---
layout: "docs"
page_title: "Auth Plugin Backend: GCP"
sidebar_current: "docs-auth-gcp"
description: |-
  The gcp backend plugin allows automated authentication of AWS entities.
---

# Auth Plugin Backend: gcp

The `gcp` plugin backend allows authentication against Vault using
Google credentials. It treats GCP as a Trusted Third Party and expects a
[JSON Web Token (JWT)](https://tools.ietf.org/html/rfc7519) signed by Google
credentials from the authenticating entity. This token can be generated through
different GCP APIs depending on the type of entity.

Currently supports authentication for:

  * GCP IAM service accounts (`iam`)
  * GCE IAM service accounts (`gce`)

We will update the documentation as we introduce more supported entities.

The following documentation assumes that the backend has been
[mounted](/docs/plugin/index.html) at `auth/gcp`.

~> Note: The `gcp` backend is implemented as a
[Vault plugin](/docs/internals/plugins.html) backend. You must be using Vault
v0.8.0+ to use plugins.

## Authentication Workflow

### IAM

The Vault authentication workflow for IAM service accounts is as follows:

  1. A client with IAM service account credentials generates a signed JWT using the IAM [projects.serviceAccounts.signJwt](https://cloud.google.com/iam/reference/rest/v1/projects.serviceAccounts/signJwt) method. See [usage](#the-iam-authentication-token) for the expected format and example code.
  2. The client sends this JWT to Vault in a login request with a role name. This role should have type `iam`.
  3. Vault grabs 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 will deny authentication.
  4. Vault authorizes the confirmed service account against the given role. See [authorization section](#authorization-workflow) to see how each type of role handles authorization.

[![IAM Login Workflow](/assets/images/vault-gcp-iam-auth-workflow.svg)](/assets/images/vault-gcp-iam-auth-workflow.svg)

#### The `iam` Authentication Token

The expected format of the JWT payload is as follows:

```json
{
  "sub" : "[SERVICE ACCOUNT IDENTIFIER]",
  "aud" : "vault/[ROLE NAME]",
  "exp" : "[EXPIRATION]"
}
```

* `[SERVICE ACCOUNT ID OR EMAIL]`: Either the email or the unique ID of a service account.
* `[ROLE NAME]`: Name of the role that this token will be used to login against. The full expected `aud` string should end in "vault/$roleName".
* `[EXPIRATION]` : A [NumericDate](https://tools.ietf.org/html/rfc7519#section-2) value (seconds from Epoch). This value must be before the max JWT expiration allowed for a role (see `max_jwt_exp` parameter for creating a role). This defaults to 15 minutes and cannot be more than a hour.

**Note:** By default, we enforce a shorter `exp` period than the default length
for a given token (1 hour) in order to make reuse of tokens difficult. You can
customize this value for a given role but it will be capped at an hour.

To generate this token, we use the Google IAM API method [projects.serviceAccounts.signJwt](https://cloud.google.com/iam/reference/rest/v1/projects.serviceAccounts/signJwt).
See an [example of how to generate this token](#generating-iam-token).

### GCE

The Vault authentication workflow for GCE instances is as follows:

  1. A client logins into a GCE instances and [obtains an instance identity metadata token](https://cloud.google.com/compute/docs/instances/verifying-instance-identity).
  2. The client request to login using this token (a JWT) and gives a role name to Vault.
  3. Vault uses 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. See the [authorization section](#authorization-workflow) to see how each type of role handles authorization.

[![GCE Login Workflow](/assets/images/vault-gcp-gce-auth-workflow.svg)](/assets/images/vault-gcp-gce-auth-workflow.svg)

#### The `gce` Authentication Token

The token can be obtained from the `service-accounts/default/identity` endpoint for a instance's 
[metadata server](https://cloud.google.com/compute/docs/storing-retrieving-metadata). You can use the 
[example of how to obtain an instance metadata token](#generating-gce-token) to get started. 

Learn more about the JWT format from the 
[documentation](https://cloud.google.com/compute/docs/instances/verifying-instance-identity#token_format) 
for the identity metadata token. The params the user provides are:

* `[AUD]`: The full expected `aud` string should end in "vault/$roleName". Note that Google requires the `aud` 
    claim to contain a scheme or authority but Vault will only check for a suffix.
* `[FORMAT]`: MUST BE `full` for Vault. Format of the metadata token generated (`standard` or `full`).

