auth/README.md

# auth

This GitHub Action establishes authentication to Google Cloud. It supports
traditional authentication via a Google Cloud Service Account Key JSON and
authentication via [Workload Identity Federation][wif].

Workload Identity Federation is the recommended approach as it obviates the need
to export a long-lived Google Cloud service account key and establishes a trust
delegation relationship between a particular GitHub Actions workflow invocation
and permissions on Google Cloud.

#### With Service Account Key JSON

1.  Create a Google Cloud service account and grant IAM permissions
1.  Export the long-lived JSON service account key
1.  Upload the JSON service account key to a GitHub secret

#### With Workload Identity Federation

1.  Create a Google Cloud service account and grant IAM permissions
1.  Create and configure a Workload Identity Provider for GitHub
1.  Exchange the GitHub Actions OIDC token for a short-lived Google Cloud access
    token


## Prerequisites

-   For authenticating via Google Cloud Service Account Keys, you must create an
    export a Google Cloud Service Account Key in JSON format.

-   For authenticating via Workload Identity Federation, you must create and
    configure a Google Cloud Workload Identity Provider. See [setup](#setup)
    for instructions.


## Usage

```yaml
jobs:
  job_id:
    # ...

    # Add "id-token" with the intended permissions.
    permissions:
      contents: 'read'
      id-token: 'write'

    steps:
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: 'google-github-actions/auth@v0.3.1'
      with:
        token_format: 'access_token'
        workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
        service_account: 'my-service-account@my-project.iam.gserviceaccount.com'

    # Example of using the token:
    - name: 'Access secret'
      run: |-
        curl https://secretmanager.googleapis.com/v1/projects/my-project/secrets/my-secret/versions/1:access \
          --header "Authorization: Bearer ${{ steps.auth.outputs.access_token }}"        
```

Note that changing the `permissions` block may remove some default permissions.
See the [permissions documentation][github-perms] for more information.

See [Examples](#examples) for more examples.


## Inputs

### Authenticating via Workload Identity Federation

The following inputs are for _authenticating_ to Google Cloud via Workload
Identity Federation.

-   `workload_identity_provider`: (Required) The full identifier of the Workload Identity
    Provider, including the project number, pool name, and provider name. If
    provided, this must be the full identifier which includes all parts:

    ```text
    projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider
    ```

-   `service_account`: (Required) Email address or unique identifier of the Google Cloud
    service account for which to generate credentials. For example:

    ```text
    my-service-account@my-project.iam.gserviceaccount.com
    ```

-   `audience`: (Optional) The value for the audience (`aud`) parameter in the
    generated GitHub Actions OIDC token. This value defaults to the value of
    `workload_identity_provider`, which is also the default value Google Cloud
    expects for the audience parameter on the token. We do not recommend
    changing this value.

### Authenticating via Service Account Key JSON

The following inputs are for _authenticating_ to Google Cloud via a Service
Account Key JSON. **We recommend using Workload Identity Federation instead as
exporting a long-lived Service Account Key JSON credential poses a security
risk.**

-   `credentials_json`: (Required) The Google Cloud JSON service account key to
    use for authentication. To generate access tokens or ID tokens using this
    service account, you must grant the underlying service account
    `roles/iam.serviceAccountTokenCreator` permissions on itself.

### Generating OAuth 2.0 access tokens

The following inputs are for _generating_ OAuth 2.0 access tokens for
authenticating to Google Cloud as an output for use in future steps in the
workflow.

-   `token_format`: This value must be `"access_token"` to generate OAuth 2.0
    access tokens. To skip token generation, omit or set to the empty string "".

-   `access_token_lifetime`: (Optional) Desired lifetime duration of the access
    token, in seconds. This must be specified as the number of seconds with a
    trailing "s" (e.g. 30s). The default value is 1 hour (3600s).

-   `access_token_scopes`: (Optional) List of OAuth 2.0 access scopes to be
    included in the generated token. This is only valid when "token_format" is
    "access_token". The default value is:

    ```text
    https://www.googleapis.com/auth/cloud-platform
    ```

### Generating ID tokens

The following inputs are for _generating_ ID tokens for authenticating to Google
Cloud as an output for use in future steps in the workflow.

