Using Recaptcha in an MVC3 / Razor application

// First, install the reCaptcha library from Nuget https://nuget.org/packages/recaptcha.
// Then, create your public and private keys at http://www.google.com/recaptcha.

// 1. Create an extension method to genereate the Recaptcha javascript
public static class Extension
{
    public static MvcHtmlString GenerateCaptcha(this HtmlHelper htmlHelper)
    {
        var html = Recaptcha.RecaptchaControlMvc.GenerateCaptcha(htmlHelper);
        return MvcHtmlString.Create(html);
    }
}

// 2. In the controller, add the CaptchaValidatorAttribute attribute and the captchaValid and captchaErrorMessage parameters to the method
[HttpPost]
[Recaptcha.RecaptchaControlMvc.CaptchaValidatorAttribute]
public ActionResult PostMessage(Message message, bool captchaValid, string captchaErrorMessage)
{
    if(captchaValid)
    {
        /// ... do something ... ///
    }
}

// 3. In the Global.asax's Application_Start method, assign you public and private keys
protected void Application_Start()
{
    Recaptcha.RecaptchaControlMvc.PublicKey = "0123456789";
    Recaptcha.RecaptchaControlMvc.PrivateKey = "9876543210";
}

// 4. In the view, add the following to generate the Recaptcha content
<tr>
    <td></td>
    <td>@Html.GenerateCaptcha()</td>
</tr>