The process usually involves one computer asking a user to complete a simple test which the computer is able to generate and grade.
In SITEFORUM we use 2 different methods and a total of 3 different designs.
The reCAPTCHA is Googles own CAPTCHA tool, that is widely used.
A fun fact about the reCAPTCHA:
It is actually a tool that not only keeps your site safe from bots, but also helps to digitize books and newspapers.
There are multiple projects which currently digitize books and transform them into text using "Optical Character Recognition" ( OCR). Unfortunately OCR is not a perfect system and that is where the reCAPTCHA comes into place:
Each word that cannot be read correctly by OCR and has caused an alert in the system is placed on an image and used as a CAPTCHA, so that a person can decipher it.
Now you may ask yourself how the computer knows if you entered the correct word if the words could not have been deciphered by same computer before - it´s quite simple actually and you might already have recognized this:
Each new word that cannot be read correctly by OCR is given to a user along with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one.
The same word is then given to a number of other people to determine, with higher confidence, whether the original answer was correct.
And that is why the CAPTCHA is marked as solved if you only enter one of the words:
In short: One of the words comes from a book, so in theory you could type anything for that word, but the other word is a computer generated word, so you have to type in the correct word.