Using OAuth 2.0 for Server to Server Applications

The Google OAuth 2.0 system supports server-to-server interactions such as those between a web application and a Google service. For this scenario you need a service account, which is an account that belongs to your application instead of to an individual end user. Your application calls Google APIs on behalf of the service account, so users aren't directly involved. This scenario is sometimes called "two-legged OAuth," or "2LO." (The related term "three-legged OAuth" refers to scenarios in which your application calls Google APIs on behalf of end users, and in which user consent is sometimes required.)

Typically, an application uses a service account when the application uses Google APIs to work with its own data rather than a user's data. For example, an application that uses Google Cloud Datastore for data persistence would use a service account to authenticate its calls to the Google Cloud Datastore API.

Google Workspace domain administrators can also grant service accounts domain-wide authority to access user data on behalf of users in the domain.

This document describes how an application can complete the server-to-server OAuth 2.0 flow by using either a Google APIs client library (recommended) or HTTP.

Overview

To support server-to-server interactions, first create a service account for your project in the API Console. If you want to access user data for users in your Google Workspace account, then delegate domain-wide access to the service account.

Then, your application prepares to make authorized API calls by using the service account's credentials to request an access token from the OAuth 2.0 auth server.

Finally, your application can use the access token to call Google APIs.

Creating a service account

A service account's credentials include a generated email address that is unique and at least one public/private key pair. If domain-wide delegation is enabled, then a client ID is also part of the service account's credentials.

If your application runs on Google App Engine, a service account is set up automatically when you create your project.

If your application runs on Google Compute Engine, a service account is also set up automatically when you create your project, but you must specify the scopes that your application needs access to when you create a Google Compute Engine instance. For more information, see Preparing an instance to use service accounts.

If your application doesn't run on Google App Engine or Google Compute Engine, you must obtain these credentials in the Google API Console. To generate service-account credentials, or to view the public credentials that you've already generated, do the following:

First, create a service account:

Open the Service accounts page.
If prompted, select a project, or create a new one.
Click Create service account.
Under Service account details, type a name, ID, and description for the service account, then click Create and continue.
Optional: Under Grant this service account access to project, select the IAM roles to grant to the service account.
Click Continue.
Optional: Under Grant users access to this service account, add the users or groups that are allowed to use and manage the service account.
Click Done.

Next, create a service account key:

Click the email address for the service account you created.
Click the Keys tab.
In the Add key drop-down list, select Create new key.
Click Create.

Your new public/private key pair is generated and downloaded to your machine; it serves as the only copy of the private key. You are responsible for storing it securely. If you lose this key pair, you will need to generate a new one.

You can return to the API Console at any time to view the email address, public key fingerprints, and other information, or to generate additional public/private key pairs. For more details about service account credentials in the API Console, see Service accounts in the API Console help file.

Take note of the service account's email address and store the service account's private key file in a location accessible to your application. Your application needs them to make authorized API calls.

Delegating domain-wide authority to the service account

Using a Google Workspace account, a Workspace administrator of the organization can authorize an application to access Workspace user data on behalf of users in the Google Workspace domain. For example, an application that uses the Google Calendar API to add events to the calendars of all users in a Google Workspace domain would use a service account to access the Google Calendar API on behalf of users. Authorizing a service account to access data on behalf of users in a domain is sometimes referred to as "delegating domain-wide authority" to a service account.

To delegate domain-wide authority to a service account, a super administrator of the Google Workspace domain must complete the following steps:

From your Google Workspace domain's Admin console, go to Main menu > Security > Access and data control > API Controls.
In the Domain wide delegation pane, select Manage Domain Wide Delegation.
Click Add new.
In the Client ID field, enter the service account's Client ID. You can find your service account's client ID in the Service accounts page.
In the OAuth scopes (comma-delimited) field, enter the list of scopes that your application should be granted access to. For example, if your application needs domain-wide full access to the Google Drive API and the Google Calendar API, enter: https://www.googleapis.com/auth/drive, https://www.googleapis.com/auth/calendar.
Click Authorize.

