Continuous evolution of CAPTCHA techniques is necessary to combat modern generation of AI enabled bots. Designing a new CAPTCHA scheme requires a careful review of existing CAPTCHA techniques. But existing reviews of current CAPTCHA techniques lack systematic evaluation of the current trends in CAPTCHA development. Existing reviews focus on mere enlisting of current CAPTCHA schemes in several categories and explaining their working schema. Hence systematic evaluation and analysis of existing CAPTCHA techniques in several categories is necessary. In this paper we highlight the facts and flaws of existing CAPTCHA techniques in order to provide insights for future improvements in current CAPTCHA techniques. We have focused on providing simple and clear understanding of existing CAPTCHA techniques in a systematic way. This will help researchers to overcome the drawbacks of current CAPTCHA schemes and work on improvement of weaker aspects of existing CAPTCHA techniques.

CAPTCHA, HCI, Web security, Human Interactive Proof (HIP), Bots

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