Game Center authentication with third party server.

GameViewController.m

#pragma mark - GameKit related stuff

// See documentation:
// https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/GameKit_Guide/Users/Users.html#//apple_ref/doc/uid/TP40008304-CH8-SW17

// Call this method in (void)viewDidAppear:(BOOL)animated
- (void) authenticateLocalPlayer {
    [GKLocalPlayer localPlayer].authenticateHandler = ^(UIViewController *viewController, NSError *error) {
        if (viewController != nil) {
            [self showAuthenticationDialog: viewController];
            return;
        }
        GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
        if (localPlayer.isAuthenticated) {
            [self authenticatedPlayer: localPlayer];
        } else {
            NSLog(@"Not authenticated!\n");
        }
    };
}

- (void) showAuthenticationDialog:(UIViewController*)viewController {
    [self presentViewController: viewController animated: YES completion:nil];
}

- (void) authenticatedPlayer:(GKLocalPlayer*)localPlayer {
    NSLog(@"Authenticated player: %@\n", localPlayer.playerID);
    
    // See documentation:
    // https://developer.apple.com/library/ios/documentation/GameKit/Reference/GKLocalPlayer_Ref/index.html#//apple_ref/occ/instm/GKLocalPlayer/generateIdentityVerificationSignatureWithCompletionHandler:
    [localPlayer generateIdentityVerificationSignatureWithCompletionHandler:^(NSURL *publicKeyUrl, NSData *signature, NSData *salt, uint64_t timestamp, NSError *error) {
        if (error) {
            NSLog(@"ERROR: %@", error);
            return;
        }

        // Data to be sent to your own server for authentication
        NSLog(@"public_key_url=%@\n", publicKeyUrl.absoluteString);
        NSLog(@"signature=%@\n", [signature base64EncodedStringWithOptions:0]);
        NSLog(@"salt=%@\n", [salt base64EncodedStringWithOptions:0]);
        NSLog(@"timestamp=%@\n", [NSString stringWithFormat:@"%llu", timestamp]);
        NSLog(@"player_id=%@\n", localPlayer.playerID);
        NSLog(@"bundle_id=%@\n", [[NSBundle mainBundle] bundleIdentifier]);
    }];
}