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.
Many implementations of CAPTCHAs use undistorted text, or text with only minor distortions. In addition to making the images unreadable by computers, the system should ensure that there are no easy ways around it at the script level.
These implementations are vulnerable to simple automated attacks. Common examples of insecurities in this respect include: (1) Systems that pass the answer to the CAPTCHA in plain text as part of the web form.
A free, secure and accessible CAPTCHA implementation is available from the re CAPTCHA project.
Easy to install plugins and controls are available for Word Press, Media Wiki, PHP, ASP.
CAPTCHAs provide an effective mechanism to hide your email address from Web scrapers.
The idea is to require users to solve a CAPTCHA before showing your email address.NET, Perl, Python, Java, and many other environments.re CAPTCHA also comes with an audio test to ensure that blind users can freely navigate your site. Instead of typing letters, you authenticate yourself as a human by recognizing what object is common in a set of images. Up until a few years ago, most of these services suffered from a specific type of attack: "bots" that would sign up for thousands of email accounts every minute.A free and secure implementation that uses CAPTCHAs to obfuscate an email address can be found at re CAPTCHA Mail Hide. In November 1999, 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 using programs that voted for CMU thousands of times. The next day, students at MIT wrote their own 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. Not unless the poll ensures that only humans can vote. CAPTCHAs can also be used to prevent dictionary attacks in password systems.An example of such a puzzle is asking text-based questions, such as a mathematical question ("what is 1 1").