By the way, why are Captchas called Captchas?


You type regularly on the net to register for a site or submit a form. These are Captchas. But do you know what the name means?

You are about to register for a site, change your password or make a purchase, but you have received an instruction asking you to rewrite letters appearing on an image or point to images showing an object particular ? This is called a Captcha and this type of request is very common on the web.

This procedure is decked out with a very strange name at first sight, but it is in fact an English acronym. If you unfold it completely, it means ” Completely Automated Public Turing test to tell Computers and Humans Apart “, that is ” Fully Automated Public Turing Test to Distinguish Computers from Humans ” in French.

This is the central goal of the captcha: to verify that at the origin of a request (to access a form, to change the password, etc.) there is someone and not something. The goal is to weed out machines that are designed to harm the system, such as dumping spam into mailboxes — that’s why it’s crucial to prevent these bots from creating accounts.

A Captcha on a computer // Source: Adam Donkus

The name of the Captcha refers to Alan Turing, this British mathematician and computer scientist who had a considerable contribution in the conduct of the Second World War, by helping to break the Enigma mechanism which Nazi Germany used to send instructions coded to his strengths. Enigma’s resolution is said to have shortened the war by two years.

His name was also used for the Turing Prize, awarded since 1966 by the ACM (Association for Computing Machinery). It is considered the equivalent of the Nobel Prize, but in computer science. It is in any case an extremely prestigious distinction and necessarily very coveted. But it is above all for the “Turing test” that his name has passed to posterity.

This test designates an experiment consisting in seeing if the answers given by an artificial intelligence are sufficiently elaborated so that we can no longer distinguish them from those that a human would give – without obviously knowing in advance if it is a human or a machine responding. In short, the Captcha is a variation of this Turing test.

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The Captcha is used to verify that you are not a robot // Source: Gwydion M Williams

Why do sites use Captcha?

The main motivation concerns security to limit nuisance on the web. Computer programs roam the network to send spam or attempt to deceive Internet users, to collect personal data, to trick you into buying dubious products, and so on.

Hence the interest of securing the use of contact forms, access to comment and exchange spaces. Registration for a discussion forum, for example, is often conditional on the resolution of a Captcha, the validation of the account via a link sent to the email and, sometimes, the approval of the administrator. Constraining, but sometimes necessary.

Similar barriers exist on contact forms, in comment areas on blogs or sites, but also to access this or that platform – such as email providers such as Outlook, Yahoo and Gmail. These webmails are also recurring targets, because email is a first-rate tool for trying to deceive someone.

How does Captcha work?

There are several types of Captcha, all of which verify and confirm that you are indeed not a robot. These are very easy tests for humans (at least they’re supposed to be), but intractable for robots. But with advances in computing, the opposite tends to happen: they have paradoxically become harder for humans to solve.

The most common Captchas are text-based. They form a series of distorted texts and figures that must be rewritten before validating their registration on a site. Sometimes, “parasitic noise” is added to the background of the image, with the aim of distorting the optical character recognition. This is the case for example with the image below.

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Text Captchas form a group of numbers and letters // Source: Shinseungback Kimyonghun

There are also image Captchas, where you have to select a series of visuals based on a particular request – designate photos where you see a car, a pedestrian crossing or an animal. We can also mention calculation Captchas, in which you have to find the answer to a simple mathematical equation and write it down in the box provided for this purpose.

Of course, it can happen to be wrong on an equation or not to be able to identify the images that the Captcha asks you for. In this case, you will be able to request another test using an icon which is usually to the right or at the bottom of your Captcha. Or to request an audio reading of the Captcha (if it is text-based).

The thing is, the future of Captchas may be numbered. Faced with the increasingly important capacities of systems to circumvent these devices, companies such as Google and Apple have developed strategies (the first with reCaptcha, the second with Privacy Access Token) which make it possible to distinguish between humans and robots based on on other signals.

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