How to create simple mathmatical captcha in ASP.Net C#.Net and VB.Net

In this tutorial i am going to explain about how to create simple mathmatical captcha in asp.net c#.net and vb.net. The basic idea is generating two numbers using random function and storing the addition of that two numbers in the session and also converting the numbers into image using the available libraries. On the target page where the captcha is implemented we will have the textbox to get the input from the user and check the textbox value is equal to the session value. It it is equal then we consider the user is a human else it is a robot.

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Here to explain the process i have created an empty website. Then do the right click on the website and click Add then Add New Item as shown in the figure.

Then select WebForm Template and then name the form as Captcha.aspx as shown in the figure.

Leave the aspx file. We no need to do anything in that file.In the code behind file include the necessary namespaces. The entire code is given below.

C#.Net:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Drawing;

using System.Drawing.Imaging;

using System.Drawing.Text;

using System.IO;

public partial class Capcha : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

        //Defining brush color

        Color brushColor = System.Drawing.Color.Green;

        // Creating object for bitmap

        Bitmap objBitmap = new System.Drawing.Bitmap(100, 30);

        // Creating object for Graphics class

        Graphics objGraphics = System.Drawing.Graphics.FromImage(objBitmap);

        objGraphics.Clear(Color.Transparent);

        // Creating object for Font class

        Font objFont = new Font("Times New Roman", 14, FontStyle.Regular);

        string inputNumberString = "";

        Random r = new Random();

        int a = r.Next(99, 999);

        int b = r.Next(99, 999);

        int c = a + b;

        inputNumberString = a.ToString() + " + " + b.ToString() + " = ";

        //Storing the captcha value in the session

        Session["CaptchaValue"] = c.ToString();

        SolidBrush myBrush = new SolidBrush(brushColor);

        objGraphics.DrawString(inputNumberString, objFont, myBrush, 3, 3);

        //Adding the content type

        Response.ContentType = "image/png";

        System.IO.MemoryStream mem = new MemoryStream();

        //Saving the bitmap image

        objBitmap.Save(mem, ImageFormat.Png);

        //Writing the image to output screen

        mem.WriteTo(Response.OutputStream);

        // Disposing Font Object

        objFont.Dispose();

        // Disposing Graphics Object

        objGraphics.Dispose();

        // Disposing Bitmap Object

        objBitmap.Dispose();

    }

}

VB.Net:

Imports System.Collections.Generic

Imports System.Linq

Imports System.Web

Imports System.Web.UI

Imports System.Web.UI.WebControls

Imports System.Drawing

Imports System.Drawing.Imaging

Imports System.Drawing.Text

Imports System.IO

Partial Public Class Capcha

    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As EventArgs)

        'Defining brush color

        Dim brushColor As Color = System.Drawing.Color.Green

        ' Creating object for bitmap

        Dim objBitmap As Bitmap = New System.Drawing.Bitmap(100, 30)

        ' Creating object for Graphics class

        Dim objGraphics As Graphics = System.Drawing.Graphics.FromImage(objBitmap)

        objGraphics.Clear(Color.Transparent)

        ' Creating object for Font class

        Dim objFont As New Font("Times New Roman", 14, FontStyle.Regular)

        Dim inputNumberString As String = ""

        Dim r As New Random()

        Dim a As Integer = r.[Next](99, 999)

        Dim b As Integer = r.[Next](99, 999)

        Dim c As Integer = a + b

        inputNumberString = a.ToString() + " + " + b.ToString() + " = "

        'Storing the captcha value in the session

        Session("CaptchaValue") = c.ToString()

        Dim myBrush As New SolidBrush(brushColor)

        objGraphics.DrawString(inputNumberString, objFont, myBrush, 3, 3)

        'Adding the content type

        Response.ContentType = "image/png"

        Dim mem As System.IO.MemoryStream = New MemoryStream()

        'Saving the bitmap image

        objBitmap.Save(mem, ImageFormat.Png)

        'Writing the image to output screen

        mem.WriteTo(Response.OutputStream)

        ' Disposing Font Object

        objFont.Dispose()

        ' Disposing Graphics Object

        objGraphics.Dispose()

        ' Disposing Bitmap Object

        objBitmap.Dispose()

    End Sub

End Class

Now to test the captcha works correct or not i have added one more aspx file called demo.aspx in the project. In the aspx page i have a image control whose href attribute call the captcha.aspx. And i have a textbox control to get the user input and a button to validate user input with the session value. And i have a Label to display the status message. While clicking on the submit button the code will check whether the entered value and the session value are equal. If yes then it will return the status as human else as Robot. Below is the aspx code

HTML Markup:

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

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">

<head runat="server">

    <title></title>

</head>

<body>

    <form id="form2" runat="server">

        <table>

            <tr>

                <td colspan="2">What is the result of below calculation?</td>

            </tr>

            <tr>

                <td colspan="2">

                    <asp:Label ForeColor="Red" runat="server" ID="lblStatus"></asp:Label></td>

            </tr>

            <tr>

                <td>

                    <img src="Captcha.aspx" /></td>

                <td>

                    <asp:TextBox runat="server" ID="txtCaptcha"></asp:TextBox></td>

            </tr>

            <tr>

                <td colspan="2">

                    <asp:Button runat="server" ID="btnSubmit" Text="Submit" OnClick="btnSubmit_Click" /></td>

            </tr>

        </table>

    </form>

</body>

</html>

Below is the code behind file code.

C#.Net

using System;

using System.Collections.Generic;

using System.Linq;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

public partial class Demo : System.Web.UI.Page

{

    protected void Page_Load(object sender, EventArgs e)

    {

    }

    protected void btnSubmit_Click(object sender, EventArgs e)

    {

        string result = txtCaptcha.Text;

        if (result == Session["CaptchaValue"].ToString())

        {

            lblStatus.Text = "Correct.. You are a human..!";

        }

        else

        {

            lblStatus.Text = "InCorrect.. You are not a human..!";

        }

        txtCaptcha.Text = "";

    }

}

VB.Net

Imports System.Collections.Generic

Imports System.Linq

Imports System.Web

Imports System.Web.UI

Imports System.Web.UI.WebControls

Partial Public Class Demo

    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As EventArgs)

    End Sub

    Protected Sub btnSubmit_Click(sender As Object, e As EventArgs)

        Dim result As String = txtCaptcha.Text

        If result = Session("CaptchaValue").ToString() Then

            lblStatus.Text = "Correct.. You are a human..!"

        Else

            lblStatus.Text = "InCorrect.. You are not a human..!"

        End If

        txtCaptcha.Text = ""

    End Sub

End Class

The output will look like below..

Download Sourcecode:

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