Here's how we were able to validate the RECAPTCHA using .NET:
FRONT-END
<div id="rcaptcha" class="g-recaptcha" data-sitekey="[YOUR-KEY-GOES-HERE]" data-callback="onFepCaptchaSubmit"></div>
BACK-END:
public static bool IsCaptchaValid(HttpRequestBase requestBase)
{
var recaptchaResponse = requestBase.Form["g-recaptcha-response"];
if (string.IsNullOrEmpty(recaptchaResponse))
{
return false;
}
string postData = string.Format("secret={0}&response={1}&remoteip={2}", "[YOUR-KEY-GOES-HERE]", recaptchaResponse, requestBase.UserHostAddress);
byte[] data = System.Text.Encoding.ASCII.GetBytes(postData);
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://www.google.com/recaptcha/api/siteverify");
request.Method = "POST";
request.ContentType = "application/x-www-form-urlencoded";
request.ContentLength = data.Length;
using (var stream = request.GetRequestStream())
{
stream.Write(data, 0, data.Length);
}
var response = (HttpWebResponse)request.GetResponse();
var responseString = "";
using (var sr = new System.IO.StreamReader(response.GetResponseStream()))
{
responseString = sr.ReadToEnd();
}
return System.Text.RegularExpressions.Regex.IsMatch(responseString, "\"success\"(\\s*?):(\\s*?)true", System.Text.RegularExpressions.RegexOptions.Compiled);
}
Call the above method within your Controller's POST action.