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The reCAPTCHA ASP.NET Library provides a straightforward way to place a CAPTCHA on your ASP.NET website, helping you stop bots from abusing your website. The library wraps the reCAPTCHA API. You can use the library from any .NET language including C# and Visual Basic .NET.

A CAPTCHA is a program that can generate and grade 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 twist with reCAPTCHA is that the distorted text comes from flaws in the process of digitizing books:

To archive human knowledge and to make information more accessible to the world, multiple projects are currently digitizing physical books that were written before the computer age. The book pages are being photographically scanned, and then transformed into text using "Optical Character Recognition" (OCR).

The transformation into text is useful because scanning a book produces images, which are difficult to store on small devices, expensive to download, and cannot be searched. The problem is that OCR is not perfect. Example of OCR errors reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.

But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.

Currently, we are helping to digitize books from the Internet Archive and old editions of the New York Times.


ReCAPTCHA also provides an email address captcha.

The one common complaint is that the reCaptcha control does not expose a ValidationGroup property. So, some rework may be required if you intend to use it on a site with an already complicated ValidationGroup structure.

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