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1.1 Overview:
You're trying to sign up for a free email service offered by Gmail or Yahoo. Before you can submit your application, you first have to pass a test. It's not a hard test -- in fact, that's the point. For you, the test should be simple and straightforward. But for a computer, the test should be almost impossible to solve.
This sort of test is a CAPTCHA. They're also known as a type of Human Interaction Proof (HIP). You've probably seen CAPTCHA tests on lots of Web sites. The most common form of CAPTCHA is an image of several distorted letters. It's your job to type the correct series of letters into a form. If your letters match the ones in the distorted image, you pass the test.
CAPTCHAs are short for Completely Automated Public Turing test to tell Computers and Humans Apart. The term "CAPTCHA" was coined in 2000 by Luis Von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University, and John Langford (then of IBM). They are challenge-response tests to ensure that the users are indeed human. The purpose of a CAPTCHA is to block form submissions from spam bots – automated scripts that harvest email addresses from publicly available web forms. A common kind of CAPTCHA used on most websites requires the users to enter the string of characters that appear in a distorted form on the screen.
CAPTCHAs are used because of the fact that it is difficult for the computers to extract the text from such a distorted image, whereas it is relatively easy for a human to understand the text hidden behind the distortions. Therefore, the correct response to a CAPTCHA challenge is assumed to come from a human and the user is permitted into the website.
Why would anyone need to create a test that can tell humans and computers apart? It's because of people trying to game the system -- they want to exploit weaknesses in the computers running the site. While these individuals probably make up a minority of all the people on the Internet, their actions can affect millions of users and Web sites. For example, a free e-mail service might find itself bombarded by account requests from an automated program. That automated program could be part of a larger attempt to send out spam mail to millions of people. The CAPTCHA test helps identify which users are real human beings and which ones are computer programs.
Spammers are constantly trying to build algorithms that read the distorted text correctly. So strong CAPTCHAs have to be designed and built so that the efforts of the spammers are thwarted.
1.2 Motivation:
The proliferation of the publicly available services on the Web is a boon for the community at large. But unfortunately it has invited new and novel abuses. Programs (bots and spiders) are being created to steal services and to conduct fraudulent transactions. Some examples:
• Free online accounts are being registered automatically many times and are being used to distribute stolen or copyrighted material.
• Recommendation systems are vulnerable to artificial inflation or deflation of rankings. For example, EBay, a famous auction website allows users to rate a product. Abusers can easily create bots that could increase or decrease the rating of a specific product, possibly changing people’s perception towards the product.
• Spammers register themselves with free email accounts such as those provided by Gmail or Hotmail and use their bots to send unsolicited mails to other users of that email service.
• Online polls are attacked by bots and are susceptible to ballot stuffing. This gives unfair mileage to those that benefit from it.
In light of the above listed abuses and much more, a need was felt for a facility that checks users and allows access to services to only human users. It was in this direction that such a tool like CAPTCHA was created.
1.3 Background:
The need for CAPTCHAs rose to keep out the website/search engine abuse by bots. In 1997, AltaVista sought ways to block and discourage the automatic submissions of URLs into their search engines. Andrei Broder, Chief Scientist of AltaVista, and his colleagues developed a filter. Their method was to generate a printed text randomly that only humans could read and not machine readers. Their approach was so effective that in an year, “spam-add-ons’” were reduced by 95% and a patent was issued in 2001.
In 2000, Yahoo’s popular Messenger chat service was hit by bots which pointed advertising links to annoying human users of chat rooms. Yahoo, along with Carnegie Mellon University, developed a CAPTCHA called EZ-GIMPY, which chose a dictionary word randomly and distorted it with a wide variety of image occlusions and asked the user to input the distorted word.
In November 1999, slashdot.com released a poll to vote for the best CS college in the US. Students from the Carnegie Mellon University and the Massachusetts Institute of Technology created bots that repeatedly voted for their respective colleges. This incident created the urge to use CAPTCHAs for such online polls to ensure that only human users are able to take part in the polls.
1.4 CAPTCHAs and the Turing Test:
CAPTCHA technology has its foundation in an experiment called the Turing Test. Alan Turing, sometimes called the father of modern computing, proposed the test as a way to examine whether or not machines can think -- or appear to think -- like humans. The classic test is a game of imitation. In this game, an interrogator asks two participants a series of questions. One of the participants is a machine and the other is a human. The interrogator can't see or hear the participants and has no way of knowing which is which. If the interrogator is unable to figure out which participant is a machine based on the responses, the machine passes the Turing Test.
Of course, with a CAPTCHA, the goal is to create a test that humans can pass easily but machines can't. It's also important that the CAPTCHA application is able to present different CAPTCHAs to different users. If a visual CAPTCHA presented a static image that was the same for every user, it wouldn't take long before a spammer spotted the form, deciphered the letters, and programmed an application to type in the correct answer automatically.
Most, but not all, CAPTCHAs rely on a visual test. Computers lack the sophistication that human beings have when it comes to processing visual data. We can look at an image and pick out patterns more easily than a computer. The human mind sometimes perceives patterns even when none exist, a quirk we call pareidolia. Ever see a shape in the clouds or a face on the moon? That's your brain trying to associate random information into patterns and shapes.
But not all CAPTCHAs rely on visual patterns. In fact, it's important to have an alternative to a visual CAPTCHA. Otherwise, the Web site administrator runs the risk of disenfranchising any Web user who has a visual impairment. One alternative to a visual test is an audible one. An audio CAPTCHA usually presents the user with a series of spoken letters or numbers. It's not unusual for the program to distort the speaker's voice, and it's also common for the program to include background noise in the recording. This helps thwart voice recognition programs.
Another option is to create a CAPTCHA that asks the reader to interpret a short passage of text. A contextual CAPTCHA quizzes the reader and tests comprehension skills. While computer programs can pick out key words in text passages, they aren't very good at understanding what those words actually mean.