What is the difference between CAPTCHA and reCAPTCHA?


I know that reCAPTCHA is later version of CAPTCHA, and both serve up challenges that, hopefully, humans will pass but bots will not. Is there an advantage for users (by users I mean the website using them) of one versus the other?

Topic: Security
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CAPTCHA is the human validation tes (usually the blurry squiglly letters that need to be deciphered) t used by many sites to prevent spam. 

reCAPTCHA is a reversed CAPTCHA - the same test, used not only to prevent spam but to help in the book digitazion project. In other words, the reCAPTCHA tests are not meaningless combination of words, but excerpts from books that undergo digitation, while CAPTCHA uses several human validation methods including math or general knowledge questions, visual puzzles and even chess puzzles. 

Google purchased reCAPTCHA several years ago, and not it is also used to collect street view data. 

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A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a program that can tell whether its user is a human or a computer.The process involves a computer asking a user to complete a simple test which generated by computer. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human.Sometimes it is described as a reverse Turing test, because it is done by a machine and targeted to a human. reCAPTCHA does exactly that by channeling the effort spent solving CAPTCHAs online into "reading" books.

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reCaptcha is hosted by Google, and one of the more interesting things about it is that it is used to digitize text of old newspapers and books. That’s why there are two “sections” of a reCaptcha instead of the single series of characters for CAPTCHA - one is known text, the other is not. If you get the known one correct, it assumes you got the second one. Then the next time it offers up that same “unknown” text, it is considered possibly known. A few more times with the same result for the “unknown” text, and it becomes “known” and the text it originated from can be correctly digitized. Clever, eh? Also, because of frequent updates, I would expect reCAPTCHA to be slightly better at preventing bots from solving them.

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