google-cloud-recipe-auth

SKILL.md

Authenticating to Google Cloud

Authentication is the
process of proving who you are. In Google Cloud, you represent a
Principal (an identity like a user or a service). This is the first step
before Authorization
(determining what you can do).

Authentication

Clarifying Questions for the Agent

Before providing a specific solution, clarify the following with the user:

Who or what is authenticating? (A human developer, a local script, or an
application running in production?)

Where is the code running? (Local laptop, Compute
Engine,
GKE, Cloud
Run, or another cloud like
AWS/Azure?)

What is the target? (A Google Cloud API like Storage/BigQuery, or a
custom application you built?)

Are you using a high-level client library? (e.g., Python, Go, Node.js
libraries usually handle ADC automatically.)

Human Authentication

For users to access Google Cloud, they need an identity that Google Cloud can
recognize.

Types of User Identities

Google Cloud supports several ways to configure identities for your internal
workforce (developers, administrators, employees):

Google-Managed
Accounts:
You can use Cloud Identity or Google Workspace to create managed user
accounts. These are called managed accounts because your organization
controls their lifecycle and configuration.

Federation using Cloud Identity or Google
Workspace:
You can federate identities to allow users to use their existing identity
and credentials to sign in to Google services. Users authenticate against an
external identity provider (IdP), but you must keep accounts synchronized
into Google Cloud using tools like Google Cloud Directory Sync (GCDS) or an
external authoritative source like Active Directory or Microsoft Entra ID.

Workforce Identity
Federation:
This lets you use an external IdP to authenticate and authorize a workforce
using IAM directly. Unlike standard federation, you do not need to
synchronize user identities from your existing IdP to Google Cloud
identities. It supports syncless, attribute-based single sign-on.

Methods of Access for Developers and Administrators

Used for interacting with Google Cloud resources and APIs during development and
management.

Google Cloud Console: The primary
web interface. You authenticate using your Google Account (Gmail or Google
Workspace).

gcloud CLI (gcloud auth login): Used to authenticate the CLI itself so you can run
management commands (e.g., gcloud compute instances list). It uses a
Credential (like an OAuth 2.0 refresh token) stored locally.

Local Development with App Default Credentials
(ADC)
(gcloud auth application-default login): This is different from CLI
auth. It creates a local JSON file that Google Cloud Client Libraries
(Python, Java, etc.) use to act as "you" when you run code on your laptop.

Service Account
Impersonation:
For security reasons, developers should avoid downloading Service Account
keys entirely. Instead, they should authenticate as humans (gcloud auth login) and use Service Account Impersonation to run CLI commands or
generate short-lived credentials. This is a critical best practice for local
development and troubleshooting.

For End-Users and Customers

Used when a human (who is not a developer) needs to access a web application
you've deployed on Google Cloud. Note: These are distinct from workforce
identities.

Identity-Aware Proxy (IAP):
Acts as a central authorization layer for web applications. It intercepts
web requests and verifies the user's identity (via Google Workspace, Cloud
Identity, or external providers) before letting them reach the application.
It's often used to protect internal apps without a VPN, or secure customer
portals.

Identity
Platform: A
Customer Identity and Access Management (CIAM) solution for adding consumer
sign-in (email/password, phone, social) directly into the code of your
custom-built applications.

Service-to-Service Authentication

When code runs in production, it should use a Service Account rather than a
human user account.

Service Accounts and Service Agents

Service
Account:
A special identity intended for non-human users. It's like a "robot
identity" with its own email address.

Service Agent:
A service account managed by Google that allows a service (like Pub/Sub) to
access your resources on your behalf.

Best Practice: Attaching Service Accounts

Instead of using Service Account Keys (dangerous JSON files), you should
attach a custom service account to the Google Cloud resource. The resource's
environment then provides a Token (a short-lived digital object) via a local
metadata server.

Compute
Engine:
Assign a service account during VM creation.

Cloud
Run:
Assign a service account in the service configuration.

Special Cases & Advanced Topics

Kubernetes Engine (GKE)

Use Workload Identity Federation for
GKE
to map Kubernetes identities to IAM principal identifiers. This grants specific
Kubernetes workloads access to specific Google Cloud APIs. Learn more
here.

External Workloads (Workload Identity Federation)

For code running outside Google Cloud (e.g., AWS, Azure, or on-prem), do not
use keys. Instead, use Workload Identity Federation to exchange an external
token (like an AWS IAM role) for a short-lived Google Cloud access token.

API Keys

API keys are encrypted strings used for public data (e.g., Google Maps) or
simplified access like Vertex AI Express
Mode,
which allows fast testing of Gemini models without complex setup. Both humans
and services (e.g., Cloud Run-based AI agent) can use API keys, for the services
that support it.

Note: API keys should be
restricted
to specific APIs and projects to minimize security risks. Store API keys in a
secrets manager like Secret
Manager to prevent
accidental exposure.

OAuth 2.0 Access Scopes

While IAM is the modern way to handle authorization, legacy Compute Engine VMs
and GKE node pools still rely on Access Scopes alongside IAM. If a VM's
scope is restricted, the attached service account will fail to make API calls
even if it has the correct IAM permissions. Check this first if attached service
accounts are failing unexpectedly.

Short-Lived Credentials

The underlying mechanism for impersonation and secure service-to-service
communication is the IAM Service Account Credentials API. This API generates
short-lived access tokens, OpenID Connect (OIDC) ID tokens, or self-signed JSON
Web Tokens (JWTs) dynamically, removing the need for static credentials.

Authorization

After Authentication, Google Cloud uses Identity and Access Management
(IAM) to determine what the
authenticated principal can do.

Allow Policy: A record that binds a Principal to a Role on a
Resource.

Predefined
Roles:
Prebuilt roles like roles/storage.objectViewer or
roles/bigquery.dataEditor. Always try to use these first.

Custom
Roles:
User-defined collections of specific permissions if predefined roles are too
broad.

Examples

Human-to-Service (Local Python Development)

Authn: Run gcloud auth application-default login to create local
credentials (ADC).

Authz: Grant your email the roles/storage.objectViewer role on a
bucket.

Code: Use the Python storage.Client(). It automatically finds your
local credentials via ADC. Note: ADC searches in a specific order—first
checking the GOOGLE_APPLICATION_CREDENTIALS environment variable, then the
local gcloud JSON file, and finally the attached service account metadata
server.

Service-to-Service (Cloud Run to Cloud SQL)

Authn: Attach a custom Service Account to your Cloud Run service.

Authz: Grant that Service Account the roles/cloudsql.client role on
the project.

Code: The Cloud Run environment provides the token automatically to the
connection driver.

Calling a Custom Application (OIDC)

When calling a private Cloud Run service from another service, the caller
generates a Google-signed OpenID Connect (OIDC) ID Token and passes it in
the Authorization: Bearer <TOKEN> header.

Validation Checklist

 Is the user running code locally? Suggest gcloud auth application-default login or Service Account Impersonation.

 Is the user attempting to use Service Account keys locally? Strongly
discourage this and recommend impersonation.

 Is the user running in production? Recommend attaching a custom,
least-privilege service account, NOT using keys.

 Is the user relying on the Compute Engine Default Service Account?
Recommend creating a custom service account instead.

 Is the user running on another cloud? Recommend Workload Identity
Federation.

 Is the user calling a custom app? Recommend OIDC ID Tokens.

 Has the user restricted their API Keys? Check for appropriate API Key
Restrictions.

References

Authentication Overview

User Identities

Application Default Credentials

Service Account Best Practices

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