Effective CAPTCHA with Amodal Completion and Aftereffects

Abstract

Accounts on web services are always exposed to the menace of attacks. Especially, a large number of accounts can be used for unfair uses such as stealth marketing or SPAM attacks. Needless to say, acquisition of those accounts and attacks are automatically done by software programs called bots. Therefore, a technology called CAPTCHA is usually used in the acquisition of accounts for web services in order to distinguish human beings from bots. The most popular kind of CAPTCHA methods is text-based CAPTCHA in which distorted alphabets and numbers appear with obstacles or noise. However, it is known that all of text-based CAPTCHA algorithms can be analyzed by computers. In addition, too much distortion or noise prevents human beings from alphabets or numbers. There are other kinds of CAPTCHA methods such as image CAPTCHA and audio CAPTCHA. However, they also have problems in use. As a related work, an effective text-based CAPTCHA algorithm was proposed to which amodal completion is applied. The CAPTCHA provides computers a large amount of calculation cost while amodal completion helps human beings to recognize characters momentarily. On the other hand, momentary recognition is uncomfortable for human beings since extreme concentration is required within ten seconds. Therefore, in this paper, we propose an improved algorithm to which amodal completion and aftereffects are applied. The aftereffects extend time for recognition of characters from a moment to several seconds.