Derik Whittaker

For all the debate regarding if Captcha's are a good thing or a bad thing one thing is certain (in my book).  If you do not have some way to stop spam-bots your site will become overridden with junk in a hurry.

In an effort to reduce the amount of spam comments I am getting over at Dimecasts.net I finally got off my lazy ass and worked on implementing a Captcha.  The solution I ended up going with was reCaptcha.  I found that their setup was very easy to use and worked right out of the box.  However, there was not a lot of information on the net on how to use reCaptcha within an MVC site, only Asp.Net Webforms.  So I thought I would share my experiences and explain how I implemented reCaptcha on Dimecasts.

Step 1 - Signup for and download the reCaptcha dll from their site

Step 2 - Add reference to the Recaptcha.dll to your project

Step 3 - Create an Action Filter to handle the Captcha validation

public class CaptchaValidatorAttribute : ActionFilterAttribute 
{
	private const string CHALLENGE_FIELD_KEY = "recaptcha_challenge_field";
        private const string RESPONSE_FIELD_KEY = "recaptcha_response_field";
        
        public override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            var captchaChallengeValue = filterContext.HttpContext.Request.Form[CHALLENGE_FIELD_KEY];
            var captchaResponseValue = filterContext.HttpContext.Request.Form[RESPONSE_FIELD_KEY];
            var captchaValidtor = new Recaptcha.RecaptchaValidator
                                      {
                                          PrivateKey = -- PUT PRIVATE KEY HERE --,
                                          RemoteIP = filterContext.HttpContext.Request.UserHostAddress,
                                          Challenge = captchaChallengeValue,
                                          Response = captchaResponseValue
                                      };

            var recaptchaResponse = captchaValidtor.Validate();

	    // this will push the result value into a parameter in our Action
            filterContext.ActionParameters["captchaValid"] = recaptchaResponse.IsValid;

            base.OnActionExecuting(filterContext);
        }
}

Step 4 - Implement the Controller Action that will handle the form submission and Captcha validation

[CaptchaValidator]
[AcceptVerbs( HttpVerbs.Post )]
public ActionResult CreateComment( Int32 id, bool captchaValid )
{
.. Do something here
}

Step 5 - Create a Html Helper to build and render the Captcha control

      
public static string GenerateCaptcha( this HtmlHelper helper )
{
            
	var captchaControl = new Recaptcha.RecaptchaControl
        	{
	                ID = "recaptcha",
                	Theme = "blackglass",
        		PublicKey = -- Put Public Key Here --,
                        PrivateKey = -- Put Private Key Here --
		};

	var htmlWriter = new HtmlTextWriter( new StringWriter() );

	captchaControl.RenderControl(htmlWriter);

	return htmlWriter.InnerWriter.ToString();
}

Step 6 - Implement the logic in your view to actually render the Captcha control

<:%= Html.GenerateCaptcha() %>:

Step 7 - Oh wait, there is no step 7.  You are done.

There you go, that is all that is needed to setup reCaptcha for use in a MVC application

Till next time,

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