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How Google reCAPTCHA v3 Works

reCAPTCHA v3 assigns a score to each user interaction, ranging from 0.0 (likely a bot) to 1.0 (likely a human). This score is based on a complex algorithm that considers various factors, including:

• User behavior: Mouse movements, scrolling, typing patterns, and time spent on the page.

• Browser and device information: IP address, browser type, operating system, and plugins.

• Engagement: How a user interacts with your website's content.

When a user performs an action on your site (e.g., submitting a form, logging in), reCAPTCHA v3 collects data and sends it to Google for analysis. Google then returns a score, which you can use to take appropriate action. For instance, you might:

• Allow actions with high scores to proceed without interruption.

• Flag actions with low scores for further verification (e.g., two-factor authentication, a traditional CAPTCHA challenge).

• Block extremely low scores altogether.

Implementing Google reCAPTCHA v3

Proper implementation of reCAPTCHA v3 involves several steps:

1. Register Your Website

First, you need to register your website with Google reCAPTCHA to obtain API keys.

1. Go to the Google reCAPTCHA Admin Console.

2. Click on the "+" sign to register a new site.

3. Fill in the required information, including a label for your site, selecting "reCAPTCHA v3" as the type, and adding your domain(s).

4. Accept the terms of service and click "Submit".

5. You will receive a Site Key and a Secret Key. Keep these keys secure. The Site Key is used on your frontend, and the Secret Key is used on your backend.

2. Add the reCAPTCHA Script to Your Frontend

Include the reCAPTCHA v3 JavaScript library on every page where you want to use reCAPTCHA. It's recommended to load this script just before the closing </head> tag or right before the closing </body> tag.

<script src="https://www.google.com/recaptcha/api.js?render=<your_site_key>"></script>

Replace <your_site_key> with the Site Key you obtained from the Admin Console.

Now there are a couple of ways you can load recaptcha on your website. The first one is described above. SImply include it in your head or body. But that will result in slightly higher page loading time. And avoiding that would be great! 

So, where do we actually need our ReCaptcha? That is the first question. Let me give you an example: If you have a contact form at the bottom of your website users may never scroll down to your form - and hence why would you slow their load time in the first place. Here is the solution:

Load ReCaptcha only when your users have the ability to do something that needs to be validated. For example: when form becomes visible on the screen. Here is the code example:

function formVisible() {
    let elm = document.getElementById('MYFORM');
    var rect = elm.getBoundingClientRect();
    var viewHeight = Math.max(document.documentElement.clientHeight, window.innerHeight);
    return !(rect.bottom < 0 || rect.top - viewHeight >= 0);
 }
 let captchaLoaded = false;
 function loadCaptcha() {
     if(formVisible() && !captchaLoaded) {
         captchaLoaded = true;
         var recaptchaScript = document.createElement('script');
         recaptchaScript.src = 'https://www.google.com/recaptcha/api.js?render={{ $siteKey }}';
         recaptchaScript.defer = true;
         document.head.appendChild(recaptchaScript);
         document.getElementById("sendButton").disabled = false;
         window.removeEventListener("scroll", loadCaptcha);
         window.removeEventListener("load",loadCaptcha);
         window.removeEventListener("resize",loadCaptcha)
      }
 }
window.addEventListener("scroll", loadCaptcha);
window.addEventListener("load",loadCaptcha);
window.addEventListener("resize",loadCaptcha)

3. Execute reCAPTCHA on User Actions

Instead of a checkbox or puzzle, you'll execute reCAPTCHA programmatically when a user performs a specific action. For example, when a user submits a form:

grecaptcha.ready(function() {
    grecaptcha.execute('', {action: 'submit_form'}).then(function(token) {
        // Add the reCAPTCHA token to your form data
        document.getElementById('your-form-id').innerHTML += '';
        // Submit the form
        document.getElementById('your-form-id').submit();
    });
});

Replace <your_site_key> with your actual Site Key and 'your-form-id' with the ID of your form. The action parameter helps reCAPTCHA understand the context of the user's action, which can improve scoring accuracy.

4. Verify the reCAPTCHA Token on Your Backend

This is the most critical step. After the user's browser sends the reCAPTCHA token to your server, you must verify it with Google using your Secret Key. This verification should happen on your backend to prevent tampering.

Here's a conceptual example of a server-side verification request (using Node.js with axios):

const axios = require('axios');
async function verifyRecaptcha(token) {
    const secretKey = ''; // Your Secret Key
    const verificationUrl = `https://www.google.com/recaptcha/api/siteverify?secret=${secretKey}&response=${token}`;

    try {
        const response = await axios.post(verificationUrl);
        const data = response.data;

        if (data.success && data.score > 0.5) { // Adjust the score threshold as needed
            console.log("reCAPTCHA verification successful. Score:", data.score);
            // Proceed with the user's action
            return true;
        } else {
            console.log("reCAPTCHA verification failed or low score.", data);
            // Block or flag the user's action
            return false;
        }
    } catch (error) {
        console.error("Error verifying reCAPTCHA:", error);
        return false;
    }
}

Replace <your_secret_key> with your actual Secret Key. The score threshold (e.g., 0.5) should be adjusted based on your website's needs and traffic patterns. You can monitor scores in the reCAPTCHA Admin Console to fine-tune this value.

Proper Implementation Considerations

• Always Verify on the Backend: Never rely solely on frontend verification, as it can be easily bypassed.

• Choose the Right Score Threshold: Experiment with different score thresholds (data.score) to find the balance between blocking bots and not inconveniencing legitimate users.

• Contextual Actions: Use meaningful action names in grecaptcha.execute() to provide Google with more context about the user's interaction. This improves the accuracy of the score.

• Error Handling: Implement robust error handling for both frontend and backend reCAPTCHA operations.

• User Feedback: While reCAPTCHA v3 is silent, you might consider subtle visual cues or messages for users who consistently get low scores, guiding them to contact support if they are legitimate but being flagged.

• Monitor Performance: Regularly check the reCAPTCHA Admin Console to monitor traffic, scores, and any suspicious activity. This helps you adjust your implementation if needed. 

By following these guidelines, you can effectively implement Google reCAPTCHA v3 to protect your website from malicious automated traffic while providing a smooth user experience.