Google Account Linking with OAuth (Implicit Flow - Archived)

To support the OAuth 2.0 implicit flow, your service makes an authorization endpoint available by HTTPS. This endpoint is responsible for authentication and obtaining consent from users for data access. The authorization endpoint presents a sign-in UI to your users that aren't already signed in and records consent to the requested access.

When a Google application needs to call one of your service's authorized APIs, Google uses this endpoint to get permission from your users to call these APIs on their behalf.

Google Account Linking: OAuth Implicit Flow

The following sequence diagram details interactions between the User, Google, and your service's endpoints.

Figure 1. The sequence of events in the OAuth 2.0 Implicit flow for Google Account Linking.

Roles and responsibilities

The following table defines the roles and responsibilities of actors in the Google Account Linking (GAL) OAuth Implicit flow. Note that in GAL, Google acts as the OAuth Client, while your service acts as the Identity/Service Provider.

Actor / Component	GAL Role	Responsibilities
Google App / Server	OAuth Client	Initiates the flow, receives the access token using a browser redirect, and securely stores it to access your service's APIs.
Your Authorization Endpoint	Authorization Server	Authenticates your users, obtains their consent, and issues long-lived access tokens directly to Google.
Google Redirect URI	Callback Endpoint	Receives the user redirect from your authorization service with the `access_token` and `state` values in the URL fragment.

A typical OAuth 2.0 implicit flow session initiated by Google has the following flow:

Google opens your authorization endpoint in the user's browser. The user signs in, if not signed in already, and grants Google permission to access their data with your API, if they haven't already granted permission.
Your service creates an access token and returns it to Google. To do so, redirect the user's browser back to Google with the access token attached to the request.
Google calls your service's APIs and attaches the access token with each request. Your service verifies that the access token grants Google authorization to access the API and then completes the API call.

Implementation Recipe

Follow these steps to implement the Implicit flow.

Step 1: Handle authorization requests

When Google initiates account linking, it redirects the user to your authorization endpoint. For detailed protocol contracts and parameter requirements, see the Authorization Endpoint.

To handle the request, perform the following actions:

Validate the request:
- Confirm that the client_id matches the Client ID assigned to Google.
- Confirm that the redirect_uri matches the expected Google redirect URL: none https://oauth-redirect.googleusercontent.com/r/YOUR_PROJECT_ID https://oauth-redirect-sandbox.googleusercontent.com/r/YOUR_PROJECT_ID
- Verify that response_type is token.
Authenticate the user:
- Check if the user is signed in to your service.
- If the user is not signed in, prompt them to complete your sign-in or sign-up flow.
Generate access token:
- Create a unique, non-guessable access token associated with the user and client.
Redirect back to Google:
- Redirect the browser to the URL provided in redirect_uri.
- Append the following parameters in the URL fragment (hash):
  - access_token: The access token you generated.
  - token_type: Must be bearer.
  - state: The unmodified state value received from Google.

Handle userinfo requests

The userinfo endpoint is an OAuth 2.0 protected resource that return claims about the linked user. Implementing and hosting the userinfo endpoint is optional, except for the following use cases:

Linked Account Sign-In with Google One Tap.
Frictionless subscription on AndroidTV.

After the access token has been successfully retrieved from your token endpoint, Google sends a request to your userinfo endpoint to retrieve basic profile information about the linked user.

userinfo endpoint request headers
`Authorization header`	The access token of type Bearer.

For example, if your userinfo endpoint is available at https://myservice.example.com/userinfo, a request might look like the following:

GET /userinfo HTTP/1.1
Host: myservice.example.com
Authorization: Bearer ACCESS_TOKEN

For your userinfo endpoint to handle requests, do the following steps:

Extract access token from the Authorization header and return information for the user associated with the access token.
If the access token is invalid, return an HTTP 401 Unauthorized error with using the WWW-Authenticate Response Header. Below is an example of a userinfo error response:
```
HTTP/1.1 401 Unauthorized
WWW-Authenticate: error="invalid_token",
error_description="The Access Token expired"
```
If a 401 Unauthorized, or any other unsuccessful error response is returned during the linking process, the error will be non-recoverable, the retrieved token will be discarded and the user will have to initiate the linking process again.

If the access token is valid, return and HTTP 200 response with the following JSON object in the body of the HTTPS response:

{
"sub": "USER_UUID",
"email": "EMAIL_ADDRESS",
"given_name": "FIRST_NAME",
"family_name": "LAST_NAME",
"name": "FULL_NAME",
"picture": "PROFILE_PICTURE",
}

If your userinfo endpoint returns an HTTP 200 success response, the retrieved token and claims are registered against the user's Google account.

userinfo endpoint response
`sub`	A unique ID that identifies the user in your system.
`email`	Email address of the user.
`given_name`	Optional: First name of the user.
`family_name`	Optional: Last name of the user.
`name`	Optional: Full name of the user.
`picture`	Optional: Profile picture of the user.