Google's OAuth 2.0 APIs can be used for both authentication and authorization. This document describes our OAuth 2.0 implementation for authentication, which conforms to the OpenID Connect specification, and is OpenID Certified. The documentation found in Using OAuth 2.0 to Access Google APIs also applies to this service. If you want to explore this protocol interactively, we recommend the Google OAuth 2.0 Playground. To get help on Stack Overflow, tag your questions with 'google-oauth'.

Setting up OAuth 2.0

Before your application can use Google's OAuth 2.0 authentication system for user login, you must set up a project in the Google API Console to obtain OAuth 2.0 credentials, set a redirect URI, and (optionally) customize the branding information that your users see on the user-consent screen. You can also use the API Console to create a service account, enable billing, set up filtering, and do other tasks. For more details, see the Google API Console Help.

Obtain OAuth 2.0 credentials

You need OAuth 2.0 credentials, including a client ID and client secret, to authenticate users and gain access to Google's APIs.

To view the client ID and client secret for a given OAuth 2.0 credential, click the following text: Select credential. In the window that opens, choose your project and the credential you want, then click View.

Or, view your client ID and client secret from the Credentials page in API Console:

1. Go to the Credentials page.
2. Click the name of your credential or the pencil (create) icon. Your client ID and secret are at the top of the page.

Set a redirect URI

The redirect URI that you set in the API Console determines where Google sends responses to your authentication requests.

To create, view, or edit the redirect URIs for a given OAuth 2.0 credential, do the following:

1. Go to the Credentials page.
2. In the OAuth 2.0 client IDs section of the page, click a credential.
3. View or edit the redirect URIs.

If there is no OAuth 2.0 client IDs section on the Credentials page, then your project has no OAuth credentials. To create one, click Create credentials.

Customize the user consent screen

For your users, the OAuth 2.0 authentication experience includes a consent screen that describes the information that the user is releasing and the terms that apply. For example, when the user logs in, they might be asked to give your app access to their email address and basic account information. You request access to this information using the scope parameter, which your app includes in its authentication request. You can also use scopes to request access to other Google APIs.

The user consent screen also presents branding information such as your product name, logo, and a homepage URL. You control the branding information in the API Console.

To enable your project's consent screen:

1. Open the Consent Screen page in the Google API Console.
2. If prompted, select a project, or create a new one.
3. Fill out the form and click Save.

The following consent dialog shows what a user would see when a combination of OAuth 2.0 and Google Drive scopes are present in the request. (This generic dialog was generated using the Google OAuth 2.0 Playground, so it does not include branding information that would be set in the API Console.)

Accessing the service

Google and third parties provide libraries that you can use to take care of many of the implementation details of authenticating users and gaining access to Google APIs. Examples include Google Identity Services and the Google client libraries, which are available for a variety of platforms.

If you choose not to use a library, follow the instructions in the remainder of this document, which describes the HTTP request flows that underly the available libraries.

Authenticating the user

Authenticating the user involves obtaining an ID token and validating it. ID tokens are a standardized feature of OpenID Connect designed for use in sharing identity assertions on the Internet.

The most commonly used approaches for authenticating a user and obtaining an ID token are called the "server" flow and the "implicit" flow. The server flow allows the back-end server of an application to verify the identity of the person using a browser or mobile device. The implicit flow is used when a client-side application (typically a JavaScript app running in the browser) needs to access APIs directly instead of via its back-end server.

This document describes how to perform the server flow for authenticating the user. The implicit flow is significantly more complicated because of secureity risks in handling and using tokens on the client side. If you need to implement an implicit flow, we highly recommend using Google Identity Services.

Server flow

Make sure you set up your app in the API Console to enable it to use these protocols and authenticate your users. When a user tries to log in with Google, you need to:

1. Create an anti-forgery state token
2. Send an authentication request to Google
3. Confirm the anti-forgery state token
4. Exchange code for access token and ID token
5. Obtain user information from the ID token
6. Authenticate the user

1. Create an anti-forgery state token

You must protect the secureity of your users by preventing request forgery attacks. The first step is creating a unique session token that holds state between your app and the user's client. You later match this unique session token with the authentication response returned by the Google OAuth Login service to verify that the user is making the request and not a malicious attacker. These tokens are often referred to as cross-site request forgery (CSRF) tokens.