Your application now has the authority to make API calls as users in your Workspace domain (to "impersonate" users). When you prepare to make these delegated API calls, you will explicitly specify the user to impersonate.

Preparing to make a delegated API call

Java

After you obtain the client email address and private key from the API Console, use the Google APIs Client Library for Java to create a GoogleCredential object from the service account's credentials and the scopes your application needs access to. For example:

import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.services.sqladmin.SQLAdminScopes;

// ...

GoogleCredential credential = GoogleCredential.fromStream(new FileInputStream("MyProject-1234.json"))
    .createScoped(Collections.singleton(SQLAdminScopes.SQLSERVICE_ADMIN));

If you are developing an app on Google Cloud Platform, you can use the application default credentials instead, which can simplify the process.

Delegate domain-wide authority

If you have delegated domain-wide access to the service account and you want to impersonate a user account, specify the email address of the user account with the createDelegated method of the GoogleCredential object. For example:

GoogleCredential credential = GoogleCredential.fromStream(new FileInputStream("MyProject-1234.json"))
    .createScoped(Collections.singleton(SQLAdminScopes.SQLSERVICE_ADMIN))
    .createDelegated("[email protected]");

The code above uses the GoogleCredential object to call its createDelegated() method. The argument for the createDelegated() method must be a user which belongs to your Workspace account. Your code making the request will use this credential to call Google APIs using your service account.

Python

After you obtain the client email address and private key from the API Console, use the Google APIs Client Library for Python to complete the following steps:

Create a Credentials object from the service account's credentials and the scopes your application needs access to. For example:

from google.oauth2 import service_account

SCOPES = ['https://www.googleapis.com/auth/sqlservice.admin']
SERVICE_ACCOUNT_FILE = '/path/to/service.json'

credentials = service_account.Credentials.from_service_account_file(
        SERVICE_ACCOUNT_FILE, scopes=SCOPES)

If you are developing an app on Google Cloud Platform, you can use the application default credentials instead, which can simplify the process.

Delegate domain-wide authority
If you have delegated domain-wide access to the service account and you want to impersonate a user account, use the with_subject method of an existing ServiceAccountCredentials object. For example:
```
delegated_credentials = credentials.with_subject('[email protected]')
```

Use the Credentials object to call Google APIs in your application.

HTTP/REST

After you obtain the client ID and private key from the API Console, your application needs to complete the following steps:

Create a JSON Web Token (JWT, pronounced, "jot") which includes a header, a claim set, and a signature.
Request an access token from the Google OAuth 2.0 Authorization Server.
Handle the JSON response that the Authorization Server returns.

The sections that follow describe how to complete these steps.

If the response includes an access token, you can use the access token to call a Google API. (If the response does not include an access token, your JWT and token request might not be properly formed, or the service account might not have permission to access the requested scopes.)

When the access token expires, your application generates another JWT, signs it, and requests another access token.

Your server application uses a JWT to request a token from the Google
Authorization Server, then uses the token to call a Google API endpoint. No
end user is involved.

The rest of this section describes the specifics of creating a JWT, signing the JWT, forming the access token request, and handling the response.

Creating a JWT

A JWT is composed of three parts: a header, a claim set, and a signature. The header and claim set are JSON objects. These JSON objects are serialized to UTF-8 bytes, then encoded using the Base64url encoding. This encoding provides resilience against encoding changes due to repeated encoding operations. The header, claim set, and signature are concatenated together with a period (.) character.

A JWT is composed as follows:

{Base64url encoded header}.{Base64url encoded claim set}.{Base64url encoded signature}

The base string for the signature is as follows:

{Base64url encoded header}.{Base64url encoded claim set}

Forming the JWT header

The header consists of three fields that indicate the signing algorithm, the format of the assertion, and the [key ID of the service account key](https://cloud.google.com/iam/docs/reference/rest/v1/projects.serviceAccounts.keys) that was used to sign the JWT. Algorithm and format are mandatory, and each field has only one value. As additional algorithms and formats are introduced, this header will change accordingly. The key ID is optional and if an incorrect Key ID is specified GCP will try all keys associated with the service account to verify the token and reject the token if no valid key is found. Google reserves the right to reject tokens with incorrect key IDs in the future.