-   `token_format`: This value must be `"id_token"` to generate ID tokens. To
    skip token generation, omit or set to the empty string "".

-   `id_token_audience`: (Required) The audience for the generated ID Token.

-   `id_token_include_email`: (Optional) Optional parameter of whether to
    include the service account email in the generated token. If true, the token
    will contain "email" and "email_verified" claims. This is only valid when
    "token_format" is "id_token". The default value is false.

### Other inputs

The following inputs are for controlling the behavior of this GitHub Actions,
regardless of the authentication mechanism.

-   `project_id`: (Optional) Custom project ID to use for authentication and
    exporting into other steps. If unspecified, the project ID will be extracted
    from the Workload Identity Provider or the Service Account Key JSON.

-   `create_credentials_file`: (Optional) If true, the action will securely
     generate a credentials file which can be used for authentication via gcloud
     and Google Cloud SDKs in other steps in the workflow. The default is true.

-   `delegates`: (Optional) List of additional service account emails or unique
    identities to use for impersonation in the chain. By default there are no
    delegates.


## Outputs

-   `project_id`: Provided or extracted value for the Google Cloud project ID.

-   `credentials_file_path`: Path on the local filesystem where the generated
    credentials file resides. This is only available if
    "create_credentials_file" was set to true.

-   `access_token`: The Google Cloud access token for calling other Google Cloud
    APIs. This is only available when "token_format" is "access_token".

-   `access_token_expiration`: The RFC3339 UTC "Zulu" format timestamp for the
    access token. This is only available when "token_format" is "access_token".

-   `id_token`: The Google Cloud ID token. This is only available when
    "token_format" is "id_token".

## Examples

### Authenticating via Workload Identity Federation

This example demonstrates authenticating via Workload Identity Federation. For
more information on how to setup and configure Workload Identity Federation, see
[#setup](#setup).

```yaml
jobs:
  job_id:
    # ...

    # Add "id-token" with the intended permissions.
    permissions:
      contents: 'read'
      id-token: 'write'

    steps:
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: `google-github-actions/auth@v0.3.1'
      with:
        workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
        service_account: 'my-service-account@my-project.iam.gserviceaccount.com'
```

### Authenticating via Service Account Key JSON

This example demonstrates authenticating via a Google Cloud Service Account Key
JSON. **We recommend using Workload Identity Federation instead as exporting a
long-lived Service Account Key JSON credential poses a security risk.**

This example assumes you have created a GitHub Secret named 'GOOGLE_CREDENTIALS'
with the contents being an export Google Cloud Service Account Key JSON. See
[Creating and managing Google Cloud Service Account
Keys](https://cloud.google.com/iam/docs/creating-managing-service-account-keys)
for more information.

```yaml
jobs:
  job_id:
    # ...

    steps:
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: `google-github-actions/auth@v0.3.1'
      with:
        credentials_json: '${{ secrets.GOOGLE_CREDENTIALS }}'
```

### Configuring gcloud

This example demonstrates using this GitHub Action to configure authentication
for the `gcloud` CLI tool. Note this does **NOT** work for the `gsutil` tool.

```yaml
jobs:
  job_id:
    # ...

    # Add "id-token" with the intended permissions.
    permissions:
      contents: 'read'
      id-token: 'write'

    steps:
    # Install gcloud, do not specify authentication.
    - uses: 'google-github-actions/setup-gcloud@master'
      with:
        project_id: 'my-project'

    # Configure Workload Identity Federation via a credentials file.
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: 'google-github-actions/auth@v0.3.1'
      with:
        workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
        service_account: 'my-service-account@my-project.iam.gserviceaccount.com'

    # Authenticate using the created credentials file.
    #
    # WARNING: The --cred-file flag is in preview and is subject to change.
    - id: 'gcloud'
      name: 'gcloud'
      run: |-
        gcloud auth login --brief --cred-file="${{ steps.auth.outputs.credentials_file_path }}"

        # Now you can run gcloud commands authenticated as the impersonated service account.
        gcloud secrets versions access "latest" --secret "my-secret"        
```

### Generating an OAuth 2.0 Access Token

This example demonstrates using this GitHub Action to generate an OAuth 2.0
Access Token for authenticating to Google Cloud. Most Google Cloud APIs accept
this access token as authentication.

