Auto Log out from an iPhone app (when app goes in the background)

The Settings bundle of my iPhone has the user login and password credentials saved.(similar to the Mail app). The application in its current state does not have a separate login/password view page.

Now, when the app enters the background, I would want it to wait for 40 seconds and after that automatically log off. I found similar posts, and thus found that I could use the methods in the app delegate:

- (void) applicationDidEnterBackground:(UIApplication *)application
{
    currentTimeBackground = CACurrentMediaTime();  
} 

- (void) applicationDidBecomeActive:(UIApplication *)application
{
    currentTimeActive = CACurrentMediaTime();
}

Then I plan to use the bottom logic somewhere in my code to trigger a lo开发者_开发问答gout.

 if ((currentTimeActive - currentTimeBackground)> 40 ) {
           NSLog(@"Need to prompt for re login!");
           // Logic to logout the application.
 }

Now, I found that it is not possible to alter the Settings bundle (.plist) file through a program (at run time). (http://stackoverflow.com/questions/4921890/how-can-i-modify-a-settings-bundle-in-real-time)

So please suggest me how would I log out of this application? This application fetches data from a server (I have a local server set up for development).

Create a background task and have it wait 40 seconds and then log off. If the user re-opens the app, cancel the background task.