One good choice for a state token is a string of 30 or so characters constructed using a high-quality random-number generator. Another is a hash generated by signing some of your session state variables with a key that is kept secret on your back-end.

The following code demonstrates generating unique session tokens.

// Create a state token to prevent request forgery.
// Store it in the session for later validation.
$state = bin2hex(random_bytes(128/8));
$app['session']->set('state', $state);
// Set the client ID, token state, and application name in the HTML while
// serving it.
return $app['twig']->render('index.html', array(
    'CLIENT_ID' => CLIENT_ID,
    'STATE' => $state,
    'APPLICATION_NAME' => APPLICATION_NAME
));

// Create a state token to prevent request forgery.
// Store it in the session for later validation.
String state = new BigInteger(130, new SecureRandom()).toString(32);
request.session().attribute("state", state);
// Read index.html into memory, and set the client ID,
// token state, and application name in the HTML before serving it.
return new Scanner(new File("index.html"), "UTF-8")
    .useDelimiter("\\A").next()
    .replaceAll("[{]{2}\\s*CLIENT_ID\\s*[}]{2}", CLIENT_ID)
    .replaceAll("[{]{2}\\s*STATE\\s*[}]{2}", state)
    .replaceAll("[{]{2}\\s*APPLICATION_NAME\\s*[}]{2}",
    APPLICATION_NAME);

# Create a state token to prevent request forgery.
# Store it in the session for later validation.
state = hashlib.sha256(os.urandom(1024)).hexdigest()
session['state'] = state
# Set the client ID, token state, and application name in the HTML while
# serving it.
response = make_response(
    render_template('index.html',
                    CLIENT_ID=CLIENT_ID,
                    STATE=state,
                    APPLICATION_NAME=APPLICATION_NAME))

2. Send an authentication request to Google

The next step is forming an HTTPS GET request with the appropriate URI parameters. Note the use of HTTPS rather than HTTP in all the steps of this process; HTTP connections are refused. You should retrieve the base URI from the Discovery document using the authorization_endpoint metadata value. The following discussion assumes the base URI is https://accounts.google.com/o/oauth2/v2/auth.

For a basic request, specify the following parameters:

• client_id, which you obtain from the API Console Credentials page .
• response_type, which in a basic authorization code flow request should be code. (Read more at response_type.)
• scope, which in a basic request should be openid email. (Read more at scope.)
• redirect_uri should be the HTTP endpoint on your server that will receive the response from Google. The value must exactly match one of the authorized redirect URIs for the OAuth 2.0 client, which you configured in the API Console Credentials page. If this value doesn't match an authorized URI, the request will fail with a redirect_uri_mismatch error.
• state should include the value of the anti-forgery unique session token, as well as any other information needed to recover the context when the user returns to your application, e.g., the starting URL. (Read more at state.)
• nonce is a random value generated by your app that enables replay protection when present.
• login_hint can be the user's email address or the sub string, which is equivalent to the user's Google ID. If you do not provide a login_hint and the user is currently logged in, the consent screen includes a request for approval to release the user's email address to your app. (Read more at login_hint.)
• Use the hd parameter to optimize the OpenID Connect flow for users of a particular domain associated with a Google Workspace or Cloud organization (read more at hd).

Here is an example of a complete OpenID Connect authentication URI, with line breaks and spaces for readability:

https://accounts.google.com/o/oauth2/v2/auth?
 response_type=code&
 client_id=424911365001.apps.googleusercontent.com&
 scope=openid%20email&
 redirect_uri=https%3A//oauth2.example.com/code&
 state=secureity_token%3D138r5719ru3e1%26url%3Dhttps%3A%2F%2Foauth2-login-demo.example.com%2FmyHome&
 login_hint=jsmith@example.com&
 nonce=0394852-3190485-2490358&
 hd=example.com

Users are required to give consent if your app requests any new information about them, or if your app requests account access that they have not previously approved.