The default lifetime is 1 hour, but you can request up to 12 hours if you set
the [`constraints/iam.allowServiceAccountCredentialLifetimeExtension` organization policy](https://cloud.google.com/resource-manager/docs/organization-policy/org-policy-constraints).

Note: If you authenticate via `credentials_json`, the service account must have
`roles/iam.serviceAccountTokenCreator` on itself.

```yaml
jobs:
  job_id:
    # ...

    # Add "id-token" with the intended permissions.
    permissions:
      contents: 'read'
      id-token: 'write'

    steps:
    # Configure Workload Identity Federation and generate an access token.
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: 'google-github-actions/auth@v0.3.1'
      with:
        token_format: 'access_token'
        workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
        service_account: 'my-service-account@my-project.iam.gserviceaccount.com'
        access_token_lifetime: '300s' # optional, default: '3600s' (1 hour)

    # Example of using the output. The token is usually provided as a Bearer
    # token.
    - id: 'access-secret'
      run: |-
        curl https://secretmanager.googleapis.com/v1/projects/my-project/secrets/my-secret/versions/1:access \
          --header "Authorization: Bearer ${{ steps.auth.outputs.access_token }}"        
```

### Generating an ID Token (JWT)

This example demonstrates using this GitHub Action to generate a Google Cloud ID
Token for authenticating to Google Cloud. This is most commonly used when
invoking a Cloud Run service.

Note: If you authenticate via `credentials_json`, the service account must have
`roles/iam.serviceAccountTokenCreator` on itself.

```yaml
jobs:
  job_id:
    # ...

    # Add "id-token" with the intended permissions.
    permissions:
      contents: 'read'
      id-token: 'write'

    steps:
    # Configure Workload Identity Federation and generate an access token.
    - id: 'auth'
      name: 'Authenticate to Google Cloud'
      uses: 'google-github-actions/auth@v0.3.1'
      with:
        token_format: 'access_token'
        workload_identity_provider: 'projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider'
        service_account: 'my-service-account@my-project.iam.gserviceaccount.com'
        id_token_audience: 'https://myapp-uvehjacqzq.a.run.app' # required, value depends on target
        id_token_include_email: true # optional

    # Example of using the output. The token is usually provided as a Bearer
    # token.
    - id: 'invoke-service'
      run: |-
        curl https://myapp-uvehjacqzq.a.run.app \
          --header "Authorization: Bearer ${{ steps.auth.outputs.id_token }}"        
```


<a id="setup"></a>

## Setting up Workload Identity Federation

To exchange a GitHub Actions OIDC token for a Google Cloud access token, you
must create and configure a Workload Identity Provider. These instructions use
the [gcloud][gcloud] command-line tool.

1.  Create or use an existing Google Cloud project. You must have privileges to
    create Workload Identity Pools, Workload Identity Providers, and to manage
    Service Accounts and IAM permissions. Save your project ID as an environment
    variable. The rest of these steps assume this environment variable is set:

    ```sh
    export PROJECT_ID="my-project" # update with your value
    ```

1.  (Optional) Create a Google Cloud Service Account. If you already have a
    Service Account, take note of the email address and skip this step.

    ```sh
    gcloud iam service-accounts create "my-service-account" \
      --project "${PROJECT_ID}"
    ```

1.  (Optional) Grant the Google Cloud Service Account permissions to access
    Google Cloud resources. This step varies by use case. For demonstration
    purposes, you could grant access to a Google Secret Manager secret or Google
    Cloud Storage object.

