OAuth2 / JWT / OpenID Connect for mocking auth... which isn't that different from doing it for real, actually.

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README.md

go-mockid

OAuth2 / JWT / OpenID Connect for mocking auth... which isn't that different from doing it for real, actually.

Enabling Google OAuth2 (Mid-2020)

Create an account at https://console.developers.google.com/apis/dashboard
Go back to https://console.developers.google.com/apis/dashboard
Create a New Project from the dropdown in the upper left that lists the current project name
Give the project a name such as Example Web App and accept its generated ID
Click "Create"

Add your test domain

Go back to https://console.developers.google.com/apis/dashboard
Select your new project from the upper-left drop-down
Select Domain Verification from the left hand side of the screen
Add your test domain (i.e. beta.example.com), but a domain that you actually own
Select Verify Ownership
Follow the specific instructions for adding a txt record to the subdomain you chose
Add a collaborator / co-owner if you wish

Enable OAuth2

Go back to https://console.developers.google.com/apis/dashboard
Select OAuth consent screen
Select External
Complete the consent screen form

Create Google Credentials

Go back to https://console.developers.google.com/apis/dashboard
Select Credentials from the left sidebar
Select OAuth ID
Select Web Application
Fill out the same test domain and test app name as before
Save the ID and Secret to a place you won't forget (perhaps a .gitignored .env)

Update your signin page.

You need to put your default scopes (i.e. profile email) and client ID in the meta tag of your login page HTML. profile is the minimum scope and is always returned.

<head>
  <meta name="google-signin-scope" content="email">
  <meta
    name="google-signin-client_id"
    content="xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.apps.googleusercontent.com"
  />
</head>

Although it should be possible to use an thin OAuth client, you'll probably want to start by including the (huge) Google platform.js

<script src="https://apis.google.com/js/platform.js" async defer></script>

You can start off with the Google's sign in button, but you need your own data-onsuccess callback. You can also adjust the data-scope per button to include more stuff. Scopes are defined at https://developers.google.com/identity/protocols/oauth2/scopes

<div
  class="g-signin2"
  data-onsuccess="ongsignin"
  data-scope="profile email https://www.googleapis.com/auth/spreadsheets.readonly https://www.googleapis.com/auth/drive.readonly"
></div>
<script>
  window.ongsignin = function (gauth) {
    // Note: this is a special prototype-style instance object with few
    // enumerable properties (which don't make sense). Requires API docs.
    // See https://developers.google.com/identity/sign-in/web
    console.log(goauth)
  };
</script>

Despite the documentation stating that passing a token as a query is deprecated and to use the Authorization header, the inspect token URL only supports the query parameter: GET https://oauth2.googleapis.com/tokeninfo?id_token=<token>

You can also validate the token with Google's public key
https://accounts.google.com/.well-known/openid-configuration
https://www.googleapis.com/oauth2/v3/certs (note that one of the Key IDs will match that of your kid)

While testing you'll probably want to revoke the app's permissions

Go to https://myaccount.google.com/permissions
Under "Third-party apps with account access" click "Manage third-party access" and search in the long list and click "Remove access".
Under "Signing in to other sites" click "Signing in with Google" and search in the list to revoke access
Active tokens will persist until they expire (1 hour), so you may need to clear cache, cookies, etc, which can be a pain

Sign out can be accomplished with a button that calls gapi.auth2.getAuthInstance().signOut().then(function() { });