AuthiOSGameCenter.js

function AuthiOSGameCenter(data, callback)
{
	if(data.bundle_id !== 'com.example.game')
	{
		callback(false);
		return;
	}
	
	function handleResponse(key)
	{
		var crypto = require('crypto'),
			ref = require('ref');
		
		var valid = false;
		var user = null;
		
		if(null !== key)
		{
			var verifier = crypto.createVerify('sha256');
			verifier.update(data.player_id, 'utf8');
			verifier.update(data.bundle_id, 'utf8');
			
			var buf = ref.alloc('uint64');
			ref.writeUInt64BE(buf, 0, data.timestamp.toString());
			
			verifier.update(buf);
			verifier.update(data.salt, 'base64');
			
			var pmd = '-----BEGIN CERTIFICATE-----';

			var base64 = key.toString('base64');
			var size = base64.length;
			
			for (var i = 0; i < size; i = i + 64) {
				var end = i + 64 < size ? i + 64 : size;
				pmd = pmd + '\n' + base64.substring(i, end);
			}
			
			pmd = pmd + '\n-----END CERTIFICATE-----';
			
			valid = verifier.verify(pmd, data.signature, 'base64');
			user = valid ? data.player_id : false;
		}
		callback(valid, user);
	}

	var url = data.public_key_url;
	var http = 0 === url.indexOf('https') ?
			require('https') :
			require('http');
		
	http.get(
		url,
		function(resp)
		{
			if(resp.statusCode !== 200)
			{
				callback();
				return;
			}
			resp.on('data', handleResponse);
		}
	);
}

AuthiOSGameCenter(
	{
		public_key_url: 'https://sandbox.gc.apple.com/public-key/gc-sb-2.cer',
		timestamp: 1234567890,
		signature: 'nahtin==',
		salt: 'nahtin==',
		player_id: 'G:1234567890',
		bundle_id: 'com.example.game'
	},
	console.log
);

AuthiOSGameCenter.mm

-(void)authenticateLocalPlayer
{
    __weak GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
    
    localPlayer.authenticateHandler = ^(UIViewController *viewController, NSError *error)
    {
        if (viewController != nil)
        {
            [ [ [ [ [UIApplication sharedApplication] delegate] window] rootViewController]
             presentViewController:viewController animated:YES completion:nil];
        }
        else if (localPlayer.authenticated)
        {
            NSLog(@"%@", localPlayer);
            [localPlayer generateIdentityVerificationSignatureWithCompletionHandler:
             ^(NSURL *publicKeyUrl, NSData *signature, NSData *salt, uint64_t timestamp, NSError *error)
             {
                 NSDictionary *params = @{@"public_key_url": [publicKeyUrl absoluteString],
                                          @"timestamp": [NSString stringWithFormat:@"%llu", timestamp],
                                          @"signature": [signature base64EncodedStringWithOptions:0],
                                          @"salt": [salt base64EncodedStringWithOptions:0],
                                          @"player_id": [GKLocalPlayer localPlayer].playerID,
                                          @"bundle_id": [[NSBundle mainBundle] bundleIdentifier]};
                 
                 NSLog(@"%@", params);
             }
            ];
        }
    };
}