I am trying to develop a server-side validation of my users' in-app purchases and subscriptions as recommended, and I want to use Firebase Functions for that. Basically it has to be an HTTP trigger function that receives a purchase token, calls the Play Developer API to verify the purchase, and then does something with the result.
However, calling many of the Google APIs (including Play Developer API) requires non-trivial authorization. Here's how I understand the required setup:
The problem is that absolutely no human-readable documentation or guidance on that is existent. Given that ingress traffic in Firebase is included in the free plan (so I assume they encourage using Google APIs from Firebase Functions), that fact is pretty disappointing. I've managed to find some bits of info here and there, but having too little experience with Google APIs (most of which required simply using an api key), I need help with putting it together.
Here's what I figured out so far:
I'm pretty surprised this hasn't been asked or documented, because verifying in-app purchases from Firebase Functions seems like a common task. Has anyone successfully done it before, or maybe the Firebase team will step in to answer?
I figured it out myself. I also ditched the heavyweight client library and just coded those few requests manually.
Notes:
scope
field of the JWT.Authentication: Bearer
, without a round trip to OAuth backend.After you've got the JSON file with the private key for a Service Account that's linked to Play Store, the code to call the API is like this (adjust to your needs). Note: I used request-promise
as a nicer way to do http.request
.
const functions = require('firebase-functions');
const jwt = require('jsonwebtoken');
const keyData = require('./key.json'); // Path to your JSON key file
const request = require('request-promise');
/**
* Exchanges the private key file for a temporary access token,
* which is valid for 1 hour and can be reused for multiple requests
*/
function getAccessToken(keyData) {
// Create a JSON Web Token for the Service Account linked to Play Store
const token = jwt.sign(
{ scope: 'https://www.googleapis.com/auth/androidpublisher' },
keyData.private_key,
{
algorithm: 'RS256',
expiresIn: '1h',
issuer: keyData.client_email,
subject: keyData.client_email,
audience: 'https://www.googleapis.com/oauth2/v4/token'
}
);
// Make a request to Google APIs OAuth backend to exchange it for an access token
// Returns a promise
return request.post({
uri: 'https://www.googleapis.com/oauth2/v4/token',
form: {
'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer',
'assertion': token
},
transform: body => JSON.parse(body).access_token
});
}
/**
* Makes a GET request to given URL with the access token
*/
function makeApiRequest(url, accessToken) {
return request.get({
url: url,
auth: {
bearer: accessToken
},
transform: body => JSON.parse(body)
});
}
// Our test function
exports.testApi = functions.https.onRequest((req, res) => {
// TODO: process the request, extract parameters, authenticate the user etc
// The API url to call - edit this
const url = `https://www.googleapis.com/androidpublisher/v2/applications/${packageName}/purchases/subscriptions/${subscriptionId}/tokens/${token}`;
getAccessToken(keyData)
.then(token => {
return makeApiRequest(url, token);
})
.then(response => {
// TODO: process the response, e.g. validate the purchase, set access claims to the user etc.
res.send(response);
return;
})
.catch(err => {
res.status(500).send(err);
});
});
These are the docs I followed.