Service accounts rely on the RSA SHA-256 algorithm and the JWT token format. As a result, the JSON representation of the header is as follows:

{"alg":"RS256","typ":"JWT", "kid":"370ab79b4513eb9bad7c9bd16a95cb76b5b2a56a"}

The Base64url representation of this is as follows:

          eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCIsICJraWQiOiIzNzBhYjc5YjQ1MTNlYjliYWQ3YzliZDE2YTk1Y2I3NmI1YjJhNTZhIn0=

Forming the JWT claim set

The JWT claim set contains information about the JWT, including the permissions being requested (scopes), the target of the token, the issuer, the time the token was issued, and the lifetime of the token. Most of the fields are mandatory. Like the JWT header, the JWT claim set is a JSON object and is used in the calculation of the signature.

Required claims

The required claims in the JWT claim set are shown below. They may appear in any order in the claim set.

Name	Description
`iss`	The email address of the service account.
`scope`	A space-delimited list of the permissions that the application requests.
`aud`	A descriptor of the intended target of the assertion. When making an access token request this value is always `https://oauth2.googleapis.com/token`.
`exp`	The expiration time of the assertion, specified as seconds since 00:00:00 UTC, January 1, 1970. This value has a maximum of 1 hour after the issued time.
`iat`	The time the assertion was issued, specified as seconds since 00:00:00 UTC, January 1, 1970.

The JSON representation of the required fields in a JWT claim set is shown below:

{
  "iss": "761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com",
  "scope": "https://www.googleapis.com/auth/devstorage.read_only",
  "aud": "https://oauth2.googleapis.com/token",
  "exp": 1328554385,
  "iat": 1328550785
}

Additional claims

In some enterprise cases, an application can use domain-wide delegation to act on behalf of a particular user in an organization. Permission to perform this type of impersonation must be granted before an application can impersonate a user, and is usually handled by a super administrator. For more information, see Control API access with domain-wide delegation.

To obtain an access token that grants an application delegated access to a resource, include the email address of the user in the JWT claim set as the value of the sub field.

Name	Description
`sub`	The email address of the user for which the application is requesting delegated access.

If an application does not have permission to impersonate a user, the response to an access token request that includes the sub field will be an error.

An example of a JWT claim set that includes the sub field is shown below:

{
  "iss": "761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com",
  "sub": "[email protected]",
  "scope": "https://www.googleapis.com/auth/prediction",
  "aud": "https://oauth2.googleapis.com/token",
  "exp": 1328554385,
  "iat": 1328550785
}

Encoding the JWT claim set

Like the JWT header, the JWT claim set should be serialized to UTF-8 and Base64url-safe encoded. Below is an example of a JSON representation of a JWT Claim set:

{
  "iss": "761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com",
  "scope": "https://www.googleapis.com/auth/prediction",
  "aud": "https://oauth2.googleapis.com/token",
  "exp": 1328554385,
  "iat": 1328550785
}

Computing the signature

JSON Web Signature (JWS) is the specification that guides the mechanics of generating the signature for the JWT. The input for the signature is the byte array of the following content:

{Base64url encoded header}.{Base64url encoded claim set}

The signing algorithm in the JWT header must be used when computing the signature. The only signing algorithm supported by the Google OAuth 2.0 Authorization Server is RSA using SHA-256 hashing algorithm. This is expressed as RS256 in the alg field in the JWT header.

Sign the UTF-8 representation of the input using SHA256withRSA (also known as RSASSA-PKCS1-V1_5-SIGN with the SHA-256 hash function) with the private key obtained from the Google API Console. The output will be a byte array.

