Text-Based CAPTCHA Using Phonemic Restoration Effect and Similar Sounds

In Recent years, bot (robot) program has been one of the problems on the web. Some kinds of the bots acquire accounts of web services in order to use the accounts for SPAM mails, phishing, etc. CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart) is one of the countermeasures for preventing bots from acquiring the accounts. Text-based CAPTCHA is especially implemented on almost all famous web services. However, CAPTCHA faces a problem that evolution of algorithms for analysis of printed characters disarms text-based CAPTCHA. Of course, stronger distortion of characters is the easiest solution of the problem. However, it makes recognition of characters difficult not only for bots but also for human beings. Therefore, in this paper, we propose a new CAPTCHA with higher safety and convenience. Especially, we focus on the human abilities of phonemic restoration and recognition of similar sounds, and adopt the abilities in the propose CAPTCHA. The proposed CAPTCHA makes machinery presumption difficult for bots, while providing easy recognition for human beings.