CAPTCHA – Telling Humans and Computers Apart Automatically

 

First of all, you make ask me what is CAPTCHA ? In our daily life, CAPTCHA is everywhere especially when we surf online. In Chinese, CAPTCHA means “图片验证码”. Here I will talk about some interesting aspects on CAPTCHA. Most ideas come from Xiaojun Zhu’s blog: http://bbs.nju.edu.cn/b/041221 , where he discusses many interesting research issues. More details are described in Manuel Blum’s paper “TELLING HUMANS AND COMPUTERS APART AUTOMATICALLY ”  published in COMMUNICATIONS OF

THE ACM, Vol.47, No. 2, February 2004 (Manuel Blum is the Turing Award Winner in 1995, whose adviser Marvin Minsky is the Turing Award Winner in 1969).

 

It is no doubt that CAPTCHA is a great application, because it’s a real application of a Turing Test. CAPTCHA can be automatically generated, which most humans can pass, but that current computer programs cannot pass. The only difference is that the judge in CAPTCHAs is now a computer, while the judge of Turing Test is a person. The strength of CAPTCHA is that it can tell us a action in Internet even computer world is real human’s behaviors or machines. It let me think about the world in Matrix where you cannot know who is a real man or just an agent of Matrix. Don’t look down on the role of CAPTCHA, let’s see a case of it.

 

In Nov. 1999, the Web site slashdot.com released an online poll asking which was the best graduate school in computer science – a dangerous question to ask over the Web. As is the case with most online polls, IP addresses of voters were recorded in order to prevent single users from voting more than once. However, students at Carnegie Mellon found a way to stuff the ballots by using programs that voted for CMU thousands of times: CMU’s score started growing rapidly. The next day, students at MIT wrote their own voting program and the poll became a contest between voting “bots”. MIT finished with 21,156 votes, Carnegie Mellon with 21,032 and every other school with less than 1,000. Can the result of any online poll be trusted? Not unless the poll requires that only humans can vote.

 

Now, CAPTCHA is used in most of votes, entries, e.g., QQ now need users have a CAPTCHA when they login. From CAPTCHA, we can see the ideal combination between theory and practice. As known to all, Turing Test is a classic theory in Artificial Intelligence, and now it in the form of CAPTCHA is running in millions of computers to order our e-life. While in my opinion, it may lose its power when someday advanced computers can identify what CAPTCHAs mean.