CAPTCHA introction (cont. 5)

Captcha的研究分為兩大主軸：一是設計Captcha，另一是破解Captcha，這也是本論文的兩大研究議題[1]。針對第一個研究議題，設計Captcha應注意的重點是所設計的Captcha需方便人類閱讀(Friendly)並且不易被電腦破解(Security)。實際上這兩個需求是互相牴觸的，以無名小站的Captcha為例（如圖1.6 (a.)），它很Friendly，人們可以輕易的辨識出6185，同時電腦也可以辨識出他是6185(Security不足)，所以這不是一個好的Captcha。再以Hotmail的Captcha為例（如圖1.6 (b.)），這組Captcha雖然不易被電腦辨識出來(Security不足)，但因雜線過多，人們也搞不清楚其內容(不Friendly)。所以要設計一個好的Captcha需在Friendly與Security這兩個需求間取得一個平衡，特別是要了解人類視覺辨識能力的優勢在哪裡，電腦辨識能力的缺點又是在何處，這樣才能設計出一個理想的Captcha。

The research of CAPTCHA has two areas, one is to design a system and the other is to break a exist system. At the first area, when we design a new system, we should consider both human friendly and security. The people should easy to answer the question, and the computer can not find the answer automatically. In actually, these considerations are conflict with each other. For example, Fig. **.** shows a CAPTCHA which is used by the www.wretch.cc. Human can recognize this picture is 6185 easily, and so do the computer. Otherwise, Fig. **.** shows a sample of the Hotmail CAPTCHA system. This CAPTCHA has high security, but the too much clutters will cause the people can hardly to read it. Because these considerations can hardly to balance each other, we should find out where is the advantage of the human recognition ability, and where is the drawback of the computer’s recognition algorithm. According to these, we can try to find out a better way to design a new system.