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Hash PII Data with Google Tag Manager

I thought Google Tag Manager (GTM) would have a built-in method to hash Personally Identifiable Information (PII) – but it doesn’t, and I couldn’t for the life of me find a simple guide on how to do it.

Plenty of guides on how to obfuscate PII in URLs and query strings, even a couple guides on encrypting – but nothing on hashing.

Isn’t encrypting and hashing the same?

In short: no. Encrypting implies that there is a method to decrypt the data again, where hashing is a one-way operation. Keep in mind that with GTM, any data manipulation happens client-side using JavaScript. Because it’s happening client-side – you have to hand over the encryption mechanism and key to the end user. Not exactly fantastic for security…

Why would I want to do this?

Any time you want to send PII data to a 3rd party vendor, it should be hashed using an agreed-upon hash algorithm. As an example: Facebook needs a hashed version of a user’s email address to do advanced matching. The Facebook Pixel tag will hash the email address for you – but if you’re using the pixel image instead, you need to hash that yourself.

Note: Google Analytics does not allow the use of PII data, hashed or otherwise. This is probably not a rule you want to break.

How do I do this?

There are three steps involved:

  1. Declare the incoming data as a Data Layer Variable
  2. Load a library for the hash algorithm you want to use
  3. Create a Custom JavaScript Variable to hash the data

Declare a Data Layer Variable

Your CMS will need to surface the PII data (such as an email address) into the dataLayer object somehow. How that happens is outside the scope of this article, but the Google Tag Manager for WordPress plugin happily adds the email of the logged-in user in a data point called visitorEmail.

Once the PII data is in the Data Layer, it needs to be declared in GTM:

  • Go to the container for your site and select Variables on the left
  • Create a new User-Defined Variable
  • Select Data Layer Variable from the list of variable types
  • For Data Layer Variable Name, enter the name of the variable exactly as it appears in the dataLayer object

For PII with alpha characters (such as email addresses), you also need to lower-case the string. While you’re in the variable configuration screen:

  • Open the Format Value section
  • Check the box for Change Case to…
  • Select Lowercase in the dropdown

Load a JS Library to Hash Data

Note: Do not write your own library unless you are a cryptographer with years of experience. Do validate that the library you choose does what you expect.

I have minimally tested the js-sha256 library and have confirmed it correctly hashes data. It doesn’t appear to do anything nefarious, but I am not a programmer – so you should do your own testing and code review!

Next, whatever you’re using needs to be hosted somewhere – hosting it yourself is an option, but I prefer using an established CDN. The js-sha256 library is available on Cloudflare’s cdnjs CDN:

https://cdnjs.cloudflare.com/ajax/libs/js-sha256/0.9.0/sha256.min.js

If you’re using a CDN-hosted library, you should make sure you’re using a specific version of the code and that there’s an SRI Hash in your tag configuration to make sure someone doesn’t tamper with the code.

Here’s how to load a library in GTM:

  • Go to the container for your site and select Tags on the left
  • Create a new Custom HTML tag
  • Go to the SRI Hash tool linked above and create an SRI Hash for your library
  • Edit the output for use in a Custom HTML tag (see second code block below)

For the js-sha256 library hosted by cdnjs, the SRI-hashed version looks something like:

<script src="https://cdnjs.cloudflare.com/ajax/libs/js-sha256/0.9.0/sha256.min.js" integrity="sha384-2epjwyVj8M4n8AweIsY7SKPSJmqBBBkmksXvkmtYORfxPS1I4NZE/+Ttk/9gCELG" crossorigin="anonymous"></script>

To use this in a Custom HTML tag, you have to use JavaScript to construct the script tag (for some reason, GTM thinks the above isn’t valid HTML).

Here’s what the result looks like – note how the src, integrity and crossorigin attributes are set:

<script>
  (function() {
  var script = document.createElement('script');
  script.type = 'text/javascript';
  script.src = 'https://cdnjs.cloudflare.com/ajax/libs/js-sha256/0.9.0/sha256.min.js';  
  script.setAttribute('integrity','sha384-2epjwyVj8M4n8AweIsY7SKPSJmqBBBkmksXvkmtYORfxPS1I4NZE/+Ttk/9gCELG');
  script.setAttribute('crossorigin','anonymous');
  document.getElementsByTagName('head')[0].appendChild(script);
  })();
</script>

You can optionally set this tag to load once per page – so long as the function is available, it should work fine for subsequent events.

Create a Custom JavaScript Variable

Last but not least – hash the data. When hashing data, it’s critically important to make sure that the source data is what you expect before hashing. After all, there’s no way to check after it’s been hashed!

In my example of hashing an email address in the visitorEmail variable, I want to run some very basic tests on the string to make sure it looks like an email address. In the code block below, you can see the test called out – you should do something similar for other types of data.

For the final piece – set up a Custom JavaScript variable:

  • Go to the container for your site and select Variables on the left
  • Create a new User-Defined Variable
  • Select Custom JavaScript from the list of variable types
  • In the Custom JavaScript box, enter the following:
function() {
  // Test email address first
  function emailIsValid (email) {
    return /\S+@\S+\.\S+/.test(email)
  }
  // If email address is valid, hash it
  if (emailIsValid({{visitorEmail}})) {
    var hash = sha256({{visitorEmail}});
    return hash;
  } else {
    return undefined;
  }
}

Next steps

Test things! Use the Preview function to make sure the variable you created actually contains a hash of the PII data you want to send to the vendor. Test with known bad data – does the code in the Custom JavaScript variable catch it?