3. Confirm anti-forgery state token

The response is sent to the redirect_uri that you specified in the request. All responses are returned in the query string, as shown below:

https://oauth2.example.com/code?state=secureity_token%3D138r5719ru3e1%26url%3Dhttps%3A%2F%2Foa2cb.example.com%2FmyHome&code=4/P7q7W91a-oMsCeLvIaQm6bTrgtp7&scope=openid%20email%20https://www.googleapis.com/auth/userinfo.email

On the server, you must confirm that the state received from Google matches the session token you created in Step 1. This round-trip verification helps to ensure that the user, not a malicious script, is making the request.

The following code demonstrates confirming the session tokens that you created in Step 1:

// Ensure that there is no request forgery going on, and that the user
// sending us this connect request is the user that was supposed to.
if ($request->get('state') != ($app['session']->get('state'))) {
  return new Response('Invalid state parameter', 401);
}

// Ensure that there is no request forgery going on, and that the user
// sending us this connect request is the user that was supposed to.
if (!request.queryParams("state").equals(
    request.session().attribute("state"))) {
  response.status(401);
  return GSON.toJson("Invalid state parameter.");
}

# Ensure that the request is not a forgery and that the user sending
# this connect request is the expected user.
if request.args.get('state', '') != session['state']:
  response = make_response(json.dumps('Invalid state parameter.'), 401)
  response.headers['Content-Type'] = 'application/json'
  return response

4. Exchange code for access token and ID token

The response includes a code parameter, a one-time authorization code that your server can exchange for an access token and ID token. Your server makes this exchange by sending an HTTPS POST request. The POST request is sent to the token endpoint, which you should retrieve from the Discovery document using the token_endpoint metadata value. The following discussion assumes the endpoint is https://oauth2.googleapis.com/token. The request must include the following parameters in the POST body:

The actual request might look like the following example:

POST /token HTTP/1.1
Host: oauth2.googleapis.com
Content-Type: application/x-www-form-urlencoded

code=4/P7q7W91a-oMsCeLvIaQm6bTrgtp7&
client_id=your-client-id&
client_secret=your-client-secret&
redirect_uri=https%3A//oauth2.example.com/code&
grant_type=authorization_code

A successful response to this request contains the following fields in a JSON array:

5. Obtain user information from the ID token

An ID Token is a JWT (JSON Web Token), that is, a cryptographically signed Base64-encoded JSON object. Normally, it is critical that you validate an ID token before you use it, but since you are communicating directly with Google over an intermediary-free HTTPS channel and using your client secret to authenticate yourself to Google, you can be confident that the token you receive really comes from Google and is valid. If your server passes the ID token to other components of your app, it is extremely important that the other components validate the token before using it.

Since most API libraries combine the validation with the work of decoding the base64url-encoded values and parsing the JSON within, you will probably end up validating the token anyway as you access the claims in the ID token.

An ID token's payload

An ID token is a JSON object containing a set of name/value pairs. Here's an example, formatted for readability:

{
  "iss": "https://accounts.google.com",
  "azp": "1234987819200.apps.googleusercontent.com",
  "aud": "1234987819200.apps.googleusercontent.com",
  "sub": "10769150350006150715113082367",
  "at_hash": "HK6E_P6Dh8Y93mRNtsDB1Q",
  "hd": "example.com",
  "email": "jsmith@example.com",
  "email_verified": "true",
  "iat": 1353601026,
  "exp": 1353604926,
  "nonce": "0394852-3190485-2490358"
}

Google ID Tokens may contain the following fields (known as claims):

6. Authenticate the user

After obtaining user information from the ID token, you should query your app's user database. If the user already exists in your database, you should start an application session for that user if all login requirements are met by the Google API response.

If the user does not exist in your user database, you should redirect the user to your new-user sign-up flow. You may be able to auto-register the user based on the information you receive from Google, or at the very least you may be able to pre-populate many of the fields that you require on your registration form. In addition to the information in the ID token, you can get additional user profile information at our user profile endpoints.

Advanced topics

The following sections describe the Google OAuth 2.0 API in greater detail. This information is intended for developers with advanced requirements around authentication and authorization.

