25-07-2012, 11:07 AM
CAPTCHA
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ABSTRACT
One common application of CAPTCHA is for verifying online polls. In fact, a former Slashdot poll serves as an example of what can go wrong if pollsters don't implement filters on their surveys. In 1999, Slashdot published a poll that asked visitors to choose the graduate school that had the best program in computer science. Students from two universities -- Carnegie Mellon and MIT -- created automated programs called bots to vote repeatedly for their respective schools. While those two schools received thousands of votes, the other schools only had a few hundred each. If it's possible to create a program that can vote in a poll, how can we trust online poll results at all? A CAPTCHA form can help prevent programmers from taking advantage of the polling system.
Registration forms on Web sites often use CAPTCHAs. For example, free Web-based e-mail services like Hotmail, yahoo mail or gmailallow people to create an e-mail account free of charge. Usually, users must provide some personal information when creating an account, but the services typically don't verify this information. They use CAPTCHAs to try to prevent spammers from using bots to generate hundreds of spam mail accounts.
HowStuffWorks Yahoo uses alphanumeric strings rather than words as CAPTCHAs when you sign up for a Yahoo! account. Ticket brokers like TicketMaster also use CAPTCHA applications. These applications help prevent ticket scalpers from bombarding the service with massive ticket purchases for big events. Without some sort of filter, it's possible for a scalper to use a bot to place hundreds or thousands of ticket orders in a matter of seconds. Legitimate customers become victims as events sell out minutes after tickets become available. Scalpers then try to sell the tickets above face value. While CAPTCHA applications don't prevent scalping, they do make it more difficult to scalp tickets on a large scale.
Some Web pages have message boards or contact forms that allow visitors to either post messages to the site or send them directly to the Web administrators. To prevent an avalanche of spam, many of these sites have a CAPTCHA program to filter out the noise. A CAPTCHA won't stop someone who is determined to post a rude message or harass an administrator, but it will help prevent bots from posting messages automatically.
The most common form of CAPTCHA requires visitors to type in a word or series of letters and numbers that the application has distor-
ted in some way. Some CAPTCHA creators came up with a way to increase the value of such an application: digitizing books. An application called reCAPTCHA harnesses users responses in CAPTCHA fields to verify the contents of a scanned piece of paper. Because computers aren't always able to identify words from a digital scan, humans have to verify what a printed page says. Then it's possible for search engines to search and index the contents of a scanned document.
Here's how it works: First, the administrator of the reCAPTCHA program digitally scans a book. Then, the reCAPTCHA program selects two words from the digitized image. The application already recognizes one of the words. If the visitor types that word into a field correctly, the application assumes the second word the user types is also correct. That second word goes into a pool of words that the application will present to other users. As each user types in a word, the application compares the word to the original answer. Eventually, the application receives enough responses to verify the word with a high degree of certainty. That word can then go into the verified pool.
It sounds time consuming, but remember that in this case the CAPTCHA is pulling double duty. Not only is it verifying the contents of a digitized book, it's also verifying that the people filling out the form are actually people. In turn, those people are gaining access to a service they want to use
INTRODUCTION
what is captcha?
If you try to get a new email account at Yahoo, you'll be asked to prove that you're a human and not a computer. Why? Because a single computer program can get thousands of free email accounts per second. And that's bad for Yahoo. But how do you prove to a computer that you'rea human?
Proving that you're a human to another human can be done using an idea from the 1950s: theTuring Test. A human judge asks you a bunch of questions and decides, depending on your answers, whether he's talking to a human or a computer. Proving that you're a human to a computer is another matter. It requires a test (or a set of tests) that computers can grade, humans can pass, but paradoxically, computers can't pass. In our lingo, it requires a captcha.so
A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot. For example, humans can read distorted text as the one shown below, but current computer programs can't:
The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University.
It is a contrived acronym based on the word "capture" and standing for "Completely Automated Public Turing test to tell Computers and Humans Apart". Carnegie Mellon University attempted to trademark the term,but the trademark application was abandoned on 21 April 2008.
Why they are used
CAPTCHAs are used to prevent robots from submitting forms and creating accounts, spamming and various other things. In some cases robots can cause some problems. Take for example a robot signing up for thousands of Gmail accounts. While it might not cause much stress on Gmail’s servers it would create lots of email accounts that could be used for spamming people. Another case is spammers creating accounts on forums and then spam the forum. CAPTCHAs help prevent robots from using websites and webapps.
Luis von Ahn of Carnegie Mellon University is one of the inventors of CAPTCHA, In the year of 2006.
Captcha is some times called reverse turing test.
Reverse 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.
Characteristics
A CAPTCHA is a means of automatically generating challenges which intends to:
• Provide a problem easy enough for all humans to solve.
• Prevent standard automated software from filling out a form, unless it is specially designed to circumvent specific CAPTCHA systems.
A check box in a form that reads "check this box please" is the simplest (and perhaps least effective) form of a CAPTCHA. CAPTCHAs do not have to rely on difficult problems in artificial intelligence, although they can.
This has the benefit of distinguishing humans from computers. It also creates incentive to further develop artificial intelligence of computers
Type of captcha:
There are two main type of captcha:
1.visual
3.graphics
3.audio(it is mainly used for visual imapaired people)
1.Gimpy :
Gimpy(it is text based captcha). gimpy works as follows: it picks seven words out of a dictionary, and renders a
distorted image containing the words. gimpy then presents a
test to its user, which consists of the distorted image and the directions: \type three words
appearing in the image". Given the types of deformations that gimpy uses, most humans can
read three words from the distorted image, while current computer programs can't.