Dynamic captcha

Page looks like.

Create a asp.net web site
 Add a App_Code folder inside that folder add a class that class name should be 'CatpchaImage.cs'
and write down the code
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Drawing2D;

public class CatpchaImage
{
    public static string SESSION_CAPTCHA = "CAPTCHA";
   

    const int default_width = 140;
    const int default_height = 40;

    protected Bitmap result = null;

    public int Width;
    public int Height;

    public CatpchaImage()
    {
        InitBitmap(default_width, default_height);
        rnd = new Random();
    }

    public CatpchaImage(int width, int height)
    {
        InitBitmap(width, height);
    }

    protected void InitBitmap(int width, int height)
    {
        result = new Bitmap(width, height);
        Width = width;
        Height = height;
        rnd = new Random();
    }



    public PointF Noise(PointF p, double eps)
    {
        p.X = Convert.ToSingle(rnd.NextDouble() * eps * 2 - eps) + p.X;
        p.Y = Convert.ToSingle(rnd.NextDouble() * eps * 2 - eps) + p.Y;
        return p;
    }

    public PointF Wave(PointF p, double amp, double size)
    {
        p.Y = Convert.ToSingle(Math.Sin(p.X / size) * amp) + p.Y;
        p.X = Convert.ToSingle(Math.Sin(p.X / size) * amp) + p.X;
        return p;
    }



    public GraphicsPath RandomWarp(GraphicsPath path)
    {
        // Add line //
        int PsCount = 10;
        PointF[] curvePs = new PointF[PsCount * 2];
        for (int u = 0; u < PsCount; u++)
        {
            curvePs[u].X = u * (Width / PsCount);
            curvePs[u].Y = Height / 2;
        }
        for (int u = PsCount; u < (PsCount * 2); u++)
        {
            curvePs[u].X = (u - PsCount) * (Width / PsCount);
            curvePs[u].Y = Height / 2 + 2;
        }

     
        double eps = Height * 0.05;

        double amp = rnd.NextDouble() * (double)(Height / 3);
        double size = rnd.NextDouble() * (double)(Width / 4) + Width / 8;

        double offset = (double)(Height / 3);


        PointF[] pn = new PointF[path.PointCount];
        byte[] pt = new byte[path.PointCount];

        GraphicsPath np2 = new GraphicsPath();

        GraphicsPathIterator iter = new GraphicsPathIterator(path);
        for (int i = 0; i < iter.SubpathCount; i++)
        {
            GraphicsPath sp = new GraphicsPath();
            bool closed;
            iter.NextSubpath(sp, out closed);

            Matrix m = new Matrix();
            m.RotateAt(Convert.ToSingle(rnd.NextDouble() * 30 - 15), sp.PathPoints[0]);

            m.Translate(-1 * i, 0);//uncomment

            sp.Transform(m);

            np2.AddPath(sp, true);
        }




        for (int i = 0; i < np2.PointCount; i++)
        {
            //pn[i] = Noise( path.PathPoints[i] , eps);
            pn[i] = Wave(np2.PathPoints[i], amp, size);
            pt[i] = np2.PathTypes[i];
        }

        GraphicsPath newpath = new GraphicsPath(pn, pt);

        return newpath;

    }

    Random rnd;


    public string DrawNumbers(int len)
    {
        string str = "";
        string possible = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789abcdfghjkmnpqrstvwxyz";
        char ch='A';
        for (int i = 0; i < len; i++)
        {          
            ch=possible[rnd.Next(0, possible.Length -1)];
            str = str + ch.ToString();
        }

