ASP.NET CAPTCHA Randomization C# Code Sample

The ASP.NET Captcha randomization sample project shows how to easily randomize various BotDetect CAPTCHA Captcha control parameters, beyond the basic image and sound style randomization used by default.

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Check the BotDetect Developer Crash Course for key integration steps.

Randomly using different Captcha image and sound generation algorithms and other parameter values (such as code length and style) can significantly improve the Captcha security.

This is the recommended approach to Captcha property setting, since it takes full advantage of the numerous Captcha image and sound generation algorithms shipped with BotDetect, as well as built-in randomization features.

Setting Captcha control properties in the InitializedCaptchaControl event handler used for randomization also reduces the amount of Session State used by the control.

Download the BotDetect ASP.NET CAPTCHA Component and run this sample

Visual Studio 2012 / .NET 4.5

By default, the .NET 4.5 C# version of the ASP.NET Captcha randomization sample project is installed at:
C:\Program Files\Lanapsoft\BotDetect 3 CAPTCHA Component\Asp.Net\v4.5\WebApp\CaptchaRandomizationSample\CSharp

You can also run it from the BotDetect Start Menu:
Programs > Lanapsoft > BotDetect 3 CAPTCHA Component > ASP.NET > DotNET 4.5 Web Applications > Run

The Visual Studio 2012 / .NET 4.5 source has no essential differences from the Visual Studio 2010 / .NET 4.0 source.

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Visual Studio 2010 / .NET 4.0

By default, the .NET 4.0 C# version of the ASP.NET Captcha randomization sample project is installed at:
C:\Program Files\Lanapsoft\BotDetect 3 CAPTCHA Component\Asp.Net\v4.0\WebApp\CaptchaRandomizationSample\CSharp

You can also run it from the BotDetect Start Menu:
Programs > Lanapsoft > BotDetect 3 CAPTCHA Component > ASP.NET > DotNET 4.0 Web Applications > Run

Default.aspx

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

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

<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
  <title>BotDetect CAPTCHA Randomization ASP.NET Sample</title>
  <link type="text/css" rel="Stylesheet" href="StyleSheet.css" />
</head>
<body>
  <form id="form1" runat="server">
  <h1>BotDetect CAPTCHA Randomization ASP.NET Sample</h1>
  <fieldset>
    <legend>CAPTCHA Validation</legend>
    <p class="prompt"><label for="CaptchaCodeTextBox">Retype the 
    characters from the picture:</label></p>
    <BotDetect:Captcha ID="SampleCaptcha" runat="server" />
    <div class="validationDiv">
      <asp:TextBox ID="CaptchaCodeTextBox" runat="server"></asp:
      TextBox>
      <asp:Button ID="ValidateCaptchaButton" runat="server" />
      <asp:Label ID="CaptchaCorrectLabel" runat="server" 
      CssClass="correct"></asp:Label>
      <asp:Label ID="CaptchaIncorrectLabel" runat="server" 
      CssClass="incorrect"></asp:Label>
    </div>
  </fieldset>
  </form>
</body>
</html>

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Default.aspx.cs

using System;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;

using BotDetect;
using BotDetect.Web;
using BotDetect.Web.UI;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Init(object sender, EventArgs e)
    {
        SampleCaptcha.InitializedCaptchaControl += 
            new EventHandler<InitializedCaptchaControlEventArgs>(
            SampleCaptcha_InitializedCaptchaControl);
    }

    /// <summary>
    /// all CAPTCHA randomization should be performed in these event 
    handlers instead of Page_Load 
    /// or Page_PreRender, because this event is also fired for direct 
    CAPTCHA image requests
    /// (which skip the page events since the page is never loaded), 
    for example when clicking 
    /// the Reload CAPTCHA button repeatedly
    /// </summary>
    void SampleCaptcha_InitializedCaptchaControl(object sender, 
    InitializedCaptchaControlEventArgs e)
    {
        if (e.CaptchaId != SampleCaptcha.CaptchaId)
        {
            return;
        }
        