The signature must then be Base64url encoded. The header, claim set, and signature are concatenated together with a period (.) character. The result is the JWT. It should be the following (line breaks added for clarity):

{Base64url encoded header}.
{Base64url encoded claim set}.
{Base64url encoded signature}

Below is an example of a JWT before Base64url encoding:

{"alg":"RS256","typ":"JWT"}.
{
"iss":"761326798069-r5mljlln1rd4lrbhg75efgigp36m78j5@developer.gserviceaccount.com",
"scope":"https://www.googleapis.com/auth/prediction",
"aud":"https://oauth2.googleapis.com/token",
"exp":1328554385,
"iat":1328550785
}.
[signature bytes]

Below is an example of a JWT that has been signed and is ready for transmission:

eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiI3NjEzMjY3OTgwNjktcjVtbGpsbG4xcmQ0bHJiaGc3NWVmZ2lncDM2bTc4ajVAZGV2ZWxvcGVyLmdzZXJ2aWNlYWNjb3VudC5jb20iLCJzY29wZSI6Imh0dHBzOi8vd3d3Lmdvb2dsZWFwaXMuY29tL2F1dGgvcHJlZGljdGlvbiIsImF1ZCI6Imh0dHBzOi8vd3d3Lmdvb2dsZWFwaXMuY29tL29hdXRoMi92NC90b2tlbiIsImV4cCI6MTMyODU1NDM4NSwiaWF0IjoxMzI4NTUwNzg1fQ.UFUt59SUM2_AW4cRU8Y0BYVQsNTo4n7AFsNrqOpYiICDu37vVt-tw38UKzjmUKtcRsLLjrR3gFW3dNDMx_pL9DVjgVHDdYirtrCekUHOYoa1CMR66nxep5q5cBQ4y4u2kIgSvChCTc9pmLLNoIem-ruCecAJYgI9Ks7pTnW1gkOKs0x3YpiLpzplVHAkkHztaXiJdtpBcY1OXyo6jTQCa3Lk2Q3va1dPkh_d--GU2M5flgd8xNBPYw4vxyt0mP59XZlHMpztZt0soSgObf7G3GXArreF_6tpbFsS3z2t5zkEiHuWJXpzcYr5zWTRPDEHsejeBSG8EgpLDce2380ROQ

Making the access token request

After generating the signed JWT, an application can use it to request an access token. This access token request is an HTTPS POST request, and the body is URL encoded. The URL is shown below:

https://oauth2.googleapis.com/token

The following parameters are required in the HTTPS POST request:

Name	Description
`grant_type`	Use the following string, URL-encoded as necessary: `urn:ietf:params:oauth:grant-type:jwt-bearer`
`assertion`	The JWT, including signature.

Below is a raw dump of the HTTPS POST request used in an access token request:

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

grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiI3NjEzMjY3OTgwNjktcjVtbGpsbG4xcmQ0bHJiaGc3NWVmZ2lncDM2bTc4ajVAZGV2ZWxvcGVyLmdzZXJ2aWNlYWNjb3VudC5jb20iLCJzY29wZSI6Imh0dHBzOi8vd3d3Lmdvb2dsZWFwaXMuY29tL2F1dGgvcHJlZGljdGlvbiIsImF1ZCI6Imh0dHBzOi8vYWNjb3VudHMuZ29vZ2xlLmNvbS9vL29hdXRoMi90b2tlbiIsImV4cCI6MTMyODU3MzM4MSwiaWF0IjoxMzI4NTY5NzgxfQ.ixOUGehweEVX_UKXv5BbbwVEdcz6AYS-6uQV6fGorGKrHf3LIJnyREw9evE-gs2bmMaQI5_UbabvI4k-mQE4kBqtmSpTzxYBL1TCd7Kv5nTZoUC1CmwmWCFqT9RE6D7XSgPUh_jF1qskLa2w0rxMSjwruNKbysgRNctZPln7cqQ

