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Authenticating users of your web app is often necessary, and usually requires special programming in your app. For Google Cloud Platform apps you can hand those responsibilities off to the Identity-Aware Proxy service. If you only need to restrict access to selected users there are no changes necessary to the application. Should the application need to know the user's identity (such as for keeping user preferences server-side) Identity-Aware Proxy can provide that with minimal application code.

What is Identity-Aware Proxy?

Identity-Aware Proxy (IAP) is a Google Cloud Platform service that intercepts web requests sent to your application, authenticates the user making the request using the Google Identity Service, and only lets the requests through if they come from a user you authorize. In addition, it can modify the request headers to include information about the authenticated user.

This codelab will walk you through creating your own application, restricting access to it, and getting user identity from IAP.

What you will build

In this codelab, you're going to build a minimal web application with Google App Engine, then explore various ways to use Identity-Aware Proxy to restrict access to the application and provide user identity information to it. Your app will:

  • Display a welcome page
  • Access user identity information provided by IAP
  • Use cryptographic verification to prevent spoofing of user identity information

What you'll learn

  • How to write and deploy a simple App Engine app using Python 3.7
  • How to enable and disable IAP to restrict access to your app
  • How to get user identity information from IAP into your app
  • How to cryptographically verify information from IAP to protect against spoofing

What you'll need

  • A modern web browser such as Chrome.
  • Basic knowledge of the Python programming language

This codelab is focused on Google App Engine and IAP. Non-relevant concepts and code blocks are glossed over and are provided for you to simply copy and paste.

You will work in the Cloud Shell command line environment. Start by opening that environment and fetching the sample code to it.

Launch the Console and Cloud Shell

In the upper left-hand part of the lab page, click the Open Google Console button. You will have to log in with the Username and Password shown below that button.

All commands in this codelab will be executed within a Cloud Shell for the project that was created and opened for you. Open the Cloud Shell by clicking the Activate Cloud Shell icon found at the right side of the console page header. The lower half of the page will allow you to enter and run commands.

The commands could be run from your own PC, but you would have to install and configure needed development software first. The Cloud Shell already has all the software tools you need.

Download the code

Click the command line area in the Cloud Shell so you can type commands. Fetch the code from Github and then change to the code folder:

git clone https://github.com/googlecodelabs/user-authentication-with-iap.git
cd iap-codelab

This folder contains one subfolder for each step of this codelab. You will change to the correct folder to perform each step.

This is an App Engine Standard application written in Python 3.7 that simply displays a "Hello, World" welcome page. We will deploy and test it, then restrict access to it using IAP.

Review the application code

Change from the main project folder to the 1-HelloWorld subfolder that contains code for this step.

cd 1-HelloWorld

The application code is in the main.py file. It uses the Flask web framework to respond to web requests with the contents of a template. That template file is in templates/index.html, and for this step contains only plain HTML. A second template file contains a skeletal example privacy policy in templates/privacy.html.

There are two other files: requirements.txt lists all the non-default Python libraries the application uses, and app.yaml tells Google Cloud Platform that this is a Python 3.7 App Engine application.

You can list each file in the shell using the cat command, as in:

cat main.py

Or you can open the Cloud Shell code editor by clicking the Pencil icon at the top right-hand side of the Cloud Shell window, and examine the code that way.

You do not need to change any files for this step.

Deploy to App Engine

Now deploy the app to the App Engine Standard environment for Python 3.7

gcloud app deploy

You may be asked to choose a region to deploy to. Select any one near to you that says it "supports standard". When you are asked if you want to continue, enter Y for yes.

In a few minutes the deploy should complete and you will see a message that you can view your application with gcloud app browse. Enter that command. If a new tab does not open in your browser, click the displayed link to open it in a new tab, or copy it to a manually opened new tab if necessary. Since this is the first time this app is run, it will take a few seconds to appear while a cloud instance is started, and you should see the following window.

You can open that same URL from any computer connected to the Internet to see that web page. Access is not yet restricted.

Restrict access with IAP

In the cloud console window, click the menu icon at the top left of the page, click on Security and then on Identity-Aware Proxy.

Since this is the first time you have enabled an authentication option for this project, you will see a message that you must configure your OAuth consent screen before you can use IAP.

Click the CONFIGURE CONSENT SCREEN BUTTON. A new tab will open to configure the consent screen.

Fill in the required blanks with appropriate values:

Application name

IAP Example

Support email

your email address. it may already be filled in for you.

