Recaptcha

reCAPTCHA is a free service from Google that protects websites from spam and abuse by distinguishing human users from automated bots. It combines automated risk analysis with optional challenges to determine whether a user is likely human, and it can adapt to different risk levels and use cases.

Since its inception, reCAPTCHA has evolved through several versions. reCAPTCHA v2 presents a checkbox labeled “I’m not a robot” and may require solving image-based challenges, such as identifying traffic lights or crosswalks. The invisible reCAPTCHA runs a background risk assessment and only prompts a challenge if the system detects suspicious activity. reCAPTCHA v3 does not present a challenge; instead it returns a score from 0.0 to 1.0 indicating the probability that a request is from a human, letting site owners decide how to respond. Google also offers reCAPTCHA for Enterprise with additional controls and features for organizations.

Implementation involves registering a site to obtain a site key and a secret key. The reCAPTCHA JavaScript library is loaded on the client side, and the site verifies tokens on the server using the secret key. The service may collect data about devices, interactions, and other signals to assess risk. It provides accessibility options, including an audio challenge for users with visual impairments.

History and scope: reCAPTCHA was developed at Carnegie Mellon University and acquired by Google in 2009. It has since become one of the most widely used bot-prevention tools for websites and apps, with multiple versions and configuration options to fit different platforms and privacy considerations.