Below is the same request, using curl:

curl -d 'grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.eyJpc3MiOiI3NjEzMjY3OTgwNjktcjVtbGpsbG4xcmQ0bHJiaGc3NWVmZ2lncDM2bTc4ajVAZGV2ZWxvcGVyLmdzZXJ2aWNlYWNjb3VudC5jb20iLCJzY29wZSI6Imh0dHBzOi8vd3d3Lmdvb2dsZWFwaXMuY29tL2F1dGgvcHJlZGljdGlvbiIsImF1ZCI6Imh0dHBzOi8vYWNjb3VudHMuZ29vZ2xlLmNvbS9vL29hdXRoMi90b2tlbiIsImV4cCI6MTMyODU3MzM4MSwiaWF0IjoxMzI4NTY5NzgxfQ.RZVpzWygMLuL-n3GwjW1_yhQhrqDacyvaXkuf8HcJl8EtXYjGjMaW5oiM5cgAaIorrqgYlp4DPF_GuncFqg9uDZrx7pMmCZ_yHfxhSCXru3gbXrZvAIicNQZMFxrEEn4REVuq7DjkTMyCMGCY1dpMa8aWfTQFt3Eh7smLchaZsU
' https://oauth2.googleapis.com/token

Handling the response

If the JWT and access token request are properly formed and the service account has permission to perform the operation, then the JSON response from the Authorization Server includes an access token. The following is an example response:

{
  "access_token": "1/8xbJqaOZXSUZbHLl5EOtu1pxz3fmmetKx9W8CV4t79M",
  "scope": "https://www.googleapis.com/auth/prediction"
  "token_type": "Bearer",
  "expires_in": 3600
}

Access tokens can be reused during the duration window specified by the expires_in value.

Calling Google APIs

Java

Use the GoogleCredential object to call Google APIs by completing the following steps:

Create a service object for the API that you want to call using the GoogleCredential object. For example:

SQLAdmin sqladmin =
    new SQLAdmin.Builder(httpTransport, JSON_FACTORY, credential).build();

Make requests to the API service using the interface provided by the service object. For example, to list the instances of Cloud SQL databases in the exciting-example-123 project:
```
SQLAdmin.Instances.List instances =
    sqladmin.instances().list("exciting-example-123").execute();
```

Python

Use the authorized Credentials object to call Google APIs by completing the following steps:

Build a service object for the API that you want to call. You build a service object by calling the build function with the name and version of the API and the authorized Credentials object. For example, to call version 1beta3 of the Cloud SQL Administration API:
```
import googleapiclient.discovery

sqladmin = googleapiclient.discovery.build('sqladmin', 'v1beta3', credentials=credentials)
```
Make requests to the API service using the interface provided by the service object. For example, to list the instances of Cloud SQL databases in the exciting-example-123 project:
```
response = sqladmin.instances().list(project='exciting-example-123').execute()
```

HTTP/REST

After your application obtains an access token, you can use the token to make calls to a Google API on behalf of a given service account or user account if the scope(s) of access required by the API have been granted. To do this, include the access token in a request to the API by including either an access_token query parameter or an Authorization HTTP header Bearer value. When possible, the HTTP header is preferable, because query strings tend to be visible in server logs. In most cases you can use a client library to set up your calls to Google APIs (for example, when calling the Drive Files API).

You can try out all the Google APIs and view their scopes at the OAuth 2.0 Playground.

HTTP GET examples

A call to the drive.files endpoint (the Drive Files API) using the Authorization: Bearer HTTP header might look like the following. Note that you need to specify your own access token:

GET /drive/v2/files HTTP/1.1
Host: www.googleapis.com
Authorization: Bearer access_token

Here is a call to the same API for the authenticated user using the access_token query string parameter:

GET https://www.googleapis.com/drive/v2/files?access_token=access_token

`curl` examples

You can test these commands with the curl command-line application. Here's an example that uses the HTTP header option (preferred):

curl -H "Authorization: Bearer access_token" https://www.googleapis.com/drive/v2/files

Or, alternatively, the query string parameter option:

curl https://www.googleapis.com/drive/v2/files?access_token=access_token

When access tokens expire

Access tokens issued by the Google OAuth 2.0 Authorization Server expire after the duration provided by the expires_in value. When an access token expires, then the application should generate another JWT, sign it, and request another access token.