Authorized domain

the hostname portion of the application's URL, e.g. iap-example-999999.appspot.com. You can see this in the address bar of the Hello World web page you previously opened. Do not include the starting https:// or trailing / from that URL.

You must press Enter after filling in this value.

Application homepage link

the URL you used to view your app

Application privacy Policy link

the privacy page link in the app, same as the homepage link with /privacy added to the end

Click Save. You will be prompted to create credentials. You do not need to create credentials for this codelab, so you can simply close this browser tab.

Return to the Identity-Aware Proxy page and refresh it. You should now see a list of resources you can protect.

Click the toggle button in the IAP column in the App Engine app row to turn IAP on.

You will see the domain names that will be protected by IAP. Click TURN ON.

Now open a browser tab and navigate to the URL for your app. You will be presented with a Sign in with Google screen requiring you to log in to access the app.

Sign in with a Google or GSuite account. You will see a screen denying you access.

You have successfully protected your app with IAP, but you have not yet told IAP which accounts to allow through.

Return to the Identity-Aware Proxy page of the console, select the checkbox next to App Engine app, and see the sidebar at the right of the page.

Each email address (or Google Group address, or GSuite domain name) that should be allowed access needs to be added as a Member. Click ADD MEMBER. Enter your email address, then pick the Cloud IAP/IAP-Secured Web App User role to assign to that address. You may enter more addresses or GSuite domains in the same way.

Click Save. The message "Policy Updated" will appear at the bottom of the window.

Navigate back to your app and reload the page. You should now see your web app, since you already logged in with a user you authorized. However, you may still see the "You don't have access" page since IAP may not recheck your authorization. In that case, do the following steps:

  • Open your web browser to the home page address with /_gcp_iap/clear_login_cookie added to the end of the URL, as in https://iap-example-999999.appspot.com/_gcp_iap/clear_login_cookie.
  • You will see a new Sign in with Google screen, with your account already showing. Do not click the account. Instead, click Use another account, and re-enter your credentials.
  • These steps cause IAP to recheck your access and you should now see your application's home screen.

If you have access to another browser or can use Incognito Mode in your browser, and have another valid GMail or GSuite account, you can use that browser to navigate to your app page and log in with the other account. Since that account has not been authorized, it will see the "You Don't Have Access" screen instead of your app.

Once an app is protected with IAP, it can use the identity information that IAP provides in the web request headers it passes through. In this step, the application will get the logged-in user's email address and a persistent unique user ID assigned by the Google Identity Service to that user. That data will be displayed to the user in the welcome page.

This is step 2, and the last step ended with your Cloud Shell open in the iap-codelab/1-HelloWorld folder. Change to the folder for this step:

cd ~/iap-codelab/2-HelloUser

Deploy to App Engine

Since deployment takes a few minutes, start by deploying the app to the App Engine Standard environment for Python 3.7:

gcloud app deploy

When you are asked if you want to continue, enter Y for yes. In a few minutes the deploy should complete. While you are waiting you can examine the application files as described below.

When the deployment is ready you will see a message that you can view your application with gcloud app browse. Enter that command. If a new tab does not open on your browser, copy the displayed link and open it in a new tab normally. You should see a page similar to the following:

You may need to wait a few minutes for the new version of your application to replace the prior version. Refresh the page if needed to see a page similar to the above.

Examine the application files

This folder contains the same set of files as seen in Step 1, but two of the files have been changed: main.py and templates/index.html. The program has been changed to retrieve the user information that IAP provides in request headers, and the template now displays that data.

There are two lines in main.py that get the IAP-provided identity data:

user_email = request.headers.get('X-Goog-Authenticated-User-Email')
user_id = request.headers.get('X-Goog-Authenticated-User-ID')

The X-Goog-Authenticated-User- headers are provided by IAP, and the names are case-insensitive, so they could be given in all lower or all upper case if preferred. The render_template statement now includes those values so they can be displayed:

page = render_template('index.html', email=user_email, id=user_id)

The index.html template can display those values by enclosing the names in doubled curly braces:

Hello, {{ email }}! Your persistent ID is {{ id }}.

As you can see, the provided data is prefixed with accounts.google.com:, showing where the information came from. Your application can remove everything up to and including the colon to get the raw values if desired.

Turn off IAP

What happens to this app if IAP is disabled, or somehow bypassed (such as by other applications running in your same cloud project)? Turn off IAP to see.

