» Google Cloud Auth Method

The gcp auth method allows authentication against Vault using Google credentials. It treats Google Cloud Platform (GCP) as a Trusted Third Party and expects a JSON Web Token (JWT) signed by Google credentials from the authenticating entity. This token can be generated through different GCP APIs depending on the type of entity.

This plugin is developed in a separate GitHub repository at hashicorp/vault-plugin-auth-gcp, but is automatically bundled in Vault releases. Please file all feature requests, bugs, and pull requests specific to the GCP plugin under that repository.

» Authentication

» Via the CLI

The default path is /gcp. If this auth method was enabled at a different path, specify -path=/my-path in the CLI.

$ vault login -method=gcp \
    role="my-role" \
    jwt="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."

In this example, the "role" is the name of a configured role. The "jwt" is a self-signed or Google-signed JWT token obtained using the signJwt API call.

Because the process to sign a service account JWT can be tedious, Vault includes a CLI helper to generate the JWT token given the service account and parameters. This process only applies to iam-type roles!

$ vault login -method=gcp \
    role="my-role" \
    jwt_exp="15m" \
    credentials=@path/to/credentials.json \
    project="my-project" \
    service_account="service-account@my-project.iam.gserviceaccounts.com"

This signs a properly formatted service account JWT and authenticates to Vault directly. For details on each field, please run vault auth help gcp.

» Via the API

$ curl \
    --request POST \
    --data '{"role":"my-role", "jwt":"eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..."}' \
    http://127.0.0.1:8200/v1/auth/gcp/login

The response will be in JSON. For example:

{
  "auth": {
    "client_token": "f33f8c72-924e-11f8-cb43-ac59d697597c",
    "accessor": "0e9e354a-520f-df04-6867-ee81cae3d42d",
    "policies": [
      "default",
      "dev",
      "prod"
    ],
    "metadata": {
      "role": "my-role",
      "service_account_email": "dev1@project-123456.iam.gserviceaccount.com",
      "service_account_id": "111111111111111111111"
    },
    "lease_duration": 2764800,
    "renewable": true
  }
}

» Configuration

Auth methods must be configured in advance before users or machines can authenticate. These steps are usually completed by an operator or configuration management tool.

  1. Enable the Google Cloud auth method:

    $ vault auth enable gcp
    
  2. Configure the auth method credentials:

    $ vault write auth/gcp/config \
        credentials=@/path/to/credentials.json
    

    If you are using instance credentials or want to specify credentials via an environment variable, you can skip this step. To learn more, see the Google Cloud Authentication section below.

  3. Create a named role:

    For an iam-type role:

    $ vault write auth/gcp/role/my-iam-role \
        type="iam" \
        project_id="my-project" \
        policies="dev,prod" \
        bound_service_accounts="my-service@my-project.iam.gserviceaccount.com"
    

    For a gce-type role:

    $ vault write auth/gcp/role/my-gce-role \
        type="gce" \
        project_id="my-project" \
        policies="dev,prod" \
        bound_zones="us-east1-b" \
        bound_labels="foo:bar,zip:zap"
    

    For the complete list of configuration options for each type, please see the API documentation.

» Google Cloud Authentication

The Google Cloud Vault auth method uses the official Google Cloud Golang SDK. This means it supports the common ways of providing credentials to Google Cloud.

  1. The environment variable GOOGLE_APPLICATION_CREDENTIALS. This is specified as the path to a Google Cloud credentials file, typically for a service account. If this environment variable is present, the resulting credentials are used. If the credentials are invalid, an error is returned.

  2. Default instance credentials. When no environment variable is present, the default service account credentials are used.

For more information on service accounts, please see the Google Cloud Service Accounts documentation.

To use this storage backend, the service account must have the following minimum scope(s):

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

» Workflow

This section describes the implementation details for how Vault communicates with Google Cloud to authenticate and authorize JWT tokens. This information is provided for those who are curious, but these implementation details are not required knowledge for using the auth method.

» IAM Login

IAM login applies only to roles of type iam. The Vault authentication workflow for IAM service accounts looks like this:

Vault Google Cloud IAM Login Workflow

  1. The client generates a signed JWT using the IAM projects.serviceAccounts.signJwt method. For examples of how to do this, see the Obtaining JWT Tokens section.

  2. The client sends this signed JWT to Vault along with a role name.

  3. Vault extracts the kid header value, which contains the ID of the key-pair used to generate the JWT, and the sub ID/email to find the service account key. If the service account does not exist or the key is not linked to the service account, Vault denies authentication.

  4. Vault authorizes the confirmed service account against the given role. If that is successful, a Vault token with the proper policies is returned.

» GCE Login

GCE login only applies to roles of type gce and must be completed on an instance running in GCE. These steps will not work from your local laptop or another cloud provider.

Vault Google Cloud GCE Login Workflow

  1. The client obtains an instance identity metadata token on a GCE instance.

  2. The client sends this JWT to Vault along with a role name.

  3. Vault extracts the kid header value, which contains the ID of the key-pair used to generate the JWT, to find the OAuth2 public cert to verify this JWT.

  4. Vault authorizes the confirmed instance against the given role, ensuring the instance matches the bound zones, regions, or instance groups. If that is successful, a Vault token with the proper policies is returned.

» Obtaining JWT Tokens

Vault expects a signed JWT token to verify against. There are a few ways to acquire a JWT token.

» Generating IAM Tokens

Vault includes a CLI helper for generating the signed JWT token and submitting it to Vault for iam-type roles. If you want to generate the JWT token yourself, follow this section.

» Shell Example

The expected format of the JWT request payload is:

{
  "sub": "$SERVICE_ACCOUNT",
  "aud": "vault/$ROLE",
  "exp": "$EXPIRATION" // optional
}

If specified, the expiration must be a NumericDate value (seconds from Epoch). This value must be before the max JWT expiration allowed for a role. This defaults to 15 minutes and cannot be more than 1 hour.

One you have all this information, the JWT token can be signed using curl and oauth2l:

ROLE="my-role"
PROJECT="my-project"
SERVICE_ACCOUNT="service-account@my-project.iam.gserviceaccount.com"
OAUTH_TOKEN="$(oauth2l header cloud-platform)"

curl \
  --header "${OAUTH_TOKEN}" \
  --header "Content-Type: application/json" \
  --request POST \
  --data "{\"aud\":\"vault/${ROLE}\", \"sub\": \"${SERVICE_ACCOUNT}\"}" \
  "https://iam.googleapis.com/v1/projects/${PROJECT}/serviceAccounts/${SERVICE_ACCOUNT}:signJwt"

» gcloud Example

gcloud beta iam service-accounts sign-jwt credentials.json - \
  --iam-account=service-account@my-project.iam.gserviceaccount.com \
  --project=my-project

» Golang Example

Read more on the Google Open Source blog.

» Generating GCE Tokens

GCE tokens can only be generated from a GCE instance. You must run these commands from the GCE instance. The JWT token can be obtained from the service-accounts/default/identity endpoint for a instance's metadata server.

ROLE="my-gce-role"
SERVICE_ACCOUNT="service-account@my-project.iam.gserviceaccount.com"

curl \
  --header "Metadata-Flavor: Google" \
  --get \
  --data-urlencode "aud=http://vault/${ROLE}" \
  --data-urlencode "format=full" \
  "http://metadata/computeMetadata/v1/instance/service-accounts/${SERVICE_ACCOUNT}/identity"

» API

The GCP Auth Plugin has a full HTTP API. Please see the API docs for more details.