JWT error codes

`error` field	`error_description` field	Meaning	How to resolve
`unauthorized_client`	`Unauthorized client or scope in request.`	If you're trying to use domain-wide delegation, the service account is not authorized in the Admin console of the user's domain.	Ensure that the service account is authorized in the Domain-wide delegation page of the Admin console for the user in the `sub` claim (field). While it usually takes a few minutes, it might take up to 24 hours for authorization to propagate to all users in your Google Account.
`unauthorized_client`	`Client is unauthorized to retrieve access tokens using this method, or client not authorized for any of the scopes requested.`	A service account was authorized using the client email address rather than the client ID (numeric) in the Admin console.	In the Domain-wide delegation page in the Admin console, remove the client, and re-add it with the numeric ID.
`access_denied`	(any value)	If you're using Domain-wide delegation, one or more requested scopes aren't authorized in the Admin console.	Ensure that the service account is authorized in the Domain-wide delegation page of the Admin console for the user in the `sub` claim (field), and that it includes all of the scopes you're requesting in the `scope` claim of your JWT. While it usually takes a few minutes, it might take up to 24 hours for authorization to propagate to all users in your Google Account.
`admin_policy_enforced`	(any value)	The Google Account is unable to authorize one or more scopes requested due to the policies of their Google Workspace administrator.	See the Google Workspace Admin help article Control which third-party & internal apps access Google Workspace data for more information about how an administrator may restrict access to all scopes or sensitive and restricted scopes until access is explicitly granted to your OAuth client ID.
`invalid_client`	(any value)	The OAuth client or JWT token is invalid or incorrectly configured. Refer to the error description for details.	Make sure the JWT token is valid and contains correct claims. Check that the OAuth client and service account are configured correctly and that you are using the correct email address. Check that the JWT token is correct and was issued for the client ID in the request.
`invalid_grant`	`Not a valid email.`	The user doesn't exist.	Check that the email address in the `sub` claim (field) is correct.
`invalid_grant`	`Invalid JWT: Token must be a short-lived token (60 minutes) and in a reasonable timeframe. Check your 'iat' and 'exp' values and use a clock with skew to account for clock differences between systems.`	Usually, it means that the local system time is not correct. It could also happen if the `exp` value is more than 65 mins in the future from the `iat` value, or the `exp` value is lower than `iat` value.	Make sure that the clock on the system where the JWT is generated is correct. If necessary, sync your time with Google NTP.
`invalid_grant`	`Invalid JWT Signature.`	The JWT assertion is signed with a private key not associated with the service account identified by the client email or the key that was used has been deleted, disabled, or has expired. Alternatively, the JWT assertion might be encoded incorrectly - it must be Base64-encoded, without newlines or padding equal signs.	Decode the JWT claim set and verify the key that signed the assertion is associated with the service account. Try to use a Google-provided OAuth library to make sure the JWT is generated correctly.
`invalid_scope`	`Invalid OAuth scope or ID token audience provided.`	No scopes were requested (empty list of scopes), or one of the requested scopes doesn't exist (i.e. is invalid).	Ensure that the `scope` claim (field) of the JWT is populated, and compare the scopes that it contains with the documented scopes for the APIs you want to use, to ensure there are no errors or typos. Note that the list of scopes in the `scope` claim needs to be separated by spaces, not commas.
`disabled_client`	`The OAuth client was disabled.`	The key used to sign the JWT assertion is disabled.	Go to the Google API Console, and under IAM & Admin > Service Accounts, enable the service account which contains the "Key ID" used to sign the assertion.
`org_internal`	`This client is restricted to users within its organization.`	The OAuth client ID in the request is part of a project limiting access to Google Accounts in a specific Google Cloud Organization.	Use a service account from the organization to authenticate. Confirm the user type configuration for your OAuth application.

Addendum: Service account authorization without OAuth

With some Google APIs, you can make authorized API calls using a signed JWT directly as a bearer token, rather than an OAuth 2.0 access token. When this is possible, you can avoid having to make a network request to Google's authorization server before making an API call.

