CAPTCHA introduction (cont. 2)

目前網路上較常見的Captcha又可以大略的分成以下三種類型：
圖形式Captcha(Image-based Captcha)：
圖1.3是此種Captcha的一個範例，它會顯示一至數張圖片，要求使用者回答相關的問題。較簡單的作法是將具有某樣特定景物的圖片做輕微的扭曲，之後詢問使用者：這是張什麼東西的照片？或是將數張具有共同元素的圖片放在一起，詢問使用者：這些圖片的共通點在哪裡？雖然電腦很難去分辨一張照片裡到底是什麼東西，但是這種Captcha卻會受到語言的隔閡。舉例來說：當系統要求使用者輸入英文時，對於某些不太懂英文的使用者來說，語言就是道很深的鴻溝。除此之外，同樣的東西在每個人的主觀認定上也可能有不同的結果，我認為那是塊牛排(Steak)，你可能認為那只是塊肉(Meet)。因此，圖形式Captcha在系統的設計上，通常會增加一個下拉式選單讓使用者選取其中的標準答案，但如果系統只有三十個標準答案，那電腦就有1/30通過Test的機會，這將大幅降低Captcha的安全性。


There are three common types on the internet.
Image-based CAPTCHA:
Fig. 1.3 is a sample of this kind of CAPTCHA. This type will give you one or more pictures, and ask you some questions. An easy method is to warp an image and then ask the user to identify what it is. Another method may generate a few pictures with the same object, and then ask the user to find out the common point of them. Although this kind of CAPTCHA is hardly to be recognized by the computer, the human may not across the language gap to answer the question. For example, when the system asks an American who don’t know the Chinese enter Chinese word, the user may not pass the test. Otherwise, someone may recognize the same picture to different answer. You may think that is a meet, but I say that is a steak. Because of the reason, the CAPTCHA system may increase a combo box to help user answer the right object. However, the security is decreased by the limited answer in this method. If a system has only 30 answers in the combo box, the bots have 1/30 chance to pass the test.