To add an API to an existing Google Sign-In integration the best option is to implement incremental authorization. For this, you need to use both google-auth-library and googleapis, so that users can have this workflow:
- Authenticate with Google Sign-In.
- Authorize your application to use their information to integrate it with a Google API. For instance, Google Calendar.
For this, your client-side JavaScript for authentication might require some changes to request
offline access:
$('#signinButton').click(function() {
auth2.grantOfflineAccess().then(signInCallback);
});
In the response, you will have a JSON object with an authorization code:
{"code":"4/yU4cQZTMnnMtetyFcIWNItG32eKxxxgXXX-Z4yyJJJo.4qHskT-UtugceFc0ZRONyF4z7U4UmAI"}
After this, you can use the one-time code to exchange it for an access token and refresh token.
Here are some workflow details:
The code is your one-time code that your server can exchange for its own access token and refresh token. You can only obtain a refresh token after the user has been presented an authorization dialog requesting offline access. If you've specified the select-account prompt in the OfflineAccessOptions [...], you must store the refresh token that you retrieve for later use because subsequent exchanges will return null for the refresh token
Therefore, you should use google-auth-library to complete this workflow in the back-end. For this,
you'll use the authentication code to get a refresh token. However, as this is an offline workflow,
you also need to verify the integrity of the provided code as the documentation explains:
If you use Google Sign-In with an app or site that communicates with a backend server, you might need to identify the currently signed-in user on the server. To do so securely, after a user successfully signs in, send the user's ID token to your server using HTTPS. Then, on the server, verify the integrity of the ID token and use the user information contained in the token
The final function to get the refresh token that you should persist in your database might look like
this:
const { OAuth2Client } = require('google-auth-library');
/**
* Create a new OAuth2Client, and go through the OAuth2 content
* workflow. Return the refresh token.
*/
function getRefreshToken(code, scope) {
return new Promise((resolve, reject) => {
// Create an oAuth client to authorize the API call. Secrets should be
// downloaded from the Google Developers Console.
const oAuth2Client = new OAuth2Client(
YOUR_CLIENT_ID,
YOUR_CLIENT_SECRET,
YOUR_REDIRECT_URL
);
// Generate the url that will be used for the consent dialog.
await oAuth2Client.generateAuthUrl({
access_type: 'offline',
scope,
});
// Verify the integrity of the idToken through the authentication
// code and use the user information contained in the token
const { tokens } = await client.getToken(code);
const ticket = await client.verifyIdToken({
idToken: tokens.id_token!,
audience: keys.web.client_secret,
});
idInfo = ticket.getPayload();
return tokens.refresh_token;
})
}
At this point, we've refactored the authentication workflow to support Google APIs. However, you haven't asked the user to authorize it yet. Since you also need to grant offline access, you should request additional permissions through your client-side application. Keep in mind that you already need an active session.
const googleOauth = gapi.auth2.getAuthInstance();
const newScope = "https://www.googleapis.com/auth/calendar"
googleOauth = auth2.currentUser.get();
googleOauth.grantOfflineAccess({ scope: newScope }).then(
function(success){
console.log(JSON.stringify({ message: "success", value: success }));
},
function(fail){
alert(JSON.stringify({message: "fail", value: fail}));
});
You're done with the front-end changes and you're only missing one step. To create a Google API's client in the back-end with the googleapis library, you need to use the refresh token from the previous step.
For a complete workflow with a Node.js back-end, you might find my gist helpful.