If the API you want to call has a service definition published in the Google APIs GitHub repository, you can make authorized API calls using a JWT instead of an access token. To do so:

Create a service account as described above. Be sure to keep the JSON file you get when you create the account.

Using any standard JWT library, such as one found at jwt.io, create a JWT with a header and payload like the following example:

{
  "alg": "RS256",
  "typ": "JWT",
  "kid": "abcdef1234567890"
}
.
{
  "iss": "[email protected]",
  "sub": "[email protected]",
  "aud": "https://firestore.googleapis.com/",
  "iat": 1511900000,
  "exp": 1511903600
}

For the kid field in the header, specify your service account's private key ID. You can find this value in the private_key_id field of your service account JSON file.
For the iss and sub fields, specify your service account's email address. You can find this value in the client_email field of your service account JSON file.
For the aud field, specify the API endpoint. For example: https://SERVICE.googleapis.com/.
For the iat field, specify the current Unix time, and for the exp field, specify the time exactly 3600 seconds later, when the JWT will expire.

Sign the JWT with RSA-256 using the private key found in your service account JSON file.

For example:

Java

Using google-api-java-client and java-jwt:

GoogleCredential credential =
        GoogleCredential.fromStream(new FileInputStream("MyProject-1234.json"));
PrivateKey privateKey = credential.getServiceAccountPrivateKey();
String privateKeyId = credential.getServiceAccountPrivateKeyId();

long now = System.currentTimeMillis();

try {
    Algorithm algorithm = Algorithm.RSA256(null, privateKey);
    String signedJwt = JWT.create()
        .withKeyId(privateKeyId)
        .withIssuer("[email protected]")
        .withSubject("[email protected]")
        .withAudience("https://firestore.googleapis.com/")
        .withIssuedAt(new Date(now))
        .withExpiresAt(new Date(now + 3600 * 1000L))
        .sign(algorithm);
} catch ...

Python

Using PyJWT:

iat = time.time()
exp = iat + 3600
payload = {'iss': '[email protected]',
           'sub': '[email protected]',
           'aud': 'https://firestore.googleapis.com/',
           'iat': iat,
           'exp': exp}
additional_headers = {'kid': PRIVATE_KEY_ID_FROM_JSON}
signed_jwt = jwt.encode(payload, PRIVATE_KEY_FROM_JSON, headers=additional_headers,
                       algorithm='RS256')

Call the API, using the signed JWT as the bearer token:

GET /v1/projects/abc/databases/123/indexes HTTP/1.1
Authorization: Bearer SIGNED_JWT
Host: firestore.googleapis.com

Implementing Cross-Account Protection

An additional step you should take to protect your users' accounts is implementing Cross-Account Protection by utilizing Google's Cross-Account Protection Service. This service lets you subscribe to security event notifications which provide information to your application about major changes to the user account. You can then use the information to take action depending on how you decide to respond to events.

Some examples of the event types sent to your app by Google's Cross-Account Protection Service are:

https://schemas.openid.net/secevent/risc/event-type/sessions-revoked
https://schemas.openid.net/secevent/oauth/event-type/token-revoked
https://schemas.openid.net/secevent/risc/event-type/account-disabled

See the Protect user accounts with Cross-Account Protection page for more information on how to implement Cross Account Protection and for the full list of available events.

Using OAuth 2.0 for Server to Server Applications

Overview

Creating a service account

Delegating domain-wide authority to the service account

Preparing to make a delegated API call

Java

Delegate domain-wide authority

Python

HTTP/REST

Creating a JWT

Forming the JWT header

Forming the JWT claim set

Required claims

Additional claims

Encoding the JWT claim set

Computing the signature

Making the access token request

Handling the response

Calling Google APIs

Java

Python

HTTP/REST

HTTP GET examples

curl examples

When access tokens expire

JWT error codes

Addendum: Service account authorization without OAuth

Java

Python

Implementing Cross-Account Protection

`curl` examples