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Validating Server-Side Verification (SSV) Callbacks

Server-side verification callbacks are URL requests, with query parameters expanded by Google, that are sent by Google to an external system to notify it that a user should be rewarded for interacting with a rewarded video ad. Rewarded video SSV (server-side verification) callbacks provide an extra layer of protection against spoofing of client-side callbacks to reward users.

This guide shows you how to verify rewarded video SSV callbacks by using the Tink third-party cryptographic library to ensure that the query parameters in the callback are legitimate values. Although Tink is used for the purposes of this guide, you have the option to use any third-party library that supports ECDSA.

Prerequisites

Use RewardedAdsVerifier from Tink

The Tink GitHub repository includes a RewardedAdsVerifier helper class to reduce the code required to verify a rewarded video SSV callback. Using this class alongside the Tink third-party cryptographic library enables you to verify a callback URL with the following code.

RewardedAdsVerifier verifier = new RewardedAdsVerifier.Builder()
    .fetchVerifyingPublicKeysWith(
        RewardedAdsVerifier.KEYS_DOWNLOADER_INSTANCE_PROD)
    .build();
String rewardUrl = ...;
verifier.verify(rewardUrl);

If the verify() method executes without raising an exception, the callback URL was successfully verified. The Rewarding the user section details best practices regarding when users should be rewarded. For a breakdown of the steps performed by this class to verify rewarded video SSV callbacks, you can read through the Manual verification of rewarded video SSV section.

SSV callback parameters

Server-side verification callbacks contain query parameters that describe the rewarded video ad interaction. Parameter names, descriptions, and example values are listed below. Parameters are sent in alphabetical order.

Parameter Name Description Example value
ad_network Ad network identifier for the network that fulfilled this ad. Network names corresponding to id values are listed in the Ad network identifiers section. 1953547073528090325
ad_unit AdMob ad unit id that was used to request the rewarded video ad. 2747237135
custom_data Custom data string as provided by customRewardString.

If no custom data string is provided by the app, this query parameter value will not be present in the SSV callback.

SAMPLE_CUSTOM_DATA_STRING
key_id Key to be used to verify SSV callback. This value maps to a public key provided by the AdMob key server. 1234567890
reward_amount Reward amount as specified in the ad unit settings. 5
reward_item Reward item as specified in the ad unit settings. coins
signature Signature for SSV callback generated by AdMob. MEUCIQCLJS_s4ia_sN06HqzeW7Wc3nhZi4RlW3qV0oO-6AIYdQIgGJEh-rzKreO-paNDbSCzWGMtmgJHYYW9k2_icM9LFMY
timestamp Timestamp of when the user was rewarded as Epoch time in ms. 1507770365237823
transaction_id Unique hex encoded identifier for each reward grant event generated by AdMob. 18fa792de1bca816048293fc71035638
user_id User identifier as provided by userIdentifier.

If no user identifier is provided by the app, this query parameter will not be present in the SSV callback.

1234567

Ad network identifiers

The table below lists the corresponding ad network name for ad network identifier values in SSV callbacks.

Ad network name Ad network id
AdColony 15586990674969969776
AdMob 5450213213286189855
Applovin 1063618907739174004
Chartboost 2873236629771172317
Facebook Audience Network 10568273599589928883
Fuse 8914788932458531264
Fyber 4839637394546996422
InMobi 7681903010231960328
maio 7505118203095108657
myTarget 8450873672465271579
Nend 9383070032774777750
Tapjoy 7295217276740746030
Unity Ads 4970775877303683148
Vungle 1953547073528090325

Rewarding the user

It is important to balance user experience and reward validation when deciding when to reward a user. Server-side callbacks may experience delays before reaching external systems. Therefore, the recommended best practice is to use the client-side callback to reward the user immediately, while performing validation on all rewards upon the receipt of server-side callbacks. This approach provides a good user experience while ensuring the validity of granted rewards.

However, for applications where reward validity is critical (for example, the reward affects your app's in-game economy) and delays in granting rewards are acceptable, waiting for the verified server-side callback may be the best approach.

