[+] current value: 0e199999999 200000000 iterations, continue...
[+] current value: 0e200999999 201000000 iterations, continue...
[+] current value: 0e201999999 202000000 iterations, continue...
[+] current value: 0e202999999 203000000 iterations, continue...
[+] current value: 0e203999999 204000000 iterations, continue...
[+] current value: 0e204999999 205000000 iterations, continue...
[+] current value: 0e205999999 206000000 iterations, continue...
[+] current value: 0e206999999 207000000 iterations, continue...
[+] current value: 0e207999999 208000000 iterations, continue...
[+] current value: 0e208999999 209000000 iterations, continue...
[+] current value: 0e209999999 210000000 iterations, continue...
[+] current value: 0e210999999 211000000 iterations, continue...
[+] current value: 0e211999999 212000000 iterations, continue...
[+] current value: 0e212999999 213000000 iterations, continue...
[+] current value: 0e213999999 214000000 iterations, continue...
[+] current value: 0e214999999 215000000 iterations, continue...
[+] found! md5( 0e215962017 ) ---> 0e291242476940776845150308577824
[+] in 215962018 iterations
bl4de:~/hacking/ctf/2017/HackDatKiwi_CTF_2017 $
```

I've put __0e215962017__ into the form and:


Original writeup (https://github.com/bl4de/ctf/blob/master/2017/HackDatKiwi_CTF_2017/md5games1/md5games1.md).