are you a human?

C#/ASP.NET SMS Example

// Default.apsx

<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Bogus account creation page</title>
</head>
<body>
    <center>
    <label id="firstCode" runat="server"/>
    <form id="form1" runat="server">
        <input type="hidden" name="form_id" id="form_id" value="" runat="server" />   
        Username:<input type="text" name="form_name" id="form_user"/><br/>    
        Code:<input type="text" name="form_code" id="form_code"/><br/>
        <input type="submit" id="form_submit" />    
    </form>
    <label id="statusMsg" runat="server"/>
    </center>
</body>
</html>

// Default.aspx.cs

using System;
using System.Collections;
using System.Net;

public partial class _Default : System.Web.UI.Page 
{
    /*
     * All requests end up here. If the 'code' and 'id' are present,
     * we assume the user is attempting to pass the CAPTCHA. If the
     * 'code' is not present, it's assumed we need to present the form.
     */
 
    protected void Page_Load(object sender, EventArgs e)
    {           
        if(Request.Form["form_id"] != null && Request.Form["form_code"] != null)
        {
            if(ValidateCode(Request.Form["form_id"], Request.Form["form_code"]))
            {
                form1.Visible = false;
                firstCode.Visible = false;
                statusMsg.InnerText = "Account Created!";                
            }
            else
            {
                RefreshForm();
                statusMsg.InnerText = "Incorrect Code";               
            }
        }
        else
        {
            RefreshForm();
        }
    }

private void RefreshForm()
    {
        form_id.Value = GenerateRandomKey();
        string initialCode = GetData(String.Format("https://areyouahuman.org/captcha.pl?id={0}&type=SMS2", form_id.Value));           
        firstCode.InnerText = String.Format("Text 'VERIFY {0}' to 41411. In response to this you will receive a code. Enter it below.", initialCode);
        firstCode.Visible = true;
    }

private string GetData(string url)
    {
        /*
         * This is a simple help to retrieve the reponse 
         * from the CAPTCHA api. WebClient is declared in
         * System.Net      
         */

WebClient wc = new WebClient();
        string response = "";
        response = wc.DownloadString(url);

return response;
    }

private bool ValidateCode(string id, string code)
    {
        /*
         * The 'code' and 'id' submitted by the user end up here. We 
         * send them off to the CAPTCHA system for validation. If the 
         * code matches, the CAPTCHA system responds with 'success'.
         * 
         */

string url = String.Format("https://areyouahuman.org/captcha.pl?id={0}&code={1}&type=SMS2", id, code);
        string reponse = GetData(url);

if(reponse == "success")
            return true;
        else
            return false;

}

private string GenerateRandomKey()
    {     
        /*
         * Generate a 32 byte alphanumeric key. This value
         * and format is required by the CAPTCHA system.
         * 
         */

ArrayList alphabet = new ArrayList();
        Random rand = new Random();
        string key = "";
        int i = 0;

// 0 - 9
        for(i = (int)'0'; i <= (int)'9'; i++)
        {
            alphabet.Add((char)i);
        }

// a - z
        for(i = (int)'a'; i <= (int)'z'; i++)
        {
            alphabet.Add((char)i);
        }

// A - Z
        for(i = (int)'A'; i <= (int)'Z'; i++)
        {
            alphabet.Add((char)i);
        }

for(i = 0; i < 32; i++)
        {                        
            key += alphabet[rand.Next(0, alphabet.Count)];
        }

return key;
    }
}