"A CAPTCHA is a reverse Turing test that takes the form of a challenge-response test. For example, if it instructs users to "select all images with stairs," they must pick the stairs out from railings, driveways, and crosswalks. Alternatively, they may be asked to enter the text they see, add the sum of dice faces, or complete a sliding puzzle."
"Image-based CAPTCHAs are responsible for the most frustrating shared experiences internet users have - deciding whether to select a square when only a small sliver of the object in question is in it. Regardless of the method, a computer or algorithm ultimately determines whether the test-taker is human or machine. This authentication service has spawned many offshoots, including reCAPTCHA and hCAPTCHA."
CAPTCHAs are challenge-response tests designed to distinguish humans from bots. Methods include image classification, puzzles, audio samples, and click-based tests. Many CAPTCHA systems use opaque decision logic, leaving users uncertain what proves humanity. Image-based challenges frequently force selection of partial objects, producing common user frustration. People with disabilities often cannot complete standard CAPTCHA tasks, resulting in exclusion from web content and services. Popular implementations such as reCAPTCHA and hCAPTCHA evolved from original CAPTCHA designs. No single accessible solution exists; improving accessibility requires understanding real user needs and designing inclusive authentication alternatives.
Read at Smashing Magazine
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