How Does CAPTCHA Work?

Hey guys, it is Thisura here after some time. It feels really good to be writing again.

In today’s article, I am talking about how CAPTCHA work and how its main features work.

So, when you are signing up for a website or when you try to comment on a blog post, have you ever seen a checkbox that says “I am not a robot” and wondered how the heck it is going to decide whether I am a robot or not? It is impossible, right? How on earth can an automated program decide whether I am a robot or not?

GIF source – Medium.com

Well, then you can count yourself lucky. Because today I am going to talk about exactly that. That checkbox is a test called CAPTCHA which is what my topic for today’s post. So, let’s hop in, shall we?

So, as I told you that checkbox you saw that day was a Turing test. I hope you know what a Turing test is. If you don’t feel free to check out this 2 min video clip to get an idea. This test was designed to check whether a user is a bot or a real person.

What is CAPTCHA?

The term CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. This test has been proved to be successful in doing its job over the years.

How does CAPTCHA work?

Usually, CAPTCHAs show you a distorted set of alphanumeric characters. You have to type the set of characters in the exact order and prove that you are a real person.

The reason why this test uses a distorted set of characters is so that bots can’t read them. If the user fails to guess the characters correctly, he has to try again and again until he succeeds.

Where can we find them?

You can find these Turing tests in login forms, signup forms, feedback forms or even when you are going to check out something from an e-commerce website.

Why reCAPTCHA then?

Well, the CAPTCHAs did a great job for a long time. But with the evolution of machine learning, people (hackers to be more specific) were able to create advanced bots who could understand those distorted handwritten type of characters. Hence, ordinary CAPTCHA tests had to be replaced with more complex Turing tests. Google developed reCAPTCHA to do just that.

What is reCAPTCHA?

Well, reCAPTCHAs were developed by a set of researchers at Carnegie Mellon University. This technology was later acquired by Google in 2009.

The difference between CAPTCHA and reCAPTCHA

Instead of generating distorted characters, reCAPTCHA uses real-world images to find the bots. reCAPTCHA uses images like street address boards, texts from books and old newspapers, etc.

reCAPTCHA has been developed over the years and has 3 types of tests by now.

Image Recognition

Image Source – Medium.com

They show some kind of a photo (or a set of photos in this case) that contains 9 or 16 squares. User is asked to identify and choose the particular squares that contain a certain object such as an animal, tree or a street sign (in this case, a statue).

“I am not a robot” Checkboxes

Image source – newatlas.com

This is not about checking the checkbox at all. This is all about the way you click it. reCAPTCHA checks the way your mouse move and predicts whether you are a bot or not.

In addition to that, this type of reCAPTCHAs check for your device history and cookies in the browser to confirm its decisions.

No Interaction type

This test is invisible. There is nothing for you to interact with here. This test is based on the way you behave on the internet. For example, if it recognizes that the user is clicking on links at an abnormal rate, it will realize that this is not a real user and block him from the website.

Are CAPTCHAs and reCAPTCHAs Successful?

Yes, for now. But the thing is that there always will be a loophole somewhere in these tests. Someone will eventually come up with something that could exploit those loopholes and bypass the system. So, the technology has to keep adapting overtime to keep its standards.

Are there any drawbacks in using CAPTCHAs on your website?

The first thing that comes to my mind is that the user is going to have a bad user experience when compared to a website that does not use CAPTCHAs.

The next thing is that visually impaired individuals can’t pass the test


So guys, thank you for reading my article. I hope you learned something valuable. If you did, go ahead and subscribe to my blog so that you won’t miss my latest posts. Have a nice day.

Published by Thisura Thenuka

I am a passionate software engineering student. But cricket is my first love ❤

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