Custom data

Apps that require additional data in server-side verification callbacks should use the custom data feature of rewarded ads. Any string value set on a rewarded ad object is forwarded within the custom_data query parameter of the SSV callback. If no custom data value is set, the custom_data query parameter value will not be present in the SSV callback.

The code snippet below demonstrates how to set custom data on a rewarded ad object before requesting an ad.

GADRewardedAd *rewardedAd = [[GADRewardedAd alloc]
      initWithAdUnitID:@"ca-app-pub-3940256099942544/5224354917"];

GADServerSideVerificationOptions *options = [[GADServerSideVerificationOptions alloc] init];
options.customRewardString = @"SAMPLE_CUSTOM_DATA_STRING";
rewardedAd.serverSideVerificationOptions = options;

Manual verification of rewarded video SSV

The steps performed by the RewardedAdsVerifier class to verify a rewarded video SSV are outlined below. Although the included code snippets are in Java and leverage the Tink third-party library, these steps can be implemented by you in the language of your choice, using any any third-party library that supports ECDSA.

Fetch public keys

To verify a rewarded video SSV callback, you need a public key provided by AdMob.

A list of public keys to be used to validate the rewarded video SSV callbacks can be fetched from the AdMob key server. The list of public keys is provided as a JSON representation with a format similar to the following:

{
 "keys": [
    {
      keyId: 1916455855,
      pem: "-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEUaWMKcBHWdhUE+DncSIHhFCLLEln\nUs0LB9oanZ4K/FNICIM8ltS4nzc9yjmhgVQOlmSS6unqvN9t8sqajRTPcw==\n-----END PUBLIC KEY-----"
      base64: "MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEUaWMKcBHWdhUE+DncSIHhFCLLElnUs0LB9oanZ4K/FNICIM8ltS4nzc9yjmhgVQOlmSS6unqvN9t8sqajRTPcw=="
    },
    {
      keyId: 3901585526,
      pem: "-----BEGIN PUBLIC KEY-----\nMFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEtxg2BsK/fllIeADtLspezS6YfHFWXZ8tiJncm8LDBa/NxEC84akdWbWDCUrMMGIV27/3/e7UuKSEonjGvaDUsw==\n-----END PUBLIC KEY-----"
      base64: "MFYwEAYHKoZIzj0CAQYFK4EEAAoDQgAEtxg2BsK/fllIeADtLspezS6YfHFWXZ8tiJncm8LDBa/NxEC84akdWbWDCUrMMGIV27/3/e7UuKSEonjGvaDUsw=="
    },
  ],
}

To retrieve the public keys, connect to the AdMob key server and download the keys. The following code accomplishes this task and saves the JSON representation of the keys to the data variable.

String url = …;
NetHttpTransport httpTransport = new NetHttpTransport.Builder().build();
HttpRequest httpRequest =
    httpTransport.createRequestFactory().buildGetRequest(new GenericUrl(url));
HttpResponse httpResponse = httpRequest.execute();
if (httpResponse.getStatusCode() != HttpStatusCodes.STATUS_CODE_OK) {
  throw new IOException("Unexpected status code = " + httpResponse.getStatusCode());
}
String data;
InputStream contentStream = httpResponse.getContent();
try {
  InputStreamReader reader = new InputStreamReader(contentStream, UTF_8);
  data = readerToString(reader);
} finally {
  contentStream.close();
}

Once the public keys have been fetched, they must be parsed. The parsePublicKeysJson method below takes a JSON string, such as the example above, as input, and creates a mapping from key id values to public keys, which are encapsulated as ECPublicKey objects from the Tink library.

private static Map<Integer, ECPublicKey> parsePublicKeysJson(String publicKeysJson)
    throws GeneralSecurityException {
  Map<Integer, ECPublicKey> publicKeys = new HashMap<>();
  try {
    JSONArray keys = new JSONObject(publicKeysJson).getJSONArray("keys");
    for (int i = 0; i < keys.length(); i++) {
      JSONObject key = keys.getJSONObject(i);
      publicKeys.put(
          key.getInt("keyId"),
          EllipticCurves.getEcPublicKey(Base64.decode(key.getString("base64"))));
    }
  } catch (JSONException e) {
    throw new GeneralSecurityException("failed to extract trusted signing public keys", e);
  }
  if (publicKeys.isEmpty()) {
    throw new GeneralSecurityException("No trusted keys are available.");
  }
  return publicKeys;
}

