Add docs for OIDC with Gsuite

Author: sourishkrout

docs/GETTING_STARTED.md

@@ -453,6 +453,70 @@ $ step ca renew site.crt site.key
 error renewing certificate: Unauthorized
 ```
 
+## Leverage G-Suite's OAuth OIDC as authenticate personal certificates for users
+
+To authenticate users with the CA you can leverage services that expose OAuth OpenID
+Connect identity providers. One of the most common provider and the one we'll use in
+this example is G-Suite.
+
+Navigate to the Google APIs developer console and pick a suitable project from the
+top navbar's dropdown.
+
+![Google Dev Console](oidc1.png)
+
+In the masthead navigation click **Credentials** (key symbol) and then "OAuth consent
+screen" from the subnav. Fill out naming details, all mandatory fields, and decide if
+your app is of type **Public** or **Internal**. Internal will make sure the access scope
+is bound to your G-Suite organization. **Publi** will let anybody with a Google Account
+log in, incl. `gmail.com` accounts.
+
+Move back to **Credentials** on the subnav and choose "OAuth client ID" from the
+**Create credentials** dropdown. Since OIDC will be used from the `step CLI` pick **Other**
+from the available options and pick a name (e.g. **Step CLI**).
+
+![Create credential](oidc2.png)
+
+On successful completion, a confirmation modal with both `clientID` and `clientSecret` will
+be presented. Please note that the `clientSecret` will allow applications access to the configured
+OAuth consent screen. However, it will not allow direct authentication of users without their own
+MfA credentials per account.
+
+![OIDC credentials](oidc3.png)
+
+Now using `clientID` and `clientSecret` run following command to add G-Suite as a provisioner to
+`step certificates`. Please see [`step ca provisioner add`](https://smallstep.com/docs/cli/ca/provisioner/add/)'s docs for all available configuration options and descriptions.
+
+```bash
+$ step ca provisioner add Google --type oidc --ca-config $(step path)/config/ca.json \
+  --client-id 972437157139-ssiqna0g4ibuhafl3pkrrcb52tbroekt.apps.googleusercontent.com \
+  --client-secret RjEk-GwKBvdsFAICiJhn_RiF \
+  --configuration-endpoint https://accounts.google.com/.well-known/openid-configuration \
+  --domain yourdomain.com --domain gmail.com
+```
+
+Start up the online CA or send a HUP signal if it's already running to pick up the new provisioner.
+Now users should be able to fetch certificates using the familiar `step ca certificate` flow:
+
+```bash
+$ step ca certificate sebastian@smallstep.com personal.crt personal.key
+Use the arrow keys to navigate: ↓ ↑ → ←
+What provisioner key do you want to use?
+    fYDoiQdYueq_LAXx2kqA4N_Yjf_eybe-wari7Js5iXI (admin)
+  ▸ 972437157139-ssiqna0g4ibuhafl3pkrrcb52tbroekt.apps.googleusercontent.com (Google)
+✔ Key ID: 972437157139-ssiqna0g4ibuhafl3pkrrcb52tbroekt.apps.googleusercontent.com (Google)
+✔ CA: https://localhost
+✔ Certificate: personal.crt
+✔ Private Key: personal.key
+
+$ step certificate inspect --short localhost.crt
+X.509v3 TLS Certificate (ECDSA P-256) [Serial: 2295...5799]
+  Subject:     localhost
+  Issuer:      Local CA Intermediate CA
+  Provisioner: admin [ID: fYDo...5iXI]
+  Valid from:  2019-03-26T19:02:58Z
+          to:  2019-03-27T19:02:58Z
+```
+
 ## Notes on Securing the Step CA and your PKI.
 
 In this section we recommend a few best practices when it comes to