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April 18, 2017updated 12 Oct 2022 2:51pm

What is CAPTCHA?

How automated tests protect your security online.

By Alexander Sword

A computer user completing a CAPTCHA (Image: Shutterstock)

CAPTCHA is an automated system which tests a visitor to a website to verify that they are human. It stands for Completely Automated Public Turing Test To Tell Computers and Humans Apart.

A CAPTCHA requires the person submitting information to complete a test, usually a visual one, that only a human could complete. However, artificial intelligence is already being used to break CAPTCHAs and other security measures.

This could involve showing a picture of text, distorted enough that a human can recognise it but a machine couldn’t if scanning it automatically.

New alternatives to CAPTCHA simply ask humans to click a box, asking “are you a human?”

CAPTCHAs are used when there is a risk that an automated piece of software, called a bot, might submit information for spam purposes or to extract data from websites.

When is a  CAPTCHA required?

Some comment boards require a CAPTCHA to be entered to prevent bots from repeatedly submitting advertising content.

They may also be used when users log in to certain accounts, in case a bot is systematically submitting vast quantities of credentials to try and find successful combinations. These could then be used to gain access not only to that account but to other accounts where people have used, for example, the same email address and password.

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