Get content to be verified

The last two query parameters of rewarded video SSV callbacks are always signature and key_id, in that order. The remaining query parameters specify the content to be verified. Let's assume you configured AdMob to send reward callbacks to https://www.myserver.com/mypath. The snippet below shows an example rewarded video SSV callback with the content to be verified highlighted.


https://www.myserver.com/path?ad_network=5450213213286189855&ad_unit=12345678&reward_amount=10&reward_item=coins×tamp=1507770365237823&transaction_id=1234567890ABCDEF1234567890ABCDEF&user_id=1234567&signature=MEUCIQDGx44BZgQU6TU4iYEo1nyzh3NgDEvqNAUXlax-XPBQ5AIgCXSdjgKZvs_6QNYad29NJRqwGIhGb7GfuI914MDDZ1c&key_id=1268222887

The code below demonstrates how to parse the content to be verified from a callback URL as a UTF-8 byte array.

public static final String SIGNATURE_PARAM_NAME = "signature=";
...
URI uri;
try {
  uri = new URI(rewardUrl);
} catch (URISyntaxException ex) {
  throw new GeneralSecurityException(ex);
}
String queryString = uri.getQuery();
int i = queryString.indexOf(SIGNATURE_PARAM_NAME);
if (i == -1) {
  throw new GeneralSecurityException("needs a signature query parameter");
}
byte[] queryParamContentData =
    queryString
        .substring(0, i - 1)
        // i - 1 instead of i because of & in the query string
        .getBytes(Charset.forName("UTF-8"));

Get signature and key_id from callback URL

Using the queryString value from the previous step, parse the signature and key_id query parameters from the callback URL as shown below:

public static final String KEY_ID_PARAM_NAME = "key_id=";
...
String sigAndKeyId = queryString.substring(i);
i = sigAndKeyId.indexOf(KEY_ID_PARAM_NAME);
if (i == -1) {
  throw new GeneralSecurityException("needs a key_id query parameter");
}
String sig =
    sigAndKeyId.substring(
        SIGNATURE_PARAM_NAME.length(), i - 1 /* i - 1 instead of i because of & */);
int keyId = Integer.valueOf(sigAndKeyId.substring(i + KEY_ID_PARAM_NAME.length()));

Perform verification

The final step is to verify the content of the callback URL with the appropriate public key. Take the mapping returned from the parsePublicKeysJson method and use the key_id parameter from the callback URL to get the public key from that mapping. Then verify the signature with that public key. These steps are demonstrated below in the verify method.

private void verify(final byte[] dataToVerify, int keyId, final byte[] signature)
    throws GeneralSecurityException {
  Map<Integer, ECPublicKey> publicKeys = parsePublicKeysJson();
  if (publicKeys.containsKey(keyId)) {
    foundKeyId = true;
    ECPublicKey publicKey = publicKeys.get(keyId);
    EcdsaVerifyJce verifier = new EcdsaVerifyJce(publicKey, "SHA256WithECDSA");
    verifier.verify(signature, dataToVerify);
  } else {
    throw new GeneralSecurityException("cannot find verifying key with key id: " + keyId);
  }
}

If the method executes without throwing an exception, the callback URL was successfully verified.

FAQ

Can I cache the public key provided by the AdMob key server?
We recommend that you cache the public key provided by the AdMob key server to reduce the number of operations required to validate SSV callbacks. However, note that public keys are regularly rotated and should not be cached for longer than 24 hours.
How frequently are the public keys provided by the AdMob key server rotated?
Public keys provided by the AdMob key server are rotated on a variable schedule. To ensure that verification of SSV callbacks continues to work as intended, public keys should not be cached for longer than 24 hours.
What happens if my server can't be reached?
Google expects an HTTP 200 OK success status response code for SSV callbacks. If your server cannot be reached or does not provide the expected response, Google will re-attempt to send SSV callbacks up to five times in one-second intervals.
How can I verify that SSV callbacks are coming from Google?
Use reverse DNS lookup to verify that SSV callbacks originate from Google.