Validate server-side verification (SSV) callbacks

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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 or rewarded interstitial ad. Rewarded 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 SSV callbacks by using the Tink Java Apps 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. You can also test your server with the testing tool in the AdMob UI.

Prerequisites

• Enable rewarded server-side verification on your ad unit.

Use RewardedAdsVerifier from the Tink Java Apps library

The Tink Java Apps GitHub repository includes a RewardedAdsVerifier helper class to reduce the code required to verify a rewarded SSV callback. Using this class enables you to verify a callback URL with the following code.

RewardedAdsVerifierverifier=newRewardedAdsVerifier.Builder().fetchVerifyingPublicKeysWith(RewardedAdsVerifier.KEYS_DOWNLOADER_INSTANCE_PROD).build();StringrewardUrl=...;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 SSV callbacks, you can read through the Manual verification of rewarded SSV section.

SSV callback parameters

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

Ad source identifiers

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 extra 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 passed to the custom_data query parameter of the SSV callback. If no custom data value is set, the custom_data query parameter value won't be present in the SSV callback.

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

firebase::gma::RewardedAd*rewarded_ad;rewarded_ad=newfirebase::gma::RewardedAd();firebase::gma::RewardedAd::ServerSideVerificationOptionsoptions;options.custom_data="SAMPLE_CUSTOM_DATA_STRING";rewarded_ad->SetServerSideVerificationOptions(options);

If you want to set the custom reward string, you must do so before showing the ad.

Manual verification of rewarded SSV

The steps performed by the RewardedAdsVerifier class to verify a rewarded 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 third-party library that supports ECDSA.

Fetch public keys

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

A list of public keys to be used to validate the rewarded 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-----\nMF...YTPcw==\n-----END PUBLIC KEY-----"base64:"MFkwEwYHKoZIzj0CAQYI...ltS4nzc9yjmhgVQOlmSS6unqvN9t8sqajRTPcw=="},{keyId:3901585526,pem:"-----BEGIN PUBLIC KEY-----\nMF...aDUsw==\n-----END PUBLIC KEY-----"base64:"MFYwEAYHKoZIzj0CAQYF...4akdWbWDCUrMMGIV27/3/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.

Stringurl=...;NetHttpTransporthttpTransport=newNetHttpTransport.Builder().build();HttpRequesthttpRequest=httpTransport.createRequestFactory().buildGetRequest(newGenericUrl(url));HttpResponsehttpResponse=httpRequest.execute();if(httpResponse.getStatusCode()!=HttpStatusCodes.STATUS_CODE_OK){thrownewIOException("Unexpected status code = "+httpResponse.getStatusCode());}Stringdata;InputStreamcontentStream=httpResponse.getContent();try{InputStreamReaderreader=newInputStreamReader(contentStream,UTF_8);data=readerToString(reader);}finally{contentStream.close();}

Note that public keys are regularly rotated. You will receive an email to inform you of an upcoming rotation. If you're caching public keys, you should update the keys upon receiving this email.

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.

privatestaticMap<Integer,ECPublicKey>parsePublicKeysJson(StringpublicKeysJson)throwsGeneralSecurityException{Map<Integer,ECPublicKey>publicKeys=newHashMap<>();try{JSONArraykeys=newJSONObject(publicKeysJson).getJSONArray("keys");for(inti=0;i < keys.length();i++){JSONObjectkey=keys.getJSONObject(i);publicKeys.put(key.getInt("keyId"),EllipticCurves.getEcPublicKey(Base64.decode(key.getString("base64"))));}}catch(JSONExceptione){thrownewGeneralSecurityException("failed to extract trusted signing public keys",e);}if(publicKeys.isEmpty()){thrownewGeneralSecurityException("No trusted keys are available.");}returnpublicKeys;}

Get content to be verified

The last two query parameters of rewarded 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 SSV callback with the content to be verified highlighted.

https://www.myserver.com/path?ad_network=54...55&ad_unit=12345678&reward_amount=10&reward_item=coins &timestamp=150777823&transaction_id=12...DEF&user_id=1234567&signature=ME...Z1c&key_id=1268887

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

publicstaticfinalStringSIGNATURE_PARAM_NAME="signature=";...URIuri;try{uri=newURI(rewardUrl);}catch(URISyntaxExceptionex){thrownewGeneralSecurityException(ex);}StringqueryString=uri.getQuery();inti=queryString.indexOf(SIGNATURE_PARAM_NAME);if(i==-1){thrownewGeneralSecurityException("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:

publicstaticfinalStringKEY_ID_PARAM_NAME="key_id=";...StringsigAndKeyId=queryString.substring(i);i=sigAndKeyId.indexOf(KEY_ID_PARAM_NAME);if(i==-1){thrownewGeneralSecurityException("needs a key_id query parameter");}Stringsig=sigAndKeyId.substring(SIGNATURE_PARAM_NAME.length(),i-1/* i - 1 instead of i because of & */);intkeyId=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.

privatevoidverify(finalbyte[]dataToVerify,intkeyId,finalbyte[]signature)throwsGeneralSecurityException{Map<Integer,ECPublicKey>publicKeys=parsePublicKeysJson();if(publicKeys.containsKey(keyId)){foundKeyId=true;ECPublicKeypublicKey=publicKeys.get(keyId);EcdsaVerifyJceverifier=newEcdsaVerifyJce(publicKey,HashType.SHA256,EcdsaEncoding.DER);verifier.verify(signature,dataToVerify);}else{thrownewGeneralSecurityException("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.