## The setup

In this tutorial, I'm going to show you how simple it is to creatively
interfere with Apple Game Center traffic using mitmproxy. To set things up, I
registered my mitmproxy CA certificate with my iPhone - there's a [step by step
set of instructions](@!urlTo("certinstall/ios.html")!@) elsewhere in this manual. I then
started mitmproxy on my desktop, and configured the iPhone to use it as a
proxy. 


## Taking a look at the Game Center traffic

Lets take a first look at the Game Center traffic. The game I'll use in this
tutorial is [Super Mega
Worm](http://itunes.apple.com/us/app/super-mega-worm/id388541990?mt=8) - a
great little retro-apocalyptic sidescroller for the iPhone: 

<center>
    <img src="@!urlTo("tutorials/supermega.png")!@"/>
</center>

After finishing a game (take your time), watch the traffic flowing through
mitmproxy:

<center>
    <img src="@!urlTo("tutorials/one.png")!@"/>
</center>

We see a bunch of things we might expect - initialisation, the retrieval of
leaderboards and so forth. Then, right at the end, there's a POST to this
tantalising URL:

<pre>
https://service.gc.apple.com/WebObjects/GKGameStatsService.woa/wa/submitScore
</pre>

The contents of the submission are particularly interesting:

<!--(block|syntax("xml"))-->
<plist version="1.0">
<dict>
    <key>category</key>
    <string>SMW_Adv_USA1</string>
    <key>score-value</key>
    <integer>55</integer>
    <key>timestamp</key>
    <integer>1301553284461</integer>
</dict>
</plist>
<!--(end)-->

This is a [property list](http://en.wikipedia.org/wiki/Property_list),
containing an identifier for the game, a score (55, in this case), and a
timestamp. Looks pretty simple to mess with.


## Modifying and replaying the score submission

Lets edit the score submission. First, select it in mitmproxy, then press
__enter__ to view it. Make sure you're viewing the request, not the response -
you can use __tab__ to flick between the two. Now press __e__ for edit. You'll
be prompted for the part of the request you want to change - press __b__ for
body.  Your preferred editor (taken from the EDITOR environment variable) will
now fire up. Lets bump the score up to something a bit more ambitious:

<!--(block|syntax("xml"))-->
<plist version="1.0">
<dict>
    <key>category</key>
    <string>SMW_Adv_USA1</string>
    <key>score-value</key>
    <integer>2200272667</integer>
    <key>timestamp</key>
    <integer>1301553284461</integer>
</dict>
</plist>
<!--(end)-->

Save the file and exit your editor. 

The final step is to replay this modified request. Simply press __r__ for
replay.

## The glorious result and some intrigue

<center>
    <img src="@!urlTo("tutorials/leaderboard.png")!@"/>
</center>

And that's it - according to the records, I am the greatest Super Mega Worm
player of all time. 

Curiously, the top competitors' scores are all the same: 2,147,483,647. If you
think that number seems familiar, you're right: it's 2^31-1, the maximum value
you can fit into a signed 32-bit int. Now let me tell you another peculiar
thing about Super Mega Worm - at the end of every game, it submits your highest
previous score to the Game Center, not your current score.  This means that it
stores your highscore somewhere, and I'm guessing that it reads that stored
score back into a signed integer. So, if you _were_ to cheat by the relatively
pedestrian means of modifying the saved score on your jailbroken phone, then
2^31-1 might well be the maximum score you could get. Then again, if the game
itself stores its score in a signed 32-bit int, you could get the same score
through perfect play, effectively beating the game. So, which is it in this
case? I'll leave that for you to decide.