CAPTCHAs

CAPTCHAs are tests designed to distinguish humans from computers, commonly used on websites to prevent automated abuse by requiring a user to complete a task that is easy for humans but difficult for machines.

The term CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. It was introduced in the early 2000s by researchers including Luis von Ahn at Carnegie Mellon University, with later popularization and expansion through Google's reCAPTCHA, acquired in 2009.

They work by presenting challenges such as distorted text, image selection, or audio playback. Solutions may rely on human input or on automated risk analysis that scores the likelihood the interaction is human. Some systems operate invisibly, requiring no active user input unless risk is detected.

Types include text-based CAPTCHAs, image-based CAPTCHAs, audio CAPTCHAs, puzzle and click-based challenges, and invisible risk scoring systems like reCAPTCHA v3.

Security and limitations: Advances in machine learning have reduced the effectiveness of older CAPTCHAs. Attackers may outsource solving tasks. CAPTCHAs can hinder accessibility for users with visual or cognitive impairments and non-native speakers, leading to ethical considerations. Developers may implement accessibility alternatives, such as audio tests, adjustable difficulty, or non-challenge-based approaches.

Alternatives and current state: Many sites use risk-based approaches, rate limiting, or device fingerprinting. Some services offer alternative proofs of humanity like behavior analysis. The field continues to balance usability and security as automated solving improves.