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