Once you’re satisfied that the code is actually working properly, map it to the vendor in your tag configuration – safe in the knowledge you’re not sending PII data in the clear! 🙂

Easy Cookie Consent with Google Tag Manager

Google Tag Manager (GTM) should have a built-in method to manage firing tags based on user consent, but it doesn’t. While looking for a good way to set this up, I was bewildered by the insanely complex methods people were coming up with.. Thankfully, there’s an easier way that doesn’t involve editing site code!

Note: The below may help make your site more GDPR or CCPA compliant, but it’s not everything you need to do. Go talk to a lawyer and get some proper advice!

Google Tag Manager basics

First, set up Google Tag Manager on your website and use it to fire something like a Google Analytics tag. Doing so is fairly straight forward, and there are heaps of guides online to show you how it’s done. For WordPress sites, I can recommend the Google Tag Manager for WordPress plugin. You’ll need to have GTM firing at least one tag so you can see the difference between having and not having user consent.

Osano Cookie Consent basics

Next – go to Cookie Consent by Osano. It’s a completely free cookie consent tool, it looks decent and it’s very simple to set up. Click the Download button and go through options 1-6 to configure the look and feel of your consent prompt.

Here’s where we start ignoring most of their instructions 😉

All Consent Types

Irrespective of which consent compliance option you selected, you need to load the Osano code as a tag. In Google Tag Manager, create a new Custom HTML tag:

  1. Give it a name: “Cookie Consent by Osano”
  2. Copy both blocks of code from the Osano site into the HTML box in GTM
  3. Configure a firing trigger so this tag loads on All Pages as a Page View

Save and publish in GTM. What you do next depends on what type of consent you need..

  1. Just let the user know we’re using cookies
  2. Give the user the ability to opt out of cookies
  3. Require the user to opt in to cookies before using them

Consent Type 1: Inform User

If you selected the compliance option “Just tell users that we use cookies” – there’s nothing more you need to do! The consent prompt should now load on each page until the user dismisses it. 🙂


Consent Type 2: Opt Out

If you selected the compliance option “Let users opt out of cookies (Advanced)” – there’s a little more to do, but it’s easy. Ignore the warning about needing to modify your site. 😉

The Cookie Consent script stores the user choice in a cookie, so we need to tell GTM to read it and use it:

Read Cookie

In GTM, create a new Variable:

  1. Call it “Cookie Consent Status” or similar
  2. Select ‘1st-Party Cookie’ as the variable type
  3. For Cookie Name, enter cookieconsent_status

Use Consent Status

Still in GTM, create a new Trigger:

  1. Call it “Cookie Consent equals deny”
  2. Select ‘Page View’ as the trigger type
  3. Choose ‘Some Page Views’
  4. Select ‘Cookie Consent Status’ (or whatever you called the variable) in the first drop down
  5. Choose ‘equals’ in the second drop down
  6. Enter deny as the text

The line should now read “Cookie Consent Status equals deny”.

To make use of this trigger, go into each of the tags that are configured to fire using a trigger (except the Cookie Consent tag, obviously). Add an “exception” trigger, then choose the ‘Cookie Consent equals deny’ trigger you created above.

Finally, save and publish in GTM – tags should now be suppressed if your users choose to deny cookies. It’s important to note that tags will fire even if the consent prompt is ignored, so if you need to stop serving up delicious cookies until someone allows them..


Consent Type 3: Opt In

Aka – the GDPR option. This one sucks because you won’t get analytics data unless someone clicks ‘Allow cookies’, but if it’s what you need to do..

First, read through Consent Type 2 above. You need to create the same Variable in the Read Cookie section, but then things are a bit different:

Are Cookies Definitely Allowed?

In Google Tag Manager, create a new Trigger:

  1. Call it “Cookie Consent does not equal allow”
  2. Select ‘Page View’ as the trigger type
  3. Choose ‘Some Page Views’
  4. Select ‘Cookie Consent Status’ (or whatever you called the variable) in the first drop down
  5. Choose ‘does not equal’ in the second drop down
  6. Enter allow as the text

The line should now read “Cookie Consent Status does not equal allow”. This is a little confusing, but this is different to the previous trigger in that it will also block tags if the user hasn’t made a choice. Logic is fun!

As with Consent Type 2, go into each of the tags that are configured to fire using a trigger (except the Cookie Consent tag). Add an “exception” trigger, then choose the ‘Cookie Consent does not equal allow’ trigger you just created.

One last thing – and this is optional – but when someone accepts cookies, you probably want to reload the page to fire all the tags that were being suppressed until now. To do so, you need to create another trigger and a tag:

Listen for the Allow Cookies click

We need to make sure GTM is paying attention to CSS classes on elements users click:

  1. In GTM, go to ‘Variables’
  2. Click ‘Configure’ in the ‘Built-In Variables’ section
  3. Scroll through the list and ensure the ‘Click Classes’ option is checked.

Now, create a trigger:

  1. Call it something like “Allow cookies button click”
  2. Choose ‘Click – All Elements’ as the trigger type
  3. Select ‘Some Clicks’
  4. Choose ‘Click Classes’ in the first drop down
  5. Select ‘contains’ in the second drop down
  6. Enter cc-allow as the text

The line should now read “Click Classes contains cc-allow”.

Reload page when the user accepts

When the user clicks on the Allow cookies button, we need to do something with it..

Still in GTM, create another Custom HTML tag:

  1. Call it “Reload Page”
  2. In the HTML box, enter the following:
    <script>window.location.reload();</script>
  3. Select ‘Allow cookies button click’ as the trigger

At long last – save and publish in GTM. This configuration prevents tags from firing unless they’re allowed and reloads the page when the user allows them so you don’t lose the pageview.