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.
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.
True CAPTCHAs should be secure even after a significant number of websites adopt them. In general, making your own CAPTCHA script (e.g., using PHP, Perl or .
Net) is a bad idea, as there are many failure modes.
However, in order to truly guarantee that bots won't enter a web site, CAPTCHAs are needed. 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.
If your website needs protection from abuse, it is recommended that you use a CAPTCHA.Please consult with a translator for accuracy if you are relying on the translation or are using this site for official business.If you have any questions please contact: Bilingual Services Program at (916) 324-5482 A copy of this disclaimer can also be found on our Disclaimer page.(2) Systems where a solution to the same CAPTCHA can be used multiple times (this makes the CAPTCHA vulnerable to so-called "replay attacks").Most CAPTCHA scripts found freely on the Web are vulnerable to these types of attacks. There are various "CAPTCHAs" that would be insecure if a significant number of sites started using them.The economics of this attack just don't add up: every time a porn site shows a CAPTCHA before a porn image, they risk losing a customer to another site that doesn't do this.