Trivia: How reCAPTCHA Works

I’ll start with some basic facts which you may already know, but at the bottom of the article there’s a curious fact that you might have not heard before.

First things first, what is a CAPTCHA? It’s an acronum for “CompletelyAutomated Public Turing test to tell Computers and Humans Apart.” Now in simple words: it’s basically a test used to make sure that whoever is performing an action is a human.

Why would you want to verify that? To avoid having bots or automated scripts doing things inside your website, application, and so on, which could lead to security problems as well as an overload of the system.

There are several ways to implement the test, but the most used one is presenting the user a sequence of letters or words and asking him to re-type and submit those letters. The image below illustrates how it works:

The image below is the CAPTCHA test from a company called reCAPTCHA, which was originally a project at the Carnegie Mellon University and later got acquired by Google.

Now here comes the interesting trivia: on all reCAPTCHA forms (so those on all Google services) you actually just need to type the first word to pass the test and prove that you are human. The second word is one that Google’s computers could not identify on its book digitalization project, and by typing it you are helping Google to identify those words and complete their book scannings.

Neat huh?

It seems to be working as well, as every day over 200 million forms like the above one are displayed around the web.