バックエンド サーバーで認証する

バックエンド サーバーと通信するアプリやサイトで Google ログインを使用する場合、サーバーで現在ログインしているユーザーを特定する必要が生じる場合があります。これを安全に行うために、ユーザーがログインに成功したら、HTTPS を使用してユーザーの ID トークンをサーバーに送信します。次に、サーバー上で ID トークンの整合性を確認し、トークンに含まれるユーザー情報を使用してセッションを確立するか、新しいアカウントを作成します。

ID トークンをサーバーに送信する

ユーザーがログインに成功したら、ユーザーの ID トークンを取得します。

function onSignIn(googleUser) {
  var id_token = googleUser.getAuthResponse().id_token;
  ...
}

次に、その ID トークンを HTTPS POST リクエストを使ってサーバーに送信します。

var xhr = new XMLHttpRequest();
xhr.open('POST', 'https://yourbackend.example.com/tokensignin');
xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
xhr.onload = function() {
  console.log('Signed in as: ' + xhr.responseText);
};
xhr.send('idtoken=' + id_token);

ID トークンの整合性を確認する

HTTPS POST で ID トークンを受け取ったら、トークンの整合性を検証する必要があります。

To verify that the token is valid, ensure that the following criteria are satisfied:

  • The ID token is properly signed by Google. Use Google's public keys (available in JWK or PEM format) to verify the token's signature. These keys are regularly rotated; examine the Cache-Control header in the response to determine when you should retrieve them again.
  • The value of aud in the ID token is equal to one of your app's client IDs. This check is necessary to prevent ID tokens issued to a malicious app being used to access data about the same user on your app's backend server.
  • The value of iss in the ID token is equal to accounts.google.com or https://accounts.google.com.
  • The expiry time (exp) of the ID token has not passed.
  • If you want to restrict access to only members of your G Suite domain, verify that the ID token has an hd claim that matches your G Suite domain name.

Rather than writing your own code to perform these verification steps, we strongly recommend using a Google API client library for your platform, or a general-purpose JWT library. For development and debugging, you can call our tokeninfo validation endpoint.

Using a Google API Client Library

Using one of the Google API Client Libraries (e.g. Java, Node.js, PHP, Python) is the recommended way to validate Google ID tokens in a production environment.

Java

To validate an ID token in Java, use the GoogleIdTokenVerifier object. For example:

import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken.Payload;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdTokenVerifier;

...

GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(transport, jsonFactory)
    // Specify the CLIENT_ID of the app that accesses the backend:
    .setAudience(Collections.singletonList(CLIENT_ID))
    // Or, if multiple clients access the backend:
    //.setAudience(Arrays.asList(CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3))
    .build();

// (Receive idTokenString by HTTPS POST)

GoogleIdToken idToken = verifier.verify(idTokenString);
if (idToken != null) {
  Payload payload = idToken.getPayload();

  // Print user identifier
  String userId = payload.getSubject();
  System.out.println("User ID: " + userId);

  // Get profile information from payload
  String email = payload.getEmail();
  boolean emailVerified = Boolean.valueOf(payload.getEmailVerified());
  String name = (String) payload.get("name");
  String pictureUrl = (String) payload.get("picture");
  String locale = (String) payload.get("locale");
  String familyName = (String) payload.get("family_name");
  String givenName = (String) payload.get("given_name");

  // Use or store profile information
  // ...

} else {
  System.out.println("Invalid ID token.");
}

The GoogleIdTokenVerifier.verify() method verifies the JWT signature, the aud claim, the iss claim, and the exp claim.

If you want to restrict access to only members of your G Suite domain, also verify the hd claim by checking the domain name returned by the Payload.getHostedDomain() method.

Node.js

To validate an ID token in Node.js, use the Google Auth Library for Node.js. Install the library: 

npm install google-auth-library --save
Then, call the verifyIdToken() function. For example:

const {OAuth2Client} = require('google-auth-library');
const client = new OAuth2Client();
async function verify() {
  const ticket = await client.verifyIdToken({
      idToken: token,
      audience: CLIENT_ID,  // Specify the CLIENT_ID of the app that accesses the backend
      // Or, if multiple clients access the backend:
      //[CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3]
  });
  const payload = ticket.getPayload();
  const userid = payload['sub'];
  // If request specified a G Suite domain:
  // const domain = payload['hd'];
}
verify().catch(console.error);

The verifyIdToken function verifies the JWT signature, the aud claim, the exp claim, and the iss claim.

If you want to restrict access to only members of your G Suite domain, also verify the hd claim matches your G Suite domain name.

PHP

To validate an ID token in PHP, use the Google API Client Library for PHP. Install the library (for example, using Composer): 

composer require google/apiclient
Then, call the verifyIdToken() function. For example:

require_once 'vendor/autoload.php';

// Get $id_token via HTTPS POST.

