CAPTCHA introduction (cont. 4)

(1.) 閱讀式Captcha(Reading-based Captcha)：

我們在上一章所討論的Captcha是一種要求使用者辨識一張圖片當中的文字，這種類型的Captcha就是屬於閱讀式Captcha（如圖1.5）。此種Captcha的優勢在於，文字原本的設計目的就是為了要讓人類使用，而且絕大多數的人從小就開始學習它。在語言的隔閡上，若使用的是英文字母與阿拉伯數字，幾乎全世界的鍵盤上都有英文字母及阿拉伯數字，使用者就算看不懂符號所代表的意思，也可以對照著鍵盤上的符號輸入答案。在安全上，相較於圖形式Captcha可能只有數十種答案，使用大寫英文字母，以及阿拉伯數字的Captcha，三個字母長度的字串就有約四萬六千組可能的解答。最後，又因為光學文字辨識以及人類視覺的認知方法都是相當著名的研究領域，Captcha的設計者比較容易根據相關的資料找出分辨人類及電腦的方法[4]。但是，在光學文字辨識的領域不斷進步下，此種類的Captcha也勢必將面臨到嚴峻的考驗。

Reading-based CAPTCHA：

At last chapter, we showed a CAPTCHA with text box and character images, and this kind of CAPTCHA is called Reading-based CAPTCHA (as shown on Fig. 1.5). The advantage of this kind of CAPTCHA is: the character is made for human and most of us used it from our childhood. In the language gap, if we use English and Arabic numerals, even the user may not know the English, the keyboard also has these symbols to help user to answer the question. In the security, unlike Image-based CAPTCHA may has limit answer; the Reading-based CAPTCHA with 3 character length will give more than 40,000 possible answers. Finally, because the OCR is a well known field, the CAPTCHA researcher can design a more security system from reading these researches. However, the progress of the OCR field will cause this kind of CAPTCHA more serious problems.