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Simple CAPTCHA, Create Your Own in C#

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3 Aug 2010CPOL2 min read 178.2K   48   35
Create your own simple captcha in C#

Have you ever wondered how to create a Captcha for your web forms? Well, it's really simple and it won't take more than 100+ lines of code. This sample captcha that I will demonstrate will have random Font Type, Font Style, Font Color, Font Rotation, Background Style and Background Colour. Here are some samples of what this can generate:

Image 1 Image 2 Image 3 Image 4 Image 5 Image 6

Now without too much explanation, here it goes.

Step 1

In your web project, add a Generic Handler that's the file with the .ahsx extension. I'll just use this for this demo for simplicity and also it will compile on demand like a normal aspx file. It's up to you if you want to create a DLL which is better so you can reuse it in any projects you want. To do that, right click on the project on your Solution Explorer.

Image 7

Then choose your file type, in this sample, it's a Generic Handler.

Image 8

Step 2

Once added, you can start coding, here are the sample codes which you can freely copy.

C#
<%@ WebHandler Language="C#" %>
using System;
using System.Web;
using System.Drawing;
using System.IO;
using System.Web.SessionState;
using System.Drawing.Imaging;
using System.Drawing.Text;
using System.Drawing.Drawing2D;
public class Captcha : IHttpHandler, IReadOnlySessionState
{
public void ProcessRequest(HttpContext context)
{
int iHeight = 80;
int iWidth = 190;
Random oRandom = new Random();

int[] aBackgroundNoiseColor = new int[] { 150, 150, 150 };
int[] aTextColor = new int[] { 0, 0, 0 };
int[] aFontEmSizes = new int[] { 15, 20, 25, 30, 35 };

string[] aFontNames = new string[]
{
 "Comic Sans MS",
 "Arial",
 "Times New Roman",
 "Georgia",
 "Verdana",
 "Geneva"
};
FontStyle[] aFontStyles = new FontStyle[]
{  
 FontStyle.Bold,
 FontStyle.Italic,
 FontStyle.Regular,
 FontStyle.Strikeout,
 FontStyle.Underline
};
HatchStyle[] aHatchStyles = new HatchStyle[]
{
 HatchStyle.BackwardDiagonal, HatchStyle.Cross, 
	HatchStyle.DashedDownwardDiagonal, HatchStyle.DashedHorizontal,
 HatchStyle.DashedUpwardDiagonal, HatchStyle.DashedVertical, 
	HatchStyle.DiagonalBrick, HatchStyle.DiagonalCross,
 HatchStyle.Divot, HatchStyle.DottedDiamond, HatchStyle.DottedGrid, 
	HatchStyle.ForwardDiagonal, HatchStyle.Horizontal,
 HatchStyle.HorizontalBrick, HatchStyle.LargeCheckerBoard, 
	HatchStyle.LargeConfetti, HatchStyle.LargeGrid,
 HatchStyle.LightDownwardDiagonal, HatchStyle.LightHorizontal, 
	HatchStyle.LightUpwardDiagonal, HatchStyle.LightVertical,
 HatchStyle.Max, HatchStyle.Min, HatchStyle.NarrowHorizontal, 
	HatchStyle.NarrowVertical, HatchStyle.OutlinedDiamond,
 HatchStyle.Plaid, HatchStyle.Shingle, HatchStyle.SmallCheckerBoard, 
	HatchStyle.SmallConfetti, HatchStyle.SmallGrid,
 HatchStyle.SolidDiamond, HatchStyle.Sphere, HatchStyle.Trellis, 
	HatchStyle.Vertical, HatchStyle.Wave, HatchStyle.Weave,
 HatchStyle.WideDownwardDiagonal, HatchStyle.WideUpwardDiagonal, HatchStyle.ZigZag
};

//Get Captcha in Session
string sCaptchaText = context.Session["Captcha"].ToString();

//Creates an output Bitmap
Bitmap oOutputBitmap = new Bitmap(iWidth, iHeight, PixelFormat.Format24bppRgb);
Graphics oGraphics = Graphics.FromImage(oOutputBitmap);
oGraphics.TextRenderingHint = TextRenderingHint.AntiAlias;

//Create a Drawing area
RectangleF oRectangleF = new RectangleF(0, 0, iWidth, iHeight);
Brush oBrush = default(Brush);

//Draw background (Lighter colors RGB 100 to 255)
oBrush = new HatchBrush(aHatchStyles[oRandom.Next
	(aHatchStyles.Length-1)], Color.FromArgb((oRandom.Next(100, 255)), 
	(oRandom.Next(100, 255)), (oRandom.Next(100, 255))), Color.White);
oGraphics.FillRectangle(oBrush, oRectangleF);

System.Drawing.Drawing2D.Matrix oMatrix = new System.Drawing.Drawing2D.Matrix();
int i = 0;
for (i = 0; i <= sCaptchaText.Length - 1; i++)
{
 oMatrix.Reset();
 int iChars = sCaptchaText.Length;
 int x = iWidth / (iChars + 1) * i;
 int y = iHeight / 2;

