What is CAPTCHA and why we are using CAPTCHA
A CAPTCHA is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a human being. The process usually involves a computer asking a user to complete a simple test which the computer is able to grade. These tests are designed to be easy for a computer to generate but difficult for a computer to solve, but again easy for a human. If a correct solution is received, it can be presumed to have been entered by a human. A common type of CAPTCHA requires the user to type letters and/or digits from a distorted image that appears on the screen. Such tests are commonly used to prevent unwanted internet bots from accessing websites, since a normal human can easily read a CAPTCHA, while the bot cannot process the image letters and therefore, cannot answer properly, or at all.
How to implement CAPTCHA validation/CAPTCHA verification in ASP.Net/C#
CAPTCHA image can be generated using ASP.Net codes and we can validate the code that user enter with the code in the CAPTCHA image. Here we are going to do a sample application that demonstrates how to create a CAPTCHA image using C# codes and how we can validate the user inputs with the CPATCH code. We are having two aspx pages and one class .cs file in the sample application.
A very simple application sample for CPTCHA validation
In the page load, first of all we are creating a 6 digit random number and stored in the session. By using this random number we creates a image suing System.Drawing package by giving some width,height,font family and alignment of the numbers, So the user can able to view the codes in the CAPTCHA image but it is difficult to read using system.
Below is the Class that we used for creating a CAPTCHA image with random number
using System;
using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;
using System.Drawing.Text;
namespace CaptchaImage
{
/// <summary>
/// Summary description for CaptchaImage.
/// </summary>
public class CaptchaImage
{
// Public properties (all read-only).
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; }
}
// Internal properties.
private string text;
private int width;
private int height;
private string familyName;
private Bitmap image;
// For generating random numbers.
private Random random = new Random();
// ====================================================================
// Initializes a new instance of the CaptchaImage class using the
// specified text, width and height.
// ====================================================================
public CaptchaImage(string s, int width, int height)
{
this.text = s;
this.SetDimensions(width, height);
this.GenerateImage();
}
// ====================================================================
// Initializes a new instance of the CaptchaImage class using the
// specified text, width, height and font family.
// ====================================================================
public CaptchaImage(string s, int width, int height, string familyName)
{
this.text = s;
this.SetDimensions(width, height);
this.SetFamilyName(familyName);
this.GenerateImage();
}
// ====================================================================
// This member overrides Object.Finalize.
// ====================================================================
~CaptchaImage()
{
Dispose(false);
}
// ====================================================================
// Releases all resources used by this object.
// ====================================================================
public void Dispose()
{
GC.SuppressFinalize(this);
this.Dispose(true);
}
// ====================================================================
// Custom Dispose method to clean up unmanaged resources.
// ====================================================================
protected virtual void Dispose(bool disposing)
{
if (disposing)
// Dispose of the bitmap.
this.image.Dispose();
}
// ====================================================================
// Sets the image width and height.
// ====================================================================
private void SetDimensions(int width, int height)
{
// Check the width and 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;
}
// ====================================================================
// Sets the font used for the image text.
// ====================================================================
private void SetFamilyName(string familyName)
{
// If the named font is not installed, default to a system font.
try
{
Font font = new Font(this.familyName, 12F);
this.familyName = familyName;
font.Dispose();
}
catch (Exception ex)
{
this.familyName = System.Drawing.FontFamily.GenericSerif.Name;
}
}
// ====================================================================
// Creates the bitmap image.
// ====================================================================
private void GenerateImage()
{
// Create a new 32-bit bitmap image.
Bitmap bitmap = new Bitmap(this.width, this.height, PixelFormat.Format32bppArgb);
// Create a graphics object for drawing.
Graphics g = Graphics.FromImage(bitmap);
g.SmoothingMode = SmoothingMode.AntiAlias;
Rectangle rect = new Rectangle(0, 0, this.width, this.height);
// Fill in the background.
HatchBrush hatchBrush = new HatchBrush(HatchStyle.SmallConfetti, Color.LightGray, Color.White);
g.FillRectangle(hatchBrush, rect);
// Set up the text font.
SizeF size;
float fontSize = rect.Height + 1;
Font font;
// Adjust the font size until the text fits within the image.
do
{
fontSize--;
font = new Font(this.familyName, fontSize, FontStyle.Bold);
size = g.MeasureString(this.text, font);
} while (size.Width > rect.Width);
// Set up the text format.
StringFormat format = new StringFormat();
format.Alignment = StringAlignment.Center;
format.LineAlignment = StringAlignment.Center;
// Create a path using the text and warp it randomly.
GraphicsPath path = new GraphicsPath();
path.AddString(this.text, font.FontFamily, (int) font.Style, font.Size, 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);
// Draw the text.
hatchBrush = new HatchBrush(HatchStyle.LargeConfetti, Color.LightGray, Color.DarkGray);
g.FillPath(hatchBrush, path);
// Add some random noise.
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);
}
// Clean up.
font.Dispose();
hatchBrush.Dispose();
g.Dispose();
// Set the image.
this.image = bitmap;
}
}
}
