CAPTCHA in ASP.Net C# , Challange Password in C#

Have you ever wondered why people put images in the website and ask you to type it. In a simple term CAPTCHA. What is it ??
CAPTCHA stands for "Completely Automated Public Turing test to tell Computers and Humans Apart"

As the word explains, you generate a code in a Image form which no other comuter or computer program can read. A human has to typed in a text box to make sure that the image content and user input is correct. we have options either to display only numbers, Alphanumeric or image of objects like Car, bus etc. Here i am going to explain about CAPTCHA with ASP.Net

Let us build the Class which will build the Image for us.. below is the code for that

First is the Image Generation Class

CaptchaImage.cs Class


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,80F,FontStyle.Bold);
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.White, Color.WhiteSmoke);
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.DarkBlue, Color.Black);
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;
}
}



Now the code where you want to call the CAPTCHA
Generate a Random code and put it in session, Please see that this session variable is used in the JpegImage.aspx.cs class

Session["CaptchaImageText"] = GenerateRandomCode();
Code to Generate Random Code
private string GenerateRandomCode()
{
string s = "";
for (int i = 0; i < s =" String.Concat(s,">
To Validate the user input write a function like this

private bool ValidateCaptcha()
{
if (txtCaptcha.Text.Trim() == Session["CaptchaImageText"].ToString())
return true;
else
{
lblError.Visible = true;
lblError.Text = "Please enter correct code. Click on Refresh to get a new code";
return false;
}
}

Thats it. You should be able to put this image to validate the user.