Saving game score locally on iOS device...is security needed?

Question

Do I need to store the score with a hash, ie to protect it from being edited/cheated by a user? From my newb knowledge, the user is unable to view and manipulate this locally stored data, as plist files for example. Can anyone elaborate?

[Edit] I'm storing scores locally and periodically uploading them to Game Center leaderboards. Even if the device is jailbroken I'd like to have security to prevent scores being manipulated by the user. What is a good approach?

Crestfallen

Why you not store your score in NSUserDefualt? – Cassondra

@RRB My app will support multiple users. – Crestfallen

Answer 1

I actually would do this:

Save it in an NSDictionary and convert it to an NSData object. Now comes the cool part:

Encrypt it into AES using this class:
https://web.archive.org/web/20160806074714/http://iphonedevelopment.blogspot.com/2009/02/strong-encryption-for-cocoa-cocoa-touch.html

This is a military standard encryption which will return an encrypted NSData object. Then just save it to file and read/write whenever necessary. I've had really good experiences with this class and the user can't do anything to it.

Much better than just storing it in plain sight using NSUserDefaults.

Give it a go!

Hick

Answer 2

In theory, the user can't manipulate this stored data.

However, there are methods to access the whole filesystem, so people could open your game's folder, edit the plist and modify their scores.

Personally, if I was making a game without online leaderboards, I wouldn't worry about it. Let them have the score they want, as long as other players aren't affected by it.

Myrmidon

The scores are stored locally, but will be uploaded to Game Center periodically. Do you have suggestions on how to encode the score when storing it to a file for the jailbreak cases? – Crestfallen

The device doesn't need to be jailbroken to do this, just use an app like iExplorer (macroplant.com/iexplorer) that lets you access the file system. – Gunsmith

Answer 3

I'm also new to iPhone programming - just built my first small game and am currently adding Game Center feature to the next version (called Smiley Lines ;) I believe many would be in the same situation as me - just want some simple hashing so jailbroken folks won't easily modify the score in user defaults, hence preventing messing up the online leaderboard.

If so, I find the NSData conversion to AES solution quite complicated (for an iOS newbie and for a simple need of hashing an integer) and bit overkill. After some search, a great solution is found here: Matthias Plapper has subclassed the NSUserDefaults class to provide some simple hashing with a secret key. Super easy to use - copy paste the two sources files and here's an example usage:

// Configuring user defaults. It is recommended that you do this
// immediately after the app did launch.
    [NSUserDefaults setSecret:@"shh, this is secret!"];

// Write secure user defaults
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    [defaults setSecureBool:YES forKey:@"IsRegistered"];

    // Read secure user defaults
    BOOL valid = NO;
    BOOL registered = [defaults secureBoolForKey:@"IsRegistered" valid:&valid];
    if (!valid) {
        // the property has been modified, handle this situation
    } else {
        // Valid property, do whatever you need to do
    }

Thanks a ton Matthias!

Seaton

Answer 4

You can encode the scores with your own salt value and decode when required. This will prevent the user from simply changing the number that they see.

Treatise

Answer 5

I have developed many iOS games, and I just save the local scores by either:

• Using the NSUserDefaults class, or
• Writing them directly to a local config file (e.g., .txt, .plist, etc.).

Technically, other apps can't touch the files inside your app's bundle, so you don't have to worry about them being edited.

Voiceless