Implementing server-to-server redeem callbacks

Use Server-to-server (S2S) callbacks to detect and prevent cheating when rewarding players.

How it works

When a player views a video ad in its entirety, the Unity Ads server sends a signed callback to the URL you've specified. This occurs before the actual end of the video, so the game can complete the reward cycle before the player returns to gameplay.

Tip: Depending on the traffic, callbacks can take some time to arrive. To ensure a smooth gameplay experience, reward players immediately, then use S2S callbacks for sanity checks against cheating. Show the reward notification after the ad view finishes, to avoid distracting the player until the video ends.


To use S2S callbacks, you need to set a Server ID (sid) before showing an ad. S2S redeem callbacks are not available by default. If you want to enable them for your project, contact Support with your Game ID(s) and their respective callback URL(s) in the message. Unity will send you a secret hash used to sign and validate the callbacks.

Unity example

To implement a callback in your C# code, set the ShowOptions.gamerSid value to your Server ID, then pass the options object through the Show method.

using UnityEngine;
using System.Collections;
using UnityEngine.Advertisements;
public class UnityAdsManager : MonoBehaviour
    public string gameId;
    public string placement = "rewardedVideo"
    public void ShowAd() {
        ShowOptions options = new ShowOptions();
        // setting the server ID
        options.gamerSid = "your-side-id";
        Advertisement.Show(placementID, options);

Android example

To implement a callback in your Java code, set the PlayerMetaData.setServerId value to your Server ID.

PlayerMetaData playerMetaData = new PlayerMetaData(context);

iOS example

To implement a callback in your Objective-C code, set the playerMetaData.setServerId value to your Server ID.

id playerMetaData = [[UADSPlayerMetaData alloc] init];
        [playerMetaData setServerId:@"example"];
        [playerMetaData commit];
        [UnityAds show:self
          placementId: placementId
   	      showDelegate: showDelegate];

Callback information

Callback origin

The callback will come from the IP addresses/networks listed in here. The list will be updated on the first of each month. Publishers can safely ignore or block callbacks from anywhere else.

Callback URL format

The request is an HTTP/1.1 GET request to a URL of the following format:


Refer to the following table for information on query parameters:




The base URL of the callback URL, for example:

To configure this, contact Support


If ? does not exist in the URL yet, use ?. Otherwise use & is used.


The user ID or any custom data you want to send to your endpoint.


The unique Offer ID generated by Unity Ads servers.


A HDMAC-MD5 hash of a parameter string

For example: 106ed4300f91145aff6378a355fced73.

Example callback URL

Signing the Callback URL

The Callback URL request will have a signature attached to the URL parameters. The signature is a HDMAC-MD5 hash of a parameter string created by concatenating all the URL parameters in key-value form (except the HMAC) in alphabetical order, separated by commas.

For example, a callback URL with a SID and OID of

will have the following parameter string:


This, hashed with the secret key you'll receive from Support, returns a hash that the award callback will fire to a URL. For example:

Important: All the parameters included in the callback URL must be included in the signature calculation in alphabetical order. Otherwise the signatures will not match.

Callback response

If the request passes all the checks and user was awarded the item, the URL must reply with a HTTP/1.1 200 OK response and include character 1 in the body of the HTTP request. For example:

Example callback response

HTTP/1.1 200 OK
Date: Wed, 22 Feb 2012 23:59:59 GMT
Content-Length: 8

If there’s an error (for example, when the OID has already been used, or signature does not match, or any other error where the user does not end up with the promised item), the server should return an HTTP error in the 400- or the 500-range with a human readable error. For example:

Example callback error response

HTTP/1.1 400 ERROR
Date: Wed, 22 Feb 2012 23:59:59 GMT
Content-Length: 12
Duplicate order

Callback example in node.js

The following example shows how to verify the signature using node.js + express:

Example callback in node.js

// NODE.js S2S callback endpoint sample implementation
// Unity Ads
var express = require('express');
var crypto = require('crypto')
var app = express();
app.listen(process.env.PORT || 3412);
function getHMAC(parameters, secret) {
    var sortedParameterString = sortParams(parameters);
    return crypto.createHmac('md5', secret).update(sortedParameterString).digest('hex');
function sortParams(parameters) {
    var params = parameters || {};
    return Object.keys(params)
        .filter(key => key !== 'hmac')
        .map(key => params[key] === null ? `${key}=` : `${key}=${params[key]}`)
app.get('/', function (req, res) {
    var sid = req.query.sid;
    var oid = req.query.oid;
    var hmac = req.query.hmac;
    // Save the secret as an environment variable. If none is set, default to xyzKEY
    var secret = process.env.UNITYADSSECRET || 'xyzKEY';
    var newHmac = getHMAC(req.query, secret);
    if (hmac === newHmac) {
        // Signatures match
        // Check for duplicate oid here (player already received reward) and return 403 if it exists
        // If there's no duplicate - give virtual goods to player. Return 500 if it fails.
        // Save the oid for duplicate checking. Return 500 if it fails.
        // Callback passed, return 200 and include '1' in the message body
    } else {
        // no match

Callback example in PHP

The following example shows how to verify the signature in PHP:

Example callback in PHP

function generate_hash($params, $secret) {
   ksort($params); // All parameters are always checked in alphabetical order
   $s = '';
   foreach ($params as $key => $value) {
     $s .= "$key=$value,";
   $s = substr($s, 0, -1);
   $hash = hash_hmac('md5', $s, $secret);
   return $hash;
$hash = $_GET['hmac'];
$signature = generate_hash($_GET, 'xyzKEY'); // insert here the secret hash key you received from Unity Ads support
error_log("req hmac".$hash);
error_log("sig hmac".$signature);
// check signature
if($hash != $signature) { header('HTTP/1.1 403 Forbidden'); echo "Signature did not match"; exit; }
// check duplicate orders
if(check_duplicate_orders($_GET['oid']) { header('HTTP/1.1 403 Forbidden'); echo "Duplicate order"; exit; }
// if not then give the player the item and check that it succeeds.
if(!give_item_to_player($_GET['sid'], $_GET['product']) { header('HTTP/1.1 500 Internal Server Error'); echo "Failed to give item to the player"; exit; }
// save the order ID for duplicate checking
if(save_order_number($_GET['oid']) { header('HTTP/1.1 500 Internal Server Error'); echo "Order ID saving failed, user granted item"; exit; }
// everything OK, return "1"
header('HTTP/1.1 200 OK');
echo "1";