[[GKLocalPlayer localPlayer] authenticateWithCompletionHandler:^(NSError *error)
     {
         if (error != nil)
         {
             NSLog(@"LOGIN");
         } else {
             NSLog(@"CANT LOGIN");
         }
     }];

- (void) authenticateLocalPlayer
{
    GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
    localPlayer.authenticateHandler = ^(UIViewController *viewController, NSError *error){
         if (viewController != nil)
         {
             [self showAuthenticationDialogWhenReasonable: viewController
         }
         else if (localPlayer.isAuthenticated)
         {
             [self authenticatedPlayer: localPlayer];
         }
         else
         {
             [self disableGameCenter];
         }
     }];
}