        CaptchaControl captcha = sender as CaptchaControl;

        // randomize code generation properties
        captcha.CodeStyle = CaptchaRandomization.GetRandomCodeStyle();
        captcha.CodeLength = CaptchaRandomization.GetRandomCodeLength(
        4, 5);

        // use an image style randomly selected from the given subset
        ImageStyle[] imageStyles = { 
            ImageStyle.Lego, 
            ImageStyle.MeltingHeat, 
            ImageStyle.Ghostly,
            ImageStyle.Fingerprints, 
            ImageStyle.Graffiti2, 
            ImageStyle.Bullets2,
            ImageStyle.CaughtInTheNet2, 
            ImageStyle.Collage, 
            ImageStyle.Chalkboard
        };
        captcha.ImageStyle = CaptchaRandomization.GetRandomImageStyle(
        imageStyles);

        // OR: use a completely random image style
        //captcha.ImageStyle = CaptchaRandomization.
        GetRandomImageStyle();

        // use a sound style randomly selected from the given subset
        SoundStyle[] soundStyles = { 
            SoundStyle.Dispatch, 
            SoundStyle.Radio, 
            SoundStyle.Synth
        };
        captcha.SoundStyle = CaptchaRandomization.GetRandomSoundStyle(
        soundStyles);

        // OR: use a completely random sound style
        //captcha.SoundStyle = CaptchaRandomization.
        GetRandomSoundStyle();
    }

    protected void Page_PreRender(object sender, EventArgs e)
    {
        // initial page setup
        if (!IsPostBack)
        {
            // set control text
            ValidateCaptchaButton.Text = "Validate";
            CaptchaCorrectLabel.Text = "Correct!";
            CaptchaIncorrectLabel.Text = "Incorrect!";

            // these messages are shown only after validation
            CaptchaCorrectLabel.Visible = false;
            CaptchaIncorrectLabel.Visible = false;
        }

        SampleCaptcha.UserInputClientID = CaptchaCodeTextBox.ClientID;

        if (IsPostBack)
        {
            // validate the Captcha to check we're not dealing with a 
            bot
            string code = CaptchaCodeTextBox.Text.Trim().ToUpper();
            bool isHuman = SampleCaptcha.Validate(code);
            CaptchaCodeTextBox.Text = null; // clear previous user 
            input

            if (isHuman)
            {
                CaptchaCorrectLabel.Visible = true;
                CaptchaIncorrectLabel.Visible = false;
            }
            else
            {
                CaptchaCorrectLabel.Visible = false;
                CaptchaIncorrectLabel.Visible = true;
            }
        }
    }
}

In the Page_Init phase of ASP.NET page life-cycle, we register a special event handler used for Captcha randomization. Since Captcha images and sounds are generated and sent to the client in Http requests separate from the one loading the ASP.NET page (when page code-behind gets executed), we must register this event handler to ensure Captcha challenges are randomized every time they are generated, and not only once per page load.

This is important because Captcha requests don't have to be tied to the number of page loads - most notably, when using the Reload Captcha button, and when bots are accessing Captcha images and sounds directly, it's possible that many Captcha images or sounds will be generated after only a single page load (and the related Page_PreRender execution).

In the SampleCaptcha_InitializedCaptchaControl handler, the Captcha control instance is accessible as the sender parameter. To make the randomization as simple as possible, we use the CaptchaRandomization class, which allows us to get a random value of a given parameter from all available values (as the Captcha CodeStyle), or from a given range of values (as the Captcha CodeLength), or from the given set of values (as the Captcha ImageStyle).

You can also randomize other Captcha properties in a similar manner, but randomized Captcha image drawing and sound generating styles as well as the Captcha code length improve Captcha security the most. Every individual Captcha generating algorithm can theoretically and eventually be broken (given enough effort etc., as described in the Can CAPTCHA Be Broken? article), but if the bot also has to recognize which algorithm does each individual image or sound use, the code recognition task becomes an order of magnitudes harder.