 //Rotate text Random
 oMatrix.RotateAt(oRandom.Next(-40, 40), new PointF(x, y));
 oGraphics.Transform = oMatrix;

 //Draw the letters with Random Font Type, Size and Color
 oGraphics.DrawString
 (
 //Text
 sCaptchaText.Substring(i, 1),
 //Random Font Name and Style
 new Font(aFontNames[oRandom.Next(aFontNames.Length - 1)], 
	aFontEmSizes[oRandom.Next(aFontEmSizes.Length-1)], 
	aFontStyles[oRandom.Next(aFontStyles.Length - 1)]),  
 //Random Color (Darker colors RGB 0 to 100)
 new SolidBrush(Color.FromArgb(oRandom.Next(0, 100), 
	oRandom.Next(0, 100), oRandom.Next(0, 100))),
 x,
 oRandom.Next(10, 40)
 );
 oGraphics.ResetTransform();
}

MemoryStream oMemoryStream = new MemoryStream();
oOutputBitmap.Save(oMemoryStream, System.Drawing.Imaging.ImageFormat.Png);
byte[] oBytes = oMemoryStream.GetBuffer();

oOutputBitmap.Dispose();
oMemoryStream.Close();

context.Response.BinaryWrite(oBytes);
context.Response.End();
}
public bool IsReusable
{
get
{
 return false;
}
}
}

There is some commented explanation along the way so you won't get lost, but you can still ask me if you really need.

Step 3

Use that Captcha. On your aspx, add the following as an image like such:

ASP.NET
<asp:Image ID="imgCaptcha" ImageUrl="Captcha.ashx" runat="server" />

If you noticed, it's like you are using it as an image, this is because it's an image rendered by that handler which you can see on the bottom part of the Captcha code.

C#
context.Response.BinaryWrite(oBytes);

Step 4

Assign a text on that captcha on your code behind:

C#
private void SetCaptchaText()
{
 Random oRandom = new Random();
 int iNumber = oRandom.Next(100000, 999999);
 Session["Captcha"] = iNumber.ToString();
}

Note: You may notice that I am using only numbers in this regard, this would be easy so users cannot be confused with l and 1, 0 and o, and other similar looking characters.
Note: By setting the session, you are also setting the text to render on the captcha and if you look at the captcha codes, it is in this section.

C#
string sCaptchaText = context.Session["Captcha"].ToString();

Step 5

Compare that value to what the users had keyed into your text box:

C#
if (Session["Captcha"].ToString() != txtCaptcha.Text.Trim())
 {
 Response.Redirect("Failed.aspx");
 }
 else
 {
 Response.Redirect("Success.aspx");
 }

That's it. Congratulations! You have created your own captcha.

Image 9

Image 10 Image 11 Image 12 Image 13 Image 14 Image 15

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


Written By
Technical Lead
New Zealand New Zealand
http://nz.linkedin.com/in/macaalay
http://macaalay.com/

Comments and Discussions

 
GeneralRe: Error Pin
Raymund Macaalay2-Oct-10 13:39
Raymund Macaalay2-Oct-10 13:39 
GeneralRe: Error Pin
lazybit2-Oct-10 17:19
lazybit2-Oct-10 17:19 
http://msdn.microsoft.com/en-us/library/aa329893(VS.71).aspx
Return Value
A 32-bit signed integer greater than or equal to minValue and less than maxValue; that is, the range of return values includes minValue but not MaxValue. If minValue equals maxValue, minValue is returned.

Example:
int iNumber = oRandom.Next(100000, 999999); Will generate numbers from 100000 to 999998.
oRandom.Next(aHatchStyles.Length-1) Will generate styles from HatchStyle.BackwardDiagonal to HatchStyle.WideUpwardDiagonal, except HatchStyle.ZigZag
oRandom.Next(aHatchStyles.Length-1) will generate styles from FontStyle.Bold to FontStyle.Strikeout, except last FontStyle.Underline
You never get random color 255,255,255 because oRandom.Next(100, 255)
e.t.c.
GeneralRe: Error Pin
Raymund Macaalay3-Oct-10 21:06
Raymund Macaalay3-Oct-10 21:06 
GeneralRe: Error Pin
jaspadee11-Jan-13 2:49
jaspadee11-Jan-13 2:49 
Generalimage not display Pin
niravsahayata23-Sep-10 4:32
niravsahayata23-Sep-10 4:32 
GeneralRe: image not display Pin
Raymund Macaalay23-Sep-10 10:06
Raymund Macaalay23-Sep-10 10:06 
GeneralRe: image not display Pin
Member 118139134-Jul-15 17:56
Member 118139134-Jul-15 17:56 
GeneralMy vote of 5 Pin
Member 432084415-Aug-10 10:50
Member 432084415-Aug-10 10:50 
GeneralMy vote of 5 Pin
lyn.ma9-Aug-10 16:57
lyn.ma9-Aug-10 16:57 
GeneralMy Vote of 5 Pin
Mico Perez5-Aug-10 6:46
Mico Perez5-Aug-10 6:46 

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