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驗證伺服器端的 Google ID 權杖

使用 Google Identity Services 或 OAuth 2.0 授權碼流程時，Google 會透過 POST 方法將 ID 權杖傳回重新導向端點。或者，OIDC 隱含流程會使用 GET 要求。因此，應用程式有責任將收到的憑證安全地傳輸至伺服器。

取得

這是隱含流程，ID 權杖會傳回至網址片段中，用戶端 JavaScript 必須剖析該片段。應用程式有責任實作自己的驗證機制，確保要求真實性並防範 CSRF 等攻擊。

    HTTP/1.1 302 Found
    Location: https://<REDIRECT_URI>#access_token=<ACCESS_TOKEN>&token_type=bearer&expires_in=<TIME_IN_SECONDS>&scope=<SCOPE>&state=<STATE_STRING>

POST

ID 權杖會以 credential 欄位形式傳回。準備將 ID 權杖傳送至伺服器時，GIS 程式庫會自動將 g_csrf_token 新增至標頭 Cookie 和要求主體。以下是 POST 要求範例：

POST /auth/token-verification HTTP/1.1
Host: example.com
Content-Type: application/json;charset=UTF-8
Cookie: g_csrf_token=<CSRF_TOKEN>
Origin: https://example.com
Content-Length: <LENGTH_OF_JSON_BODY>
    {
      "credential": "<ID_TOKEN>",
      "g_csrf_token": "<CSRF_TOKEN>",
      "client_id": "<CLIENT_ID>"
    }

驗證 g_csrf_token，防範跨網站偽造要求 (CSRF) 攻擊：
- 從 g_csrf_token Cookie 擷取 CSRF 權杖值。
- 從要求內文中擷取 CSRF 權杖值。GIS 程式庫會在 POST 要求主體中加入這個權杖做為參數，同樣命名為 g_csrf_token。
- 比較兩個權杖值
  - 如果這兩個值都存在且完全相符，系統就會將要求視為合法，且來自您的網域。
  - 如果值不存在或不相符，伺服器必須拒絕要求。這項檢查可確保要求是從您網域上執行的 JavaScript 發出，因為只有您的網域可以存取 g_csrf_token Cookie。
驗證 ID 權杖。

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 need to validate that the ID token represents a Google Workspace or Cloud organization account, you can check the hd claim, which indicates the hosted domain of the user. This must be used when restricting access to a resource to only members of certain domains. The absence of this claim indicates that the account does not belong to a Google hosted domain.
Using the email, email_verified and hd fields, you can determine if Google hosts and is authoritative for an email address. In the cases where Google is authoritative, the user is known to be the legitimate account owner, and you may skip password or other challenge methods.

Cases where Google is authoritative:
- email has a @gmail.com suffix, this is a Gmail account.
- email_verified is true and hd is set, this is a Google Workspace account.
Users may register for Google Accounts without using Gmail or Google Workspace. When email does not contain a @gmail.com suffix and hd is absent, Google is not authoritative and password or other challenge methods are recommended to verify the user. email_verified can also be true as Google initially verified the user when the Google account was created, however ownership of the third party email account may have since changed.

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.

使用 Google API 用戶端程式庫

使用其中一個 Google API 用戶端程式庫 (例如 Java、 Node.js、 PHP、 Python) 是在正式環境中驗證 Google ID 權杖的建議做法。
Java

如要在 Java 中驗證 ID 符記，請使用 GoogleIdTokenVerifier物件。例如：
```
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 WEB_CLIENT_ID of the app that accesses the backend:
    .setAudience(Collections.singletonList(WEB_CLIENT_ID))
    // Or, if multiple clients access the backend:
    //.setAudience(Arrays.asList(WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3))
    .build();

// (Receive idTokenString by HTTPS POST)

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

  // Print user identifier. This ID is unique to each Google Account, making it suitable for
  // use as a primary key during account lookup. Email is not a good choice because it can be
  // changed by the user.
  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.");
}
```
GoogleIdTokenVerifier.verify() 方法會驗證 JWT 簽章、aud 宣告、iss 憑證和 exp著作權聲明。

如需驗證 ID 權杖是否代表 Google Workspace 或 Cloud 機構帳戶，您可以藉由檢查網域名稱來驗證 hd 擁有權聲明是由 Payload.getHostedDomain() 方法傳回的。網域 email 憑證附加資訊不足以確保帳戶是由網域管理或機構
，瞭解如何調查及移除這項存取權。
Node.js