### Examples for Obtaining Auth Tokens

#### Generating IAM Token

**HTTP Request Example**

This uses [Google API HTTP annotation](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto). 
Note the `$PAYLOAD` must be a marshaled JSON string with escaped double quotes.

```sh
#!/bin/sh
# [START PARAMS]
ROLE="test-role"
PROJECT="project-123456"
SERVICE_ACCOUNT="my-account@project-123456.iam.gserviceaccount.com"
OAUTH_TOKEN=$(oauth2l header cloud-platform)
# [END PARAMS]


PAYLOAD=$(echo "{ \"aud\": \"vault/$ROLE\", \"sub\": \"$SERVICE_ACCOUNT\"}" | sed -e 's/"/\\&/g')
curl -H "$OAUTH_TOKEN" \
    -H "Content-Type: application/json" \
    -X POST -d "{\"payload\":\"$PAYLOAD\"}" https://iam.googleapis.com/v1/projects/$PROJECT/serviceAccounts/$SERVICE_ACCOUNT:signJwt```
```

**Golang Example**

We use the Go OAuth2 libraries, GCP IAM API, and Vault API. The example generates a token valid for the `dev-role` role (as indicated by the `aud` field of `jwtPayload`).

```go
// Abbreviated imports to show libraries.
import (
	vaultapi "github.com/hashicorp/vault/api"
	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/iam/v1"
	...
)

func main() {
	// Start [PARAMS]
	project := "project-123456"
	serviceAccount := "myserviceaccount@project-123456.iam.gserviceaccount.com"
	credsPath := "path/to/creds.json"

	os.Setenv("VAULT_ADDR", "https://vault.mycompany.com")
	defer os.Setenv("VAULT_ADDR", "")
	// End [PARAMS]

	// Start [GCP IAM Setup]
	jsonBytes, err := ioutil.ReadFile(credsPath)
	if err != nil {
		log.Fatal(err)
	}
	config, err := google.JWTConfigFromJSON(jsonBytes, iam.CloudPlatformScope)
	if err != nil {
		log.Fatal(err)
	}

	httpClient := config.Client(oauth2.NoContext)
	iamClient, err := iam.New(httpClient)
	if err != nil {
		log.Fatal(err)
	}
	// End [GCP IAM Setup]

	// 1. Generate signed JWT using IAM.
	resourceName := fmt.Sprintf("projects/%s/serviceAccounts/%s", project, serviceAccount)
	jwtPayload := map[string]interface{}{
		"aud": "vault/dev-role",
		"sub": serviceAccount,
		"exp": time.Now().Add(time.Minute * 10).Unix(),
	}

	payloadBytes, err := json.Marshal(jwtPayload)
	if err != nil {
		log.Fatal(err)
	}
	signJwtReq := &iam.SignJwtRequest{
		Payload: string(payloadBytes),
	}

	resp, err := iamClient.Projects.ServiceAccounts.SignJwt(resourceName, signJwtReq).Do()
	if err != nil {
		log.Fatal(err)
	}

	// 2. Send signed JWT in login request to Vault.
	vaultClient, err := vaultapi.NewClient(vaultapi.DefaultConfig())
	if err != nil {
		log.Fatal(err)
	}

	vaultResp, err := vaultClient.Logical().Write(
		"auth/gcp/login",
		map[string]interface{}{
			"role": "test",
			"jwt":  resp.SignedJwt,
	})

	if err != nil {
		log.Fatal(err)
	}

	// 3. Use auth token from response.
	log.Println("Access token: %s", vaultResp.Auth.ClientToken)
	vaultClient.SetToken(vaultResp.Auth.ClientToken)
	// ...
}
```

#### Generating GCE Token

**HTTP Request Example**

This uses [Google API HTTP annotation](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto)
and must be run on a GCE instance.

```sh
# [START PARAMS]
VAULT_ADDR="https://127.0.0.1:8200/"
ROLE="my-gce-role"
SERVICE_ACCOUNT="default" # replace with an instance's service account if needed
# [END PARAMS]

curl -H "Metadata-Flavor: Google"\
     -G 
     --data-urlencode "audience=$VAULT_ADDR/vault/$ROLE"\
     --data-urlencode "format=full" \
     "http://metadata/computeMetadata/v1/instance/service-accounts/$SERVICE_ACCOUNT/identity"
```

## Authorization Workflow

For `gcp`, login is per-role. Each role has a specific set of restrictions that
an authorized entity must fit in order to login. These restrictions are specific
to the role type.

