CAPTCHA is a kind of challenge response test, which is used to distinguish human users from malicious computer program such as bots. However, the attack technique called relay attack as a method to avoid the CAPTCHA has been devised. This attack relays the CAPTCHA challenges to remote human-solvers, let them to decode CAPTCHA challenges. We used delay time that is caused by communications needed in relay attack. Our new CAPTCHA uses this delay time between communications to prevent relay attacks. We constructed an experimental environment in which relay attack can be simulated, made a series of experiments in order to evaluate the performance of the proposed method.

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