What is the use of CAPTCHA and how does it work

The use of captcha in different websites is for security reasons that if we are real human browsing the site. We accept CAPTCHAs every day when accessing a web page or registering for a new service, but you may not know why they are there, what function they fulfill or what differences there are between one or the other. 

What are they? What are they for? What are they based on? This is everything we need to know about CAPTCHA, its difference from Google’s ReCAPTCHA and the different versions.

What is a CAPTCHA?

CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. That is, it stands for public and automatic Turing test to distinguish computers from humans. 

And that is its best definition: it is a security or verification measure that seeks to differentiate whether there is a human or an automated robot behind the computer. It seeks to protect from spam and it takes us a few seconds to prove that we are capable of deciphering those letters or easy puzzle and that we are not a robot or a bot trying to access a personal account.

As its name suggests, CAPTCHA tells us about the Turing Test. What is it? The Turing test or the Turing Test was created by computer scientist Alan Turing in order to “ evaluate machines ”. Its creator intended to test if a machine was intelligent enough as a human and was capable of thinking like one or we could fool it. 

Therefore, they ask a series of questions to see if they can react as a person would. This Turing Test has evolved from the fifties to today and applied to machine learning or Artificial Intelligence and based on this idea the CAPTCHAs that you use almost daily are created and that seek to block or limit spam and automatic clicks.

There are many types of CAPTCHA, as we will see in the next sections. You wear them every day. You may have to choose the images that have a car or a lamppost but you also have to write a series of letters and numbers that appear in the box or you have to answer a very simple question of logic or math, such as how much is of 2 + 2, for example. 

Generally they are images of letters or numbers somewhat distorted or rotated in a text box that we must write next. If we do not see it well, we can touch to generate another. The purpose is that the test is easily passed for a human but that a machine does not know how to pass it. This way we will verify that we are not a robot. Some companies like cloudflare are seeking alternative methods to Captcha.

What is reCAPTCHA?


It is an evolution of the previous code that seeks to be more useful and that Google decided to buy in 2009 from an independent company. What reCAPTCHA did in the beginning was to show an image with two words that we had to write. One of the words was known by the system and another was not, and thus Google took advantage of the user to transcribe words that could not be automatically. 

If the one known by the system was written correctly, it was taken for granted that the unknown was also, so a manual and collaborative transcription was achieved with millions of users using this ReCAPTCHA. But it was evolving. For example, since 2012 Google Street View photographs are also used with street names or numbers that do not look good.


No captcha recaptcha

These new versions do not need you to enter anything but you can automatically verify that you are human by checking a single box. You may have seen a Google record where you should not write the aforementioned letters or numbers in the box but just tap on “I’m not a robot” together with the reCAPTCHA icon. 

Although they are generally referred to interchangeably in the same way, these reCaptcha are more advanced and use the behaviour of the user on the web page to consider whether or not it is a robot. The IP address, the mouse, the cookies. All this differentiates us from a bot. This is the most widely used version of reCAPTCHA at the moment.

Recaptcha V3

Recaptcha v3
Recaptcha v3

Since 2019 there is the third version of ReCAPTCHA used by Google and you do not even have to check the box that shows that you are a robot, but the behavior is analyzed automatically. It is not as widespread as the previous one and it is still starting to work, so it is less frequent that you see it.

More control of user activity is achieved. It is “version 3” or ReCAPTCHA v3 that uses an automatic scoring system based on previous interactions we have made with that web page. That score ranges from 0.0 to 1.0 and takes into account a number of things like the history you’ve had, mouse movements or clicks. Depending on your movements, it will suffice to say that you are not a robot. And that they believe it or not, according to that score of 0.0 to 1.0 where 1.0 is as human as possible.

CAPTCHA and ReCAPTCHA differences

We call them interchangeably “captcha” but there is a difference between the first and the second. The first is the “general” concept of the identification or differentiation system between humans and machines. The second is the proprietary technology used by Google and purchased in 2009, which, as we have seen, has evolved. 

That Google uses this system or technology does not imply that it is the only and surely that you have visited many other websites that tell you to mark the photos with traffic lights, that they ask you an easy question, that they make you solve a small mathematical operation. Everything, together, is CAPTCHA. Google’s, accompanied by the icon, is the “ReCAPTCHA” technology itself.

What are they used for?

You will have used a CAPTCHA in your day to day in all types of records on websites, in online services, changing a password… According to Google, it is used to:

  • Sign up for a new service like Gmail, like Blogger, like YouTube
  • If you sign up for any edition of Google Workspace account
  • If you are going to change the password of an account that you already have
  • Setting up Google services on another device or app

You will also find this verification method in registration forms not only from Google but from any other type of email such as Yahoo or other systems. They are also used to create accounts or to buy tickets for concerts or musicals or any event through the Internet in order to prevent them from being bought in bulk and automatically for later resale.

Another common use of captchas is when we are going to register in a forum or leave a comment on a web page, trying to avoid spam between said comments.

Types of CAPTCHA

It is common to find the code of numbers and letters that we must decipher but it is not the only one that we find. These letters are usually warped, rotated, floating in space, shrunk or transformed so that it is not easy to know what they are. But there are other options.


Captcha text type

We have the possibility of finding meaningless sets of letters and numbers, twisted or shrunken or deformed, which we must write in the corresponding box. They are used by Google, Microsoft and we see them on many other websites. You just find that code, which can have letters and numbers, and copy it. It does not always have a meaning to each other or it may be a word.

But we also have the reCAPTCHA mentioned before that serves as digitization or transcription of unreadable texts for a machine or even using Google Street View photographs so that we can verify at the same time that we transcribe the name of a street or the number of a house or what it says on a concrete sign. 

In the latter case, there are two words that we must write in the box below and Google assumes that if the one readable by them is readable by us, it can also trust the one that the machine is not capable of transcribing.


Graphics Captcha

Among the most frequent are also graphic captchas that show us photos with a common element. For example, tap on all the images that have a car or all the photos in which a street light, a traffic light, a motorcycle or a tree appears. They are very common and very easy to solve even for illiterate people, for example, because we all identify certain objects or things.

We usually find two different types in this section. On the one hand, totally independent images or photographs that have a common element. That is, nine random images and different landscapes or places in which there is a tree in the middle of them. But it can also be a great tiled photograph. A large image divided into nine parts and we must mark the areas where a traffic sign, a car, a road or any other reason appears.


Use of captcha

They allow visually impaired people to resolve this identification without the need to see anything. They are not frequent but you may find the option when you see a CAPTCHA to use the auditory method. When you activate it, it begins a kind of dictation with an understandable recording. It can be interesting if you are unable to identify well the letters that are always displayed.

Math and logic problems

They are usually easy, understandable for anyone. You will not find complicated equations to show whether you are a human or a robot but you may see operations like 2 + 2 or 3 + 3 that you must solve in order to go further.

There are also problems of logic or general culture but, like the previous ones, they are very easy and uncomplicated so you will not need to be an expert in these matters. For example, questions like puns or in which city there is a certain well-known monument. Data that anyone knows.

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