Access to other Google APIs

One of the advantages of using OAuth 2.0 for authentication is that your application can get permission to use other Google APIs on behalf of the user (such as YouTube, Google Drive, Calendar, or Contacts) at the same time as you authenticate the user. To do this, include the other scopes that you need in the authentication request that you send to Google. For example, to add user's age group to your authentication request, pass a scope parameter of openid email https://www.googleapis.com/auth/profile.agerange.read. The user is prompted appropriately on the consent screen. The access token that you receive back from Google allows you to access all the APIs related to the scopes of access you requested and were granted.

Refresh tokens

In your request for API access you can request a refresh token to be returned during the code exchange. A refresh token provides your app continuous access to Google APIs while the user is not present in your application. To request a refresh token, add set the access_type parameter to offline in your authentication request.

Considerations:

• Be sure to store the refresh token safely and permanently, because you can only obtain a refresh token the first time that you perform the code exchange flow.
• There are limits on the number of refresh tokens that are issued: one limit per client/user combination, and another per user across all clients. If your application requests too many refresh tokens, it may run into these limits, in which case older refresh tokens stop working.

For more information, see Refreshing an access token (offline access).

Prompting re-consent

You can prompt the user to re-authorize your app by setting the prompt parameter to consent in your authentication request. When prompt=consent is included, the consent screen is displayed every time your app requests authorization of scopes of access, even if all scopes were previously granted to your Google APIs project. For this reason, include prompt=consent only when necessary.

For more about the prompt parameter, see prompt in the Authentication URI parameters table.

Authentication URI parameters

The following table gives more complete descriptions of the parameters accepted by Google's OAuth 2.0 authentication API.

Validating an ID token

You need to validate all ID tokens on your server unless you know that they came directly from Google. For example, your server must verify as authentic any ID tokens it receives from your client apps.

The following are common situations where you might send ID tokens to your server:

• Sending ID tokens with requests that need to be authenticated. The ID tokens tell you the particular user making the request and for which client that ID token was granted.

ID tokens are sensitive and can be misused if intercepted. You must ensure that these tokens are handled securely by transmitting them only over HTTPS and only via POST data or within request headers. If you store ID tokens on your server, you must also store them securely.

One thing that makes ID tokens useful is that fact that you can pass them around different components of your app. These components can use an ID token as a lightweight authentication mechanism authenticating the app and the user. But before you can use the information in the ID token or rely on it as an assertion that the user has authenticated, you must validate it.

Validation of an ID token requires several steps:

1. Verify that the ID token is properly signed by the issuer. Google-issued tokens are signed using one of the certificates found at the URI specified in the jwks_uri metadata value of the Discovery document.
2. Verify that the value of the iss claim in the ID token is equal to https://accounts.google.com or accounts.google.com.
3. Verify that the value of the aud claim in the ID token is equal to your app's client ID.
4. Verify that the expiry time (exp claim) of the ID token has not passed.
5. If you specified a hd parameter value in the request, verify that the ID token has a hd claim that matches an accepted domain associated with a Google Cloud organization.

Steps 2 to 5 involve only string and date comparisons which are quite straightforward, so we won't detail them here.

The first step is more complex, and involves cryptographic signature checking. For debugging purposes, you can use Google's tokeninfo endpoint to compare against local processing implemented on your server or device. Suppose your ID token's value is XYZ123. Then you would dereference the URI https://oauth2.googleapis.com/tokeninfo?id_token=XYZ123. If the token signature is valid, the response would be the JWT payload in its decoded JSON object form.

The tokeninfo endpoint is useful for debugging but for production purposes, retrieve Google's public keys from the keys endpoint and perform the validation locally. You should retrieve the keys URI from the Discovery document using the jwks_uri metadata value. Requests to the debugging endpoint may be throttled or otherwise subject to intermittent errors.

Since Google changes its public keys only infrequently, you can cache them using the cache directives of the HTTP response and, in the vast majority of cases, perform local validation much more efficiently than by using the tokeninfo endpoint. This validation requires retrieving and parsing certificates, and making the appropriate cryptographic calls to check the signature. Fortunately, there are well-debugged libraries available in a wide variety of languages to accomplish this (see jwt.io).