Currently supported role types are:

* `iam` (Supports both IAM and inference for GCE tokens)
* `gce` (Only supports GCE tokens)

Vault validates an authenticated entity against the role and uses the role to
determine information about the lease, including Vault policies assigned and
TTLs. For a full list of accepted restrictions, see [role API docs](/api/auth/gcp/index.html#create-role).

If a GCE token is provided for login under an `iam` role, the service account associated with the token
(`sub` claim) is inferred and used to login.

## Usage

### Via the CLI.

#### Enable GCP authentication in Vault

```
$ vault auth-enable gcp
```

#### Configure the GCP authentication backend

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

**Configuration**: This includes GCP credentials Vault will use these to make calls to
GCP APIs. If credentials are not configured or if the user explicitly sets the
config with no credentials, the Vault server will attempt to use
[Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials)
as set on the Vault server.

See [API documentation](/api/auth/gcp/index.html#configure)
to learn more about parameters.

#### Create a role

```
$ vault write auth/gcp/role/dev-role \
    type="iam" \
    project_id="project-123456" \
    policies="prod,dev" \
    bound_service_accounts="serviceaccount1@project1234.iam.gserviceaccount.com,uuid123,..."
    ...
``` 

**Roles**: Roles are associated with an authentication type/entity and a set of
Vault [policies](/docs/concepts/policies.html). Roles are configured with constraints
specific to the authentication type, as well as overall constraints and
configuration for the generated auth tokens.

We also expose a helper path for updating the service accounts attached to an existing `iam` role:

    ```sh
    vault write auth/gcp/role/iam-role/service-accounts \
      add='serviceAccountToAdd,...' \
      remove='serviceAccountToRemove,...' \
    ```

and for updating the labels attached to an existing `gce` role:
    
    ```sh
    vault write auth/gcp/role/gce-role/labels \
      add='label1:value1,foo:bar,...' \
      remove='key1,key2,...' \
    ```
    
    
See [API docs](/api/auth/gcp/index.html#create-role) to view
parameters for role creation and updates.

#### Login to get a Vault Token

Once the backend is setup and roles are registered with the backend,
the user can login against a specific role.

```
$ vault write auth/gcp/login role='dev-role' jwt='eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...'
```

The `role` and `jwt` parameters are required. These map to the name of the
role to login against, and the signed JWT token for authenticating a role
respectively. The format of the provided JWT differs depending on the
authenticating entity.

### Via the API

#### Enable GCP authentication in Vault

```
$ curl $VAULT_ADDR/v1/sys/auth/gcp -d '{ "type": "gcp" }'
```

#### Configure the GCP authentication backend

```
$ curl $VAULT_ADDR/v1/auth/gcp/config \
-d '{  "credentials": "{...}" }'
```

#### Create a role

```
$ curl $VAULT_ADDR/v1/auth/gcp/role/dev-role \
-d '{ "type": "iam", "project_id": "project-123456", ...}'
```

#### Login to get a Vault Token

The endpoint for the GCP login is `auth/gcp/login`.

The `gcp` mountpoint value in the url is the default mountpoint value.
If you have mounted the `gcp` backend with a different mountpoint, use that value.

The `role` and `jwt` should be sent in the POST body encoded as JSON.

```
$ curl $VAULT_ADDR/v1/auth/gcp/login \
    -d '{ "role": "dev-role", "jwt": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." }'
```

The response will be in JSON. For example:

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

## Contributing

This plugin is developed in a separate Github repository: [`hashicorp/vault-plugin-auth-gcp`](https://github.com/hashicorp/vault-plugin-auth-gcp). Please file all feature requests, bugs, and pull requests specific to the GCP plugin under that repository.

## API

The GCP Auth Plugin has a full HTTP API. Please see the
[API docs](/api/auth/gcp/index.html) for more details.