1.  Enable the IAM Credentials API:

    ```sh
    gcloud services enable iamcredentials.googleapis.com \
      --project "${PROJECT_ID}"
    ```

1.  Create a Workload Identity Pool:

    ```sh
    gcloud iam workload-identity-pools create "my-pool" \
      --project="${PROJECT_ID}" \
      --location="global" \
      --display-name="Demo pool"
    ```

1.  Get the full ID of the Workload Identity Pool:

    ```sh
    gcloud iam workload-identity-pools describe "my-pool" \
      --project="${PROJECT_ID}" \
      --location="global" \
      --format="value(name)"
    ```

    Save this value as an environment variable:

    ```sh
    export WORKLOAD_IDENTITY_POOL_ID="..." # value from above
    ```


1.  Create a Workload Identity Provider in that pool:

    ```sh
    gcloud iam workload-identity-pools providers create-oidc "my-provider" \
      --project="${PROJECT_ID}" \
      --location="global" \
      --workload-identity-pool="my-pool" \
      --display-name="Demo provider" \
      --attribute-mapping="google.subject=assertion.sub,attribute.actor=assertion.actor,attribute.aud=assertion.aud" \
      --issuer-uri="https://token.actions.githubusercontent.com"
    ```

    The attribute mappings map claims in the GitHub Actions JWT to assertions
    you can make about the request (like the repository or GitHub username of
    the principal invoking the GitHub Action). These can be used to further
    restrict the authentication using `--attribute-condition` flags.

    For example, you can map the attribute repository values (which can be used
    later to restrict the authentication to specific repositories):

    ```sh
    --attribute-mapping="google.subject=assertion.sub,attribute.repository=assertion.repository"
    ```

1.  Allow authentications from the Workload Identity Provider to impersonate the
    Service Account created above:

    **Warning**: This grants access to any resource in the pool (all GitHub
    repos). It's **strongly recommended** that you map to a specific attribute
    such as the actor or repository name instead. See [mapping external
    identities][map-external] for more information.

    ```sh
    gcloud iam service-accounts add-iam-policy-binding "my-service-account@${PROJECT_ID}.iam.gserviceaccount.com" \
      --project="${PROJECT_ID}" \
      --role="roles/iam.workloadIdentityUser" \
      --member="principalSet://iam.googleapis.com/${WORKLOAD_IDENTITY_POOL_ID}/*"
    ```

    To map to a specific repository:

    ```sh
    gcloud iam service-accounts add-iam-policy-binding "my-service-account@${PROJECT_ID}.iam.gserviceaccount.com" \
      --role="roles/iam.workloadIdentityUser" \
      --member="principalSet://iam.googleapis.com/${WORKLOAD_IDENTITY_POOL_ID}/attribute.repository/username/repo"
    ```

1.  Use this GitHub Action with the Workload Identity Provider ID and Service
    Account email. The GitHub Action will mint a GitHub OIDC token and exchange
    the GitHub token for a Google Cloud access token (assuming the authorization
    is correct). This all happens without exporting a Google Cloud service
    account key JSON!

    Note: It can take **up to 5 minutes** from when you configure the Workload
    Identity Pool mapping until the permissions are available.

## GitHub Token Format

Here is a sample GitHub Token for reference for attribute mappings:

```json
{
  "jti": "...",
  "sub": "repo:username/reponame:ref:refs/heads/master",
  "aud": "https://iam.googleapis.com/projects/123456789/locations/global/workloadIdentityPools/my-pool/providers/my-provider",
  "ref": "refs/heads/master",
  "sha": "d11880f4f451ee35192135525dc974c56a3c1b28",
  "repository": "username/reponame",
  "repository_owner": "reponame",
  "run_id": "1238222155",
  "run_number": "18",
  "run_attempt": "1",
  "actor": "username",
  "workflow": "OIDC",
  "head_ref": "",
  "base_ref": "",
  "event_name": "push",
  "ref_type": "branch",
  "job_workflow_ref": "username/reponame/.github/workflows/token.yml@refs/heads/master",
  "iss": "https://token.actions.githubusercontent.com",
  "nbf": 1631718827,
  "exp": 1631719727,
  "iat": 1631719427
}
```

[wif]: https://cloud.google.com/iam/docs/workload-identity-federation
[gcloud]: https://cloud.google.com/sdk
[map-external]: https://cloud.google.com/iam/docs/access-resources-oidc#impersonate
[github-perms]: https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#permissions