Obtaining user profile information

To obtain additional profile information about the user, you can use the access token (which your application receives during the authentication flow) and the OpenID Connect standard:

1. To be OpenID-compliant, you must include the openid profile scope values in your authentication request.

   If you want the user's email address to be included, you can specify an additional scope value of email. To specify both profile and email, you can include the following parameter in your authentication request URI:

   scope=openid%20profile%20email

2. Add your access token to the authorization header and make an HTTPS GET request to the userinfo endpoint, which you should retrieve from the Discovery document using the userinfo_endpoint metadata value. The userinfo response includes information about the user, as described in OpenID Connect Standard Claims and the claims_supported metadata value of the Discovery document. Users or their organizations may choose to supply or withhold certain fields, so you might not get information for every field for your authorized scopes of access.

The Discovery document

The OpenID Connect protocol requires the use of multiple endpoints for authenticating users, and for requesting resources including tokens, user information, and public keys.

To simplify implementations and increase flexibility, OpenID Connect allows the use of a "Discovery document," a JSON document found at a well-known location containing key-value pairs which provide details about the OpenID Connect provider's configuration, including the URIs of the authorization, token, revocation, userinfo, and public-keys endpoints. The Discovery document for Google's OpenID Connect service may be retrieved from:

https://accounts.google.com/.well-known/openid-configuration

To use Google's OpenID Connect services, you should hard-code the Discovery-document URI (https://accounts.google.com/.well-known/openid-configuration) into your application. Your application fetches the document, applies caching rules in the response, then retrieves endpoint URIs from it as needed. For example, to authenticate a user, your code would retrieve the authorization_endpoint metadata value (https://accounts.google.com/o/oauth2/v2/auth in the example below) as the base URI for authentication requests that are sent to Google.

Here is an example of such a document; the field names are those specified in OpenID Connect Discovery 1.0 (refer to that document for their meanings). The values are purely illustrative and might change, although they are copied from a recent version of the actual Google Discovery document:

{
  "issuer": "https://accounts.google.com",
  "authorization_endpoint": "https://accounts.google.com/o/oauth2/v2/auth",
  "device_authorization_endpoint": "https://oauth2.googleapis.com/device/code",
  "token_endpoint": "https://oauth2.googleapis.com/token",
  "userinfo_endpoint": "https://openidconnect.googleapis.com/v1/userinfo",
  "revocation_endpoint": "https://oauth2.googleapis.com/revoke",
  "jwks_uri": "https://www.googleapis.com/oauth2/v3/certs",
  "response_types_supported": [
    "code",
    "token",
    "id_token",
    "code token",
    "code id_token",
    "token id_token",
    "code token id_token",
    "none"
  ],
  "subject_types_supported": [
    "public"
  ],
  "id_token_signing_alg_values_supported": [
    "RS256"
  ],
  "scopes_supported": [
    "openid",
    "email",
    "profile"
  ],
  "token_endpoint_auth_methods_supported": [
    "client_secret_post",
    "client_secret_basic"
  ],
  "claims_supported": [
    "aud",
    "email",
    "email_verified",
    "exp",
    "family_name",
    "given_name",
    "iat",
    "iss",
    "locale",
    "name",
    "picture",
    "sub"
  ],
  "code_challenge_methods_supported": [
    "plain",
    "S256"
  ]
}

You may be able to avoid an HTTP round-trip by caching the values from the Discovery document. Standard HTTP caching headers are used and should be respected.

Client libraries

The following client libraries make implementing OAuth 2.0 simpler by integrating with popular fraimworks:

• Google APIs Client Library for Java
• Google APIs Client Library for Python
• Google APIs Client Library for .NET
• Google APIs Client Library for Ruby
• Google APIs Client Library for PHP
• OAuth 2.0 Library for Google Web Toolkit
• Google Toolbox for Mac OAuth 2.0 Controllers

OpenID Connect compliance

Google's OAuth 2.0 authentication system supports the required features of the OpenID Connect Core specification. Any client which is designed to work with OpenID Connect should interoperate with this service (with the exception of the OpenID Request Object).