        DrawText(str);
        return str;
    }

    public void DrawText(string aText)
    {

        Graphics g = Graphics.FromImage(result);
        int startsize = Height;
        Font f = new Font("Verdana", startsize, FontStyle.Bold, GraphicsUnit.Pixel);

        do
        {
            f = new Font("Verdana", startsize, GraphicsUnit.Pixel);
            startsize--;
        } while ((g.MeasureString(aText, f).Width >= Width) || (g.MeasureString(aText, f).Height >= Height));
        SizeF sf = g.MeasureString(aText, f);
        int width = Convert.ToInt32(sf.Width);
        int height = Convert.ToInt32(sf.Height);

        int x = Convert.ToInt32(Math.Abs((double)width - (double)Width) * rnd.NextDouble());
        int y = Convert.ToInt32(Math.Abs((double)height - (double)Height) * rnd.NextDouble());

        //////// Paths ///
        GraphicsPath path = new GraphicsPath(FillMode.Alternate);

        FontFamily family = new FontFamily("Verdana");     
        int fontStyle = (int)(FontStyle.Regular);
        float emSize = f.Size;
        Point origin = new Point(x, y);
        StringFormat format = StringFormat.GenericDefault;

        path.AddString(aText, family, fontStyle, emSize, origin, format);

      
        g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
        Rectangle rect = new Rectangle(0, 0, Width, Height);
        g.FillRectangle(new System.Drawing.Drawing2D.LinearGradientBrush(rect, Color.White, Color.White, 0f), rect);
        g.SmoothingMode = SmoothingMode.HighQuality;


        Color noiseCol = Color.FromArgb(100, 120, 180);
        Pen p = new Pen(noiseCol);
       
       
        /* generate random dots in background */
      for( int i=0; i<(Width*Height)/3; i++ ) {
          g.FillEllipse(Brushes.Blue, rnd.Next(0, Width), rnd.Next(0, Height), 1, 1);
      }
        /*noise ends here*/

        /* generate random lines in background */
      for( int i=0; i<(Width*Height)/150; i++ ) {
         g.DrawLine(p,rnd.Next(0,Width), rnd.Next(0,Height), rnd.Next(0,Width), rnd.Next(0,Height));
      }
      
      Color textColor = Color.FromArgb(20, 40, 100);
      g.FillPath(new SolidBrush(textColor), path);
     
      g.Dispose();
    }

    public Bitmap Result
    {
        get
        {
            return result;
        }
    }

  
}
after that add a Generic Handler page .page name should be "captcha.ashx" 
and write down the code inside the Generic handler page

<%@ WebHandler Language="C#" Class="captcha" %>

using System;
using System.Web;
using System.Drawing;
using System.Web.SessionState;

public class captcha : IHttpHandler, IRequiresSessionState
{

  
    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "image/jpeg";
        CatpchaImage captcha = new CatpchaImage();
        string str = captcha.DrawNumbers(5);
        if (context.Session[ CatpchaImage.SESSION_CAPTCHA] == null) context.Session.Add(CatpchaImage.SESSION_CAPTCHA, str);
        else
        {
            context.Session[ CatpchaImage.SESSION_CAPTCHA] = str;
        }
        Bitmap bmp = captcha.Result;
        bmp.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
    }

   
   
    public bool IsReusable {
        get {
            return true;
        }
    }

}
Add a asp page and design it the code will be

<%@ 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></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
     <table align="center">
    <tr><td><asp:Image ID="im" runat="server" Height="40" Width="180" ImageUrl="~/captcha.ashx" /></td></tr>
    <tr><td><asp:TextBox ID="tb" runat="server"></asp:TextBox></td></tr>
    <tr><td> <asp:Button ID="btn" runat="server" Text="Save" onclick="btn_Click" Width="80" /></td></tr>
    <tr><td> <asp:Label ID="lb" runat="server"></asp:Label></td></tr>
    </table>
    </div>
    </form>
</body>
</html>

C#  Code  will be
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void btn_Click(object sender, EventArgs e)
    {
        if (Session["CAPTCHA"].ToString().Equals(tb.Text))
        {
            lb.ForeColor = Color.Green;
            lb.Text = "Valid Code Inputed";
        }
        else
        {
            lb.ForeColor = Color.Red;
            lb.Text = "InValid Code Inputed";
        }
           
    }
}

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