In the cloud console window, click the menu icon at the top left of the page, click on Security and then on Identity-Aware Proxy. Click the IAP toggle switch next to App Engine app to turn IAP off.

You will be warned that this will allow all users to access the app.

Refresh the application web page. You should see the same page, but without any user information:

Since the application is now unprotected, a user could send a web request that appeared to have passed through IAP. For example, you can run the following curl command from the Cloud Shell to do that (replace <your-url-here> with the correct URL for your app):

curl -X GET <your-url-here> -H "X-Goog-Authenticated-User-Email: totally fake email"

The web page will be displayed on the command line, and look like the following:

<!doctype html>
<html>
<head>
  <title>IAP Hello User</title>
</head>
<body>
  <h1>Hello World</h1>

  <p>
    Hello, totally fake email! Your persistent ID is None.
  </p>

  <p>
    This is step 2 of the <em>User Authentication with IAP</em>
    codelab.
 </p>

</body>
</html>

There is no way for the application to know that IAP has been disabled or bypassed. For cases where that is a potential risk, step 3 shows a solution.

If there is a risk of IAP being turned off or bypassed, your app can check to make sure the identity information it receives is valid. This uses a third web request header added by IAP, called X-Goog-IAP-JWT-Assertion. The value of the header is a cryptographically signed object that also contains the user identity data. Your application can verify the digital signature and use the data provided in this object to be certain that it was provided by IAP without alteration.

Digital signature verification requires several extra steps, such as retrieving the latest set of Google public keys. You can decide whether your application needs these extra steps based on the risk that someone might be able to turn off or bypass IAP, and the sensitivity of the application.

This is step 3, and the last step ended with your Cloud Shell open in the iap-codelab/2-HelloUser folder. Change to the folder for this step:

cd ~/iap-codelab/3-HelloVerifiedUser

Deploy to App Engine

Deploy the app to the App Engine Standard environment for Python 3.7:

gcloud app deploy

When you are asked if you want to continue, enter Y for yes. In a few minutes the deploy should complete. While you are waiting you can examine the application files as described below.

When the deployment is ready you will see a message that you can view your application with gcloud app browse. Enter that command. If a new tab does not open on your browser, copy the displayed link and open it in a new tab normally.

Recall that you disabled IAP in Step 2, so no IAP data is being provided to the application. You should see a page similar to the following:

As before, you may need to wait a few minutes for the newest version to be live to see the new version of the page.

Since IAP is disabled, no user information is available. Now turn IAP back on.

In the cloud console window, click the menu icon at the top left of the page, click on Security and then on Identity-Aware Proxy. Click the IAP toggle switch next to App Engine app to turn IAP on again.

Refresh the page. The page should look like the following:

Notice that the email address provided by the verified method does not have the accounts.google.com: prefix.

If IAP is turned off or bypassed, the verified data would either be missing, or invalid, since it cannot have a valid signature unless it was created by the holder of Google's private keys.

Examine the application files

This folder contains the same set of files as seen in Step 2, with two files altered and one new file. The new file is auth.py, which provides a user() method to retrieve and verify the cryptographically signed identity information. The changed files are main.py and templates/index.html, which now use the results of that method. The unverified headers as found in step 2 are also shown for comparison.

The new functionality is primarily in the user() function:

def user():
    assertion = request.headers.get('X-Goog-IAP-JWT-Assertion')
    if assertion is None:
        return None, None

    info = jwt.decode(
        assertion,
        keys(),
        algorithms=['ES256'],
        audience=audience()
    )

    return info['email'], info['sub']

The assertion is the cryptographically signed data provided in the specified request header. The code uses a library to validate and decode that data. Validation uses the public keys that Google provides for checking data it signs, and knowing the audience that the data was prepared for (essentially, the Google Cloud project that is being protected). Helper functions keys() and audience() gather and return those values.

The signed object has two pieces of data we need: the verified email address, and the unique ID value (provided in the sub, for subscriber, standard field).

This completes Step 3.

You deployed an App Engine web application. In Step 1, you restricted access to the application to only users you chose. In Step 2, you retrieved and displayed the identity of users that IAP allowed access to your application, and saw how that information might be spoofed if IAP were disabled or bypassed. In Step 3, you verified cryptographically signed assertions of the user's identity, which cannot be spoofed.

The only Google Cloud Platform resources you used in this codelab are App Engine instances. Each time you deployed the app, a new version was created and continues to exist until deleted. Exit the lab to delete the project and all the resources within it.