I'm not a robot ": This is how the Google Captcha works

Previously, you had to solve arithmetic problems or solve sequences of numbers for verification on the Internet. With "I am not a robot" Google simplifies the captcha for the visitor of a website. But how does it work? We clarify this question in this practical tip.

No Captcha reCaptcha - "I am not a robot"

Captchas are supposed to distinguish human visitors to a website from spambots. So far, it has always been necessary to do a task: solve a simple arithmetic problem, enter a sequence of numbers that is difficult to decipher, or the like. With reCaptcha, Google has made things much easier: just tick the box and confirm "I'm not a robot". Simple - but how does it work?

• Google does not provide exact details on how it works - probably also in order not to disclose any opportunities for attack to spammers. The company's blog says about technology only "... last year we developed an Advanced Risk Analysis backend for reCAPTCHA that actively considers a user's entire engagement with the CAPTCHA — before, during, and after — to determine whether that user is a human . "
• In summary, this means that Google uses an upstream algorithm. Even before entering the captcha, this calculates the likelihood that the visitor is human. Parameters such as the IP address of the user, his time on the website and his mouse movements are used for the calculation.
• So reCaptcha already knows whether you are a human or a robot before you check "I am not a robot".

Captcha provides additional security: mobile cat pictures

The hit rate is between 60 and 80%. For this reason and because mobile devices such as smartphones and tablets do not use a mouse, reCaptcha is usually still linked to a second captcha query, the well-known "cat pictures": the user has to recognize and mark all related pictures from a series of small pictures, so all cats or all mountains.

We explain in another practical tip what exactly a captcha is.