Abstract
CAPTCHA has long been used to keep bots from misusing web services. Various CAPTCHA schemes have been proposed over the years, principally to increase usability and security against emerging bots and hackers performing malicious operations. However, automated attacks have effectively cracked all common conventional schemes, and the majority of present CAPTCHA methods are also vulnerable to human-assisted relay attacks. Invisible reCAPTCHA and some approaches have not yet been cracked. However, with the introduction of fourth generation bots accurately mimicking human behavior, a secure CAPTCHA would be hardly designed without additional special devices. In this paper, we presented a short literature review of the current CAPTCHA schemes, as well as highlighting new trends and open issues, the challenges, and the opportunities as a solid starting point for designing the future secure and usable CAPTCHA schemes.
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