Attacks and Design of Image Recognition CAPTCHAs

CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) [1][2][3], also known as Human Interactive Proof (HIP),is an automated Turing test in which both generation of challenges and grading of responses are performed by computer programs. CAPTCHAs are based on Artificial Intelligence (AI) problems that cannot be solved by current computer programs or bots, but are easily solvable by humans.A client who provides a correct response to a challenge is presumed to be a human; otherwise a bot.CAPTCHAs have been widely used as a security measure to restrict access from bots.

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