如要透過 Node.js 驗證 ID 權杖，請使用 Node.js 適用的 Google 驗證程式庫。安裝程式庫：
```
npm install google-auth-library --save
```
敬上然後呼叫 verifyIdToken() 函式。例如：
```
const {OAuth2Client} = require('google-auth-library');
const client = new OAuth2Client();
async function verify() {
  const ticket = await client.verifyIdToken({
      idToken: token,
      audience: WEB_CLIENT_ID,  // Specify the WEB_CLIENT_ID of the app that accesses the backend
      // Or, if multiple clients access the backend:
      //[WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3]
  });
  const payload = ticket.getPayload();
  // This ID is unique to each Google Account, making it suitable for use as a primary key
  // during account lookup. Email is not a good choice because it can be changed by the user.
  const userid = payload['sub'];
  // If the request specified a Google Workspace domain:
  // const domain = payload['hd'];
}
verify().catch(console.error);
```
verifyIdToken 函式會驗證 JWT 簽名、aud 憑證附加資訊、exp 憑證附加資訊和 iss 聲明

如需驗證 ID 權杖是否代表 Google Workspace 或 Cloud 機構帳戶，您可以檢查 hd 憑證附加資訊，也就是使用者的網域將資源存取權授予僅限成員時，就必須使用這個引數無法歸類到特定網域沒有出現這項聲明，表示帳戶不屬於該帳戶所有 Google 代管的網域
，瞭解如何調查及移除這項存取權。
PHP

如要在 PHP 中驗證 ID 符記，請使用 PHP 適用的 Google API 用戶端程式庫。安裝程式庫 (例如使用 Composer)：
```
composer require google/apiclient
```
敬上然後呼叫 verifyIdToken() 函式。例如：
```
require_once 'vendor/autoload.php';

// Get $id_token via HTTPS POST.

$client = new Google_Client(['client_id' => $WEB_CLIENT_ID]);  // Specify the WEB_CLIENT_ID of the app that accesses the backend
$payload = $client->verifyIdToken($id_token);
if ($payload) {
  // This ID is unique to each Google Account, making it suitable for use as a primary key
  // during account lookup. Email is not a good choice because it can be changed by the user.
  $userid = $payload['sub'];
  // If the request specified a Google Workspace domain
  //$domain = $payload['hd'];
} else {
  // Invalid ID token
}
```
verifyIdToken 函式會驗證 JWT 簽名、aud 憑證附加資訊、exp 憑證附加資訊和 iss 聲明

如需驗證 ID 權杖是否代表 Google Workspace 或 Cloud 機構帳戶，您可以檢查 hd 憑證附加資訊，也就是使用者的網域將資源存取權授予僅限成員時，就必須使用這個引數無法歸類到特定網域沒有出現這項聲明，表示帳戶不屬於該帳戶所有 Google 代管的網域
，瞭解如何調查及移除這項存取權。
Python

如要在 Python 中驗證 ID 符記，請使用 verify_oauth2_token 函式。例如：
```
from google.oauth2 import id_token
from google.auth.transport import requests

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

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

    # Or, if multiple clients access the backend server:
    # idinfo = id_token.verify_oauth2_token(token, requests.Request())
    # if idinfo['aud'] not in [WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3]:
    #     raise ValueError('Could not verify audience.')

    # If the request specified a Google Workspace domain
    # if idinfo['hd'] != DOMAIN_NAME:
    #     raise ValueError('Wrong domain name.')

    # ID token is valid. Get the user's Google Account ID from the decoded token.
    # This ID is unique to each Google Account, making it suitable for use as a primary key
    # during account lookup. Email is not a good choice because it can be changed by the user.
    userid = idinfo['sub']
except ValueError:
    # Invalid token
    pass
```
verify_oauth2_token 函式會驗證 JWT 簽章、aud 聲明和 exp 聲明。並驗證hd 來識別請求 (如果適用的話) 來檢查會傳回 verify_oauth2_token。如果多位用戶端存取後端伺服器，也手動驗證 aud 憑證附加資訊。
重點： ID 權杖會傳回至 credential 欄位，而非 g_csrf_token 欄位。
確認權杖有效後，您可以使用 Google ID 權杖中的資訊，將網站的帳戶狀態與下列項目建立關聯：
- 未註冊的使用者：您可以顯示註冊使用者介面 (UI)，讓使用者提供額外的個人資料資訊 (如有需要)。使用者也可以透過這項功能，在不顯示任何提示的情況下建立新帳戶和登入使用者工作階段。
- 網站上已有的現有帳戶：您可以顯示網頁，讓使用者輸入密碼，並將舊版帳戶連結至 Google 憑證。確認使用者可以存取現有帳戶。
- 回訪的同盟使用者：您可以讓使用者無須輸入憑證即可登入。

驗證伺服器端的 Google ID 權杖 透過集合功能整理內容 你可以依據偏好儲存及分類內容。

使用 Google API 用戶端程式庫

驗證伺服器端的 Google ID 權杖