# CAPTCHA

> A small test a website uses to tell a person from an automated program.

- Level: Basic
- Topics: Security
- URL: https://tidbit.legios.com.ar/captcha/
- In Spanish: [CAPTCHA](https://tidbit.legios.com.ar/es/captcha/)

A **CAPTCHA** is the little challenge that appears before you sign up, log in or buy tickets: type the wavy letters, click every picture with a traffic light, or just tick a box that says "I'm not a robot". The name is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart". The idea is a task that is easy for people and hard for software.

It exists because bots are cheap. A single program can try thousands of stolen passwords per minute, create fake accounts by the hundred, post spam or buy a whole concert in seconds to resell it. A CAPTCHA does not stop a determined attacker, but it makes each attempt slower and more expensive, which is often enough.

The tests have changed a lot. Distorted text became easy for image-recognition software, so many sites moved to "invisible" checks such as Google's **reCAPTCHA** or Cloudflare's Turnstile: they look at signals like how the mouse moves, the browser and the network's reputation, and only show a puzzle when something looks off. The trade-offs are real: puzzles can be a wall for people with visual or motor disabilities, and the invisible kind works by observing your behavior.

## An analogy

A bouncer who asks a quick question at the door. Anyone can answer it, but it slows down the crowd of fake guests trying to rush in at once.

## Keep in mind

- Its job is telling people apart from bots, not checking who you are.
- It protects sign-ups, logins, forms and ticket sales from automated abuse.
- Modern versions are often invisible and only show a puzzle when something looks suspicious.
- Puzzles can block people with disabilities, so good sites offer accessible alternatives.

## Go deeper

- [CAPTCHA on Wikipedia](https://en.wikipedia.org/wiki/CAPTCHA)
- [What is a CAPTCHA? (Kaspersky)](https://www.kaspersky.com/resource-center/definitions/what-is-a-captcha)
- [Inaccessibility of CAPTCHA (W3C)](https://www.w3.org/TR/turingtest/)

## Related terms

- [Spam](https://tidbit.legios.com.ar/spam/): Unwanted messages sent in bulk: ads, scams and junk you never asked for.
- [Google](https://tidbit.legios.com.ar/google/): The search company that gives most products away and makes its money on ads.
- [Cloudflare](https://tidbit.legios.com.ar/cloudflare/): A global network that sits in front of websites to make them faster and safer.
- [Web browser](https://tidbit.legios.com.ar/web-browser/): The app that fetches web pages from the internet and draws them on your screen.