Using Google reCAPTCHA in ASP.NET MVC - Validate reCAPTCHA Attribute

ValidateGoogleCaptchaAttribute.cs

public class ValidateGoogleCaptchaAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        const string urlToPost = "https://www.google.com/recaptcha/api/siteverify";
        const string secretKey = SiteSettings.GoogleRecaptchaSecretKey;
        var captchaResponse = filterContext.HttpContext.Request.Form["g-recaptcha-response"];

        if (string.IsNullOrWhiteSpace(captchaResponse)) AddErrorAndRedirectToGetAction(filterContext);

        var validateResult = ValidateFromGoogle(urlToPost, secretKey, captchaResponse);
        if (!validateResult.Success) AddErrorAndRedirectToGetAction(filterContext);

        base.OnActionExecuting(filterContext);
    }

    private static void AddErrorAndRedirectToGetAction(ActionExecutingContext filterContext)
    {
        filterContext.Controller.TempData["InvalidCaptcha"] = "Invalid Captcha !";
        filterContext.Result = new RedirectToRouteResult(filterContext.RouteData.Values);
    }

    private static ReCaptchaResponse ValidateFromGoogle(string urlToPost, string secretKey, string captchaResponse)
    {
        var postData = "secret=" + secretKey + "&response=" + captchaResponse;

        var request = (HttpWebRequest)WebRequest.Create(urlToPost);
        request.Method = "POST";
        request.ContentLength = postData.Length;
        request.ContentType = "application/x-www-form-urlencoded";

        using (var streamWriter = new StreamWriter(request.GetRequestStream()))
            streamWriter.Write(postData);

        string result;
        using (var response = (HttpWebResponse)request.GetResponse())
        {
            using (var reader = new StreamReader(response.GetResponseStream()))
                result = reader.ReadToEnd();
        }

        return JsonConvert.DeserializeObject<ReCaptchaResponse>(result);
    }
}

internal class ReCaptchaResponse
{
    [JsonProperty("success")]
    public bool Success { get; set; }

    [JsonProperty("challenge_ts")]
    public string ValidatedDateTime { get; set; }

    [JsonProperty("hostname")]
    public string HostName { get; set; }

    [JsonProperty("error-codes")]
    public List<string> ErrorCodes { get; set; }
}