Nuno Silva

reCaptcha - Part 1

What is reCaptcha?

reCaptcha is a Captcha program that can tell the difference between a human and a machine. And Plus, it helps to digitalize books. And how does it do it?

Recapcha uses a 2 word challenge. A first word, that is an already known word, and a second word that comes from a digitalized book. If the user inserts the known word correctly, then the second word is assumed as correct and you helped to translate a digitalize one word.

A more complete explanation can be found here, at reCaptcha official page.

How to use reCaptcha

1. Register in the website. With your registration you will have to give the domain where you intend to use reCaptcha.
2. Then, a private and a public key will be generated (for localhost test proposes, you can use the same keys as in production).
3. Insert the following piece of javascript into your page:

   <script type="text/javascript"
   src="http://api.recaptcha.net/challenge?k=your_public_key">
   </script>
   
   <noscript>
   <iframe src="http://api.recaptcha.net/noscript?k=your_public_key"
   height="300" width="500" frameborder="0"></iframe><br>
   <textarea name="recaptcha_challenge_field" rows="3" cols="40">
   </textarea>
   <input type="hidden" name="recaptcha_response_field"
   value="manual_challenge">
   </noscript>

And a box, similar to the one below, will appear.

Then, for this process to work, a request must be made to http://api-verify.recaptcha.net/verify with the right parameters in order to validate the captcha.

The steps to do this are:

1. Insert a submit button and place the above script and that button inside a form.
2. Click on the submit button so all the input fields can be posted into the server.
3. Then make a Web Request to “http://api-verify.recaptcha.net/verify” with the right parameters. These parameters are:
   1. privatekey - the private you already generated
   2. remoteip - the DNS name of the remote client
   3. challenge - the text you wrote in the reCaptcha control. The value of “recaptcha_challenge_field” sent via the form
   4. response - The value of “recaptcha_response_field” sent via the form
4. Verify if the response is correct.

The Web request you make to the above url will retrieve a response separated by a \n. In the first part of the response, the possible values are true or false. If the first part has the value false, then the second part is the error code. You can find the list of error codes here in the section “Error Code Reference”. This page also has all the information you need make your implementation.

In Part 2 of this article i will show how to implement an ASP.NET Validator control that uses this API.

Related Posts

• reCaptcha - Part 2