docs update gcp kms vault native authentication

docs/gcp_kms.md

@@ -54,20 +54,33 @@ The key-ring name and location are required in the signatory configuration.
 
 ## **Application Access:**
 
-The below steps are for providing signatory with the permissions to access the google cloud account Key Management.
+Providing Signatory with the permissions to access GCP KMS will differ depending on whether or not Signatory is running inside or outside of GCP.
+One thing that each method has in common is creation of the IAM Service Account:
 
 * Select `IAM & ADMIN` from the menu and select `Service accounts`. Create a new service account or use an existing one with all the above permissions (Get, Sign & Import) granted.
-* Select the created/existing service account and within that create a new key and a prompt to download the application credentials will appear, select the JSON format.
-* The downloaded JSON file is needed in signatory config or can be assigned to the below environment variable.
 
-## **Environment variables**
+### **Authenticating with the Service Account from outside GCP:**
 
-`cloudkms` backend accepts GCP's standard `GOOGLE_APPLICATION_CREDENTIALS` environment variable
+* Select the created/existing service account and within that create a new key and a prompt to download the application credentials will appear, select the JSON format.
+* The downloaded JSON file is needed in signatory config or can be assigned to the below environment variable.
 
 ```sh
 export GOOGLE_APPLICATION_CREDENTIALS="signatory-testing-a7sdfew625aecb.json"
 ```
 
+### **Authenticating with the Service Account from GCP VM:**
+
+Do not download the service account credentials and place them on Signatory's file system, and do not use `GOOGLE_APPLICATION_CREDENTIALS` env var. Instead, edit the VM specifications for `Identity and API access` such that it selects the IAM Service Account.
+
+### **Authenticating with the Service Account from GKE pod:**
+
+Do not download the service account credentials and place them on Signatory's file system, and do not use `GOOGLE_APPLICATION_CREDENTIALS` env var. Best practice is to [use Workload Identity](https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity)  In short:
+
+* enable Workload Identity on the cluster
+* create a kubernetes Service Account and bind it to the IAM Service Account
+* annotate the kubernetes Service Account with the email address of the IAM Service Account
+* update the pod spec to include the `serviceAccountName` field, this is the name of the kubernetes Service Account
+
 ## **Getting a PKH**
 
 ```sh