$client = new Google_Client(['client_id' => $CLIENT_ID]);  // Specify the CLIENT_ID of the app that accesses the backend
$payload = $client->verifyIdToken($id_token);
if ($payload) {
  $userid = $payload['sub'];
  // If request specified a G Suite domain:
  //$domain = $payload['hd'];
} else {
  // Invalid ID token
}

The verifyIdToken function verifies the JWT signature, the aud claim, the exp claim, and the iss claim.

If you want to restrict access to only members of your G Suite domain, also verify the hd claim matches your G Suite domain name.

Python

To validate an ID token in Python, use the verify_oauth2_token function. For example:

from google.oauth2 import id_token
from google.auth.transport import requests

# (Receive token by HTTPS POST)
# ...

try:
    # Specify the CLIENT_ID of the app that accesses the backend:
    idinfo = id_token.verify_oauth2_token(token, requests.Request(), CLIENT_ID)

    # Or, if multiple clients access the backend server:
    # idinfo = id_token.verify_oauth2_token(token, requests.Request())
    # if idinfo['aud'] not in [CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3]:
    #     raise ValueError('Could not verify audience.')

    # If auth request is from a G Suite domain:
    # if idinfo['hd'] != GSUITE_DOMAIN_NAME:
    #     raise ValueError('Wrong hosted domain.')

    # ID token is valid. Get the user's Google Account ID from the decoded token.
    userid = idinfo['sub']
except ValueError:
    # Invalid token
    pass

The verify_oauth2_token function verifies the JWT signature, the aud claim, and the exp claim. You must also verify the hd claim (if applicable) by examining the object that verify_oauth2_token returns. If multiple clients access the backend server, also manually verify the aud claim.

Calling the tokeninfo endpoint

An easy way to validate an ID token signature for debugging is to use the tokeninfo endpoint. Calling this endpoint involves an additional network request that does most of the validation for you while you test proper validation and payload extraction in your own code. It is not suitable for use in production code as requests may be throttled or otherwise subject to intermittent errors.

To validate an ID token using the tokeninfo endpoint, make an HTTPS POST or GET request to the endpoint, and pass your ID token in the id_token parameter. For example, to validate the token "XYZ123", make the following GET request:

https://oauth2.googleapis.com/tokeninfo?id_token=XYZ123

If the token is properly signed and the iss and exp claims have the expected values, you will get a HTTP 200 response, where the body contains the JSON-formatted ID token claims. Here's an example response:

{
 // These six fields are included in all Google ID Tokens.
 "iss": "https://accounts.google.com",
 "sub": "110169484474386276334",
 "azp": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "aud": "1008719970978-hb24n2dstb40o45d4feuo2ukqmcc6381.apps.googleusercontent.com",
 "iat": "1433978353",
 "exp": "1433981953",

 // These seven fields are only included when the user has granted the "profile" and
 // "email" OAuth scopes to the application.
 "email": "testuser@gmail.com",
 "email_verified": "true",
 "name" : "Test User",
 "picture": "https://lh4.googleusercontent.com/-kYgzyAWpZzJ/ABCDEFGHI/AAAJKLMNOP/tIXL9Ir44LE/s99-c/photo.jpg",
 "given_name": "Test",
 "family_name": "User",
 "locale": "en"
}

If you are a G Suite customer, you might also be interested in the hd claim, which indicates the hosted domain of the user. This can be used to restrict access to a resource to only members of certain domains. The absence of this claim indicates that the user does not belong to a G Suite hosted domain.

アカウントまたはセッションを作成する

トークンを確認したら、ユーザーがユーザー データベースにすでに存在していることを確認します。ユーザーのログイン セッションを確立します。ユーザー データベースにユーザーが存在しない場合は、ID トークン ペイロード内の情報から新しいユーザー レコードを作成し、そのユーザーのセッションを確立します。アプリで新しく作成されたユーザーを検出すると、必要な追加のプロフィール情報をユーザーに求めることができます。

クロスアカウント保護でユーザーを保護（#39）

ユーザーのログインに Google を利用すると、ユーザーデータを保護するために Google が構築したセキュリティ機能とインフラストラクチャをすべて自動的に利用できます。ただし、ごくまれに、ユーザーの Google アカウントが侵害された場合や、重大なセキュリティ イベントが発生した場合、アプリは攻撃に対して脆弱になる可能性があります。重大なセキュリティ イベントからアカウントを保護するため、クロスアカウント保護を使用して Google からセキュリティ アラートを受け取ります。これらのイベントを受信すると、ユーザーの Google アカウントのセキュリティに関する重要な変更が自動的に検出され、サービスを保護するためにアカウントを保護できます。