CAPTCHA is a test that can, automatically, tell human and computer programmes apart. It is now almost a standard security technology, and has found widespread application on commercial websites. Robustness and usability are two fundamental aspects with CAPTCHA. The robustness of a text CAPTCHA is typically determined by the strength of its segmentation-resistance mechanism. The mechanism of Crowding Character Together (CCT) has been shown to be reasonably resistant to known attacks. On the other hand, such an approach can reduce the usability by making characters very difficult to recognize. This paper proposes an optimiser that automatically enhances the usability of a CAPTCHA design. A key point of this optimiser is that the usability of the CAPTCHA scheme is improved without sacrificing its robustness level. Applying the proposed optimiser will be shown to achieve a significant improvement in the usability of CAPTCHA.
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