CAPTCHA

CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot.


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. At the time, they developed the first CAPTCHA to be used by Yahoo. CAPTCHA is a free Anti-Bot service that helps digitize your books. It has a Strong Security, Accesible for the blind people and of course, it is FREE. 


Because CAPTCHAs rely on visual perception, users unable to view a CAPTCHA due to a disability will be unable to perform the task protected by a CAPTCHA. Therefore, sites implementing CAPTCHAs may provide an audio version of the CAPTCHA in addition to the visual method. The official CAPTCHA site recommends providing an audio CAPTCHA for accessibility reasons, but it is not usable for deafblind people or for users of text web browsers. This combination is not universally adopted, with most websites (including Wikipedia) offering only the visual CAPTCHA, with or without providing the option of generating a new image if one is too difficult to read.


Applications of CAPTCHAs

• Preventing Spam on your Blogs - Most bloggers are familiar with programs that submit bogus comments, usually for the purpose of raising search engine ranks of some website (e.g., "buy penny stocks here"). This is called comment spam. By using a CAPTCHA, only humans can enter comments on a blog. There is no need to make users sign up before they enter a comment, and no legitimate comments are ever lost!
• Protecting Website Registration - Most of the Service Companies like Yahoo & Google suffered from a terrorist attack. (not in Bombing) but by attacking of "Bots" that will create and sign up for thousands of email accounts every minute.The solution to this problem was to use CAPTCHAs to ensure that only humans obtain free accounts. In general, free services should be protected with a CAPTCHA in order to prevent abuse by automated programs.
• Online Polls - by voting in polls, IP addresses of voters were recorded in order to prevent single users from voting more than once.
• Preventing Dictionary Attacks - It also prevent a computer from being able to iterate through the entire space of passwords by requiring it to solve a CAPTCHA after a certain number of unsuccessful logins.
• Search Engine Bots - they usually belong to large companies, respect web pages that don't want to allow them in. However, in order to truly guarantee that bots won't enter a web site, CAPTCHAs are needed.
• Worms and Spam - CAPTCHAs also offer a plausible solution against email worms and spam: "I will only accept an email if I know there is a human behind the other computer." A few companies are already marketing this idea.