Captcha Image using C# in ASP.NET

Download CaptchaImage.zip - 131.44 KB

What is Captcha Image? 

Source:DotNet Stuff   

If both match then displays above message 

If both doesn't match then display above message 

In the web, whenever you register on any site, normally they ask at the end of page for entering Image code/Challenge Image/Image verification code, etc. This type of image is called Captcha Image.

What is the Use of Captcha?

(Source: http://en.wikipedia.org/wiki/Captcha)

Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human.

A best technique to restrict automatic form submissions when developing a web page is to add some kind of verification. As per my knowledge, image verification is the best way, it’s also called Captcha image. In this process, dynamically we are creating an image with random string either string, number or alphanumeric and displaying it on the web page. After that, at the time of form submission, the user is asked to enter the same code in the text box as shown in that image. Once the form is submitted, then we can compare both auto generated value and entered by user because there is no other good way to protect your webworms from spammers (auto submission).

How Can We Do That?

Here is the detailed explanation with code snippets.

This is the default.aspx (partially):

<asp:Label ID="lblmsg" runat="server" Font-Bold="True" 
	ForeColor="Red" Text=""></asp:Label>
         <br />
    </div>
    <asp:TextBox ID="txtimgcode" runat="server"></asp:TextBox>
    <br />
    <asp:Image ID="Image1" runat="server" ImageUrl="~/CImage.aspx"/>
    <br />
    <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />

In the above code, there are three control Label controls to display message whether entered code is matched or not, TextBox to take input from user and Image box to hold an image autogenerated by CImage.aspx page. Before that, we will see default.aspx.cs page code:

protected void Button1_Click(object sender, EventArgs e)
    {
        if (this.txtimgcode.Text == this.Session["CaptchaImageText"].ToString())
        {
            lblmsg.Text = "Excellent.......";
        }
        else
        {
            lblmsg.Text = "image code is not valid.";
        }
        this.txtimgcode.Text = "";
    }

In button’s click event, we are just checking entered value with stored value in session. And we display the appropriate message on the screen.

CImage.aspx Page

Remove all the code except the first line.

The main code is here which is calling a method to generate random number string and passes it to the class file which will generate image with the same string and return it as a JPEG format.

Here are the details of the code:

using System.Drawing.Imaging;
public partial class CImage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Create a random code and store it in the Session object.
        this.Session["CaptchaImageText"] = GenerateRandomCode();
        // Create a CAPTCHA image using the text stored in the Session object.
        RandomImage ci = new RandomImage(this.Session
			["CaptchaImageText"].ToString(), 300, 75);
        // Change the response headers to output a JPEG image.
        this.Response.Clear();
        this.Response.ContentType = "image/jpeg";
        // Write the image to the response stream in JPEG format.
        ci.Image.Save(this.Response.OutputStream, ImageFormat.Jpeg);
        // Dispose of the CAPTCHA image object.
        ci.Dispose();
    }
   
    // Function to generate random string with Random class.
    private string GenerateRandomCode()
    {
        Random r = new Random();
        string s = "";
        for (int j = 0; j < 5;j++)
        {
            int i = r.Next(3);
            int ch;
            switch (i)
            {
                case 1:
                    ch = r.Next(0, 9);
                    s = s + ch.ToString();
                    break;
                case 2:
                    ch = r.Next(65, 90);
                    s = s + Convert.ToChar(ch).ToString();
                    break;
                case 3:
                    ch = r.Next(97, 122);
                    s = s + Convert.ToChar(ch).ToString();
                    break;
                default:
                    ch = r.Next(97, 122);
                    s = s + Convert.ToChar(ch).ToString();
                    break;
            }
            r.NextDouble();
            r.Next(100, 1999);
        }
        return s;
    }
}

And finally, the code for RandomImage class which generates Image with the help of BitMap class.

//Extra name space
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Drawing.Text;
public class RandomImage
{
//Default Constructor 
      public RandomImage(){}
//property
      public string Text
        {
            get { return this.text; }
        }
    public Bitmap Image
    {
        get { return this.image; }
    }
    public int Width
    {
        get { return this.width; }
    }
    public int Height
    {
        get { return this.height; }
    }
//Private variable
    private string text;
    private int width;
    private int height;
    private Bitmap image;
    private Random random = new Random();
    //Methods declaration
    public RandomImage(string s, int width, int height)
    {
          this.text = s;
          this.SetDimensions(width, height);
          this.GenerateImage();
    }
    public void Dispose()
    {
        GC.SuppressFinalize(this);
        this.Dispose(true);
    }
    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
            this.image.Dispose();
    }
    private void SetDimensions(int width, int height)
    {
          if (width <= 0)
                throw new ArgumentOutOfRangeException("width", width, 
                    "Argument out of range, must be greater than zero.");
          if (height <= 0)
                throw new ArgumentOutOfRangeException("height", height, 
                    "Argument out of range, must be greater than zero.");
          this.width = width;
          this.height = height;
    }
    private void GenerateImage()
    {
          Bitmap bitmap = new Bitmap
			(this.width, this.height, PixelFormat.Format32bppArgb);
          Graphics g = Graphics.FromImage(bitmap);
          g.SmoothingMode = SmoothingMode.AntiAlias;
          Rectangle rect = new Rectangle(0, 0, this.width, this.height);
          HatchBrush hatchBrush = new HatchBrush(HatchStyle.SmallConfetti,
              Color.LightGray, Color.White);
          g.FillRectangle(hatchBrush, rect);
          SizeF size;
          float fontSize = rect.Height + 1;
        Font font;
        
          do
          {
                fontSize--;
            font = new Font(FontFamily.GenericSansSerif, fontSize, FontStyle.Bold);
                size = g.MeasureString(this.text, font);
          } while (size.Width > rect.Width);
          StringFormat format = new StringFormat();
          format.Alignment = StringAlignment.Center;
          format.LineAlignment = StringAlignment.Center;
          GraphicsPath path = new GraphicsPath();
          //path.AddString(this.text, font.FontFamily, (int) font.Style, 
          //    font.Size, rect, format);
        path.AddString(this.text, font.FontFamily, (int)font.Style, 75, rect, format);
          float v = 4F;
          PointF[] points =
          {
                new PointF(this.random.Next(rect.Width) / v, this.random.Next(
                   rect.Height) / v),
                new PointF(rect.Width - this.random.Next(rect.Width) / v, 
                    this.random.Next(rect.Height) / v),
                new PointF(this.random.Next(rect.Width) / v, 
                    rect.Height - this.random.Next(rect.Height) / v),
                new PointF(rect.Width - this.random.Next(rect.Width) / v,
                    rect.Height - this.random.Next(rect.Height) / v)
          };
          Matrix matrix = new Matrix();
          matrix.Translate(0F, 0F);
          path.Warp(points, rect, matrix, WarpMode.Perspective, 0F);
          hatchBrush = new HatchBrush(HatchStyle.Percent10, Color.Black, Color.SkyBlue);
          g.FillPath(hatchBrush, path);
          int m = Math.Max(rect.Width, rect.Height);
          for (int i = 0; i < (int) (rect.Width * rect.Height / 30F); i++)
          {
                int x = this.random.Next(rect.Width);
                int y = this.random.Next(rect.Height);
                int w = this.random.Next(m / 50);
                int h = this.random.Next(m / 50);
                g.FillEllipse(hatchBrush, x, y, w, h);
          }
          font.Dispose();
          hatchBrush.Dispose();
          g.Dispose();
          this.image = bitmap;
    }
}

Happy Coding...