Also keep in mind that several popular Captcha implementations have been broken because they used a fixed number of characters in their Captcha images – disregarding that "find 5 characters in this image" is inherently a much easier task to automate than "find an unknown number of characters in this image". So always use a random Captcha code length to keep your Captcha implementation at peak security levels.

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Web.config

<?xml version="1.0"?>
<configuration xmlns="http://schemas.microsoft.com/.
NetConfiguration/v2.0">
  <system.web>
    <httpHandlers>
      <!-- Register the HttpHandler used for BotDetect Captcha 
      requests -->
      <add verb="GET" path="BotDetectCaptcha.ashx" 
	  type="BotDetect.Web.CaptchaHandler, BotDetect"/>
    </httpHandlers>
    <!-- Register a custom SessionIDManager for BotDetect Captcha 
    requests -->
    <sessionState mode="InProc" cookieless="AutoDetect" timeout="20" 
    sessionIDManagerType="BotDetect.Web.CustomSessionIdManager, 
    BotDetect"/>
    <!-- Session state is required for BotDetect storage; you can also 
    turn if off globally and only enable for BotDetect-protected pages 
    if you prefer -->
    <pages enableSessionState="true">
      <controls>
        <!-- Register the BotDetect tag prefix for easier use in all 
        pages -->
        <add assembly="BotDetect" namespace="BotDetect.Web.UI" 
        tagPrefix="BotDetect"/>
      </controls>
    </pages>
    <compilation debug="true" targetFramework="4.0">
      <assemblies>
        <add assembly="System.Design, Version=4.0.0.0, Culture=neutral,
        PublicKeyToken=B03F5F7F11D50A3A"/>
      </assemblies>
    </compilation>
    <trace enabled="false" localOnly="true"/>
    <httpCookies httpOnlyCookies="true"/>
    <trust level="Medium" originUrl=""/>
    <authentication mode="None"/>
    <customErrors mode="RemoteOnly"></customErrors>
  </system.web>
  <system.webServer>
    <validation validateIntegratedModeConfiguration="false"/>
    <handlers>
      <!-- Register the HttpHandler used for BotDetect Captcha 
      requests (IIS 7.0+) -->
      <remove name="BotDetectCaptchaHandler"/>
      <add name="BotDetectCaptchaHandler" 
      preCondition="integratedMode" verb="GET" 
	  path="BotDetectCaptcha.ashx" 
	  type="BotDetect.Web.CaptchaHandler, BotDetect"/>
    </handlers>
  </system.webServer>
</configuration>

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Visual Studio 2008 / .NET 3.5

By default, the .NET 3.5 C# version of the ASP.NET Captcha randomization sample project is installed at:
C:\Program Files\Lanapsoft\BotDetect 3 CAPTCHA Component\Asp.Net\v3.5\WebApp\CaptchaRandomizationSample\CSharp

You can also run it from the BotDetect Start Menu:
Programs > Lanapsoft > BotDetect 3 CAPTCHA Component > ASP.NET > DotNET 3.5 Web Applications > Run

The Visual Studio 2008 / .NET 3.5 source has no essential differences from the Visual Studio 2010 / .NET 4.0 source.

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Visual Studio 2005 / .NET 2.0

By default, the .NET 2.0 C# version of the ASP.NET Captcha randomization sample project is installed at:
C:\Program Files\Lanapsoft\BotDetect 3 CAPTCHA Component\Asp.Net\v2.0\WebApp\CaptchaRandomizationSample\CSharp

You can also run it from the BotDetect Start Menu:
Programs > Lanapsoft > BotDetect 3 CAPTCHA Component > ASP.NET > DotNET 2.0 Web Applications > Run

The Visual Studio 2005 / .NET 2.0 source has no essential differences from the Visual Studio 2010 / .NET 4.0 source.

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