In
this article I will explain what is CAPTCHA? How to use CAPTCHA code in ASP.NET
web application?
A
CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a challenge-response
system test designed to differentiate humans from automated
programs. A CAPTCHA differentiates between human and bot by setting some task that is easy for most humans to
perform but is more difficult and time-consuming for current bots to complete.
HOW
TO USE CAPTCHA:
Copy the below code and
paste it in a separate class in Business Logic Layer
using
System;
using
System.Diagnostics;
using
System.Collections.Generic;
using
System.ComponentModel;
using System.Text;
using
System.Drawing;
using
System.Drawing.Imaging;
using
System.Drawing.Drawing2D;
using
System.Drawing.Design;
/// CAPTCHA Image generation, on each Image property call,
/// based on all other customizable property values.
///
[DefaultProperty("Text")]
public class CaptchaImage
{
private Random _random = new
Random();
public
CaptchaImage() { Text = ""; }
#region Bitmap
[Browsable(false)]
[Category("Bitmap")]
[Description("Generated
image")]
public
Bitmap Image
{
get
{
Bitmap bitmap = new Bitmap(_width, _height,
PixelFormat.Format32bppPArgb);
Graphics g = null;
Font font = null;
HatchBrush textBrush = null;
HatchBrush backBrush = null;
try
{
//
Fill Background
g = Graphics.FromImage(bitmap);
g.SmoothingMode =
SmoothingMode.AntiAlias;
Rectangle rect = new Rectangle(0, 0, _width, _height);
textBrush = new HatchBrush(_textStyle,
_textForeColor,
_textBackColor);
backBrush = new HatchBrush(_backStyle,
_backForeColor,
_backBackColor);
g.FillRectangle(backBrush,
rect);
//
Write Text
StringFormat stringFormat = new StringFormat();
stringFormat.Alignment =
StringAlignment.Center;
stringFormat.LineAlignment = StringAlignment.Center;
font = new
Font(_fontName, _fontSize, _fontStyle);
GraphicsPath path = new GraphicsPath();
path.AddString(_text,
font.FontFamily,
(int)font.Style,
font.Size, rect, stringFormat);
//
Warp Text
if
(_warp > 0)
{
Matrix matrix = new Matrix();
float
warp = 10 - _warp * 6 / 100;
PointF[] points = {
new
PointF(_random.Next(rect.Width) / warp,
_random.Next(rect.Height) / warp),
new
PointF(rect.Width - _random.Next(rect.Width) / warp,
_random.Next(rect.Height) / warp),
new
PointF(_random.Next(rect.Width) / warp,
rect.Height -
_random.Next(rect.Height) / warp),
new
PointF(rect.Width - _random.Next(rect.Width) / warp,
rect.Height -
_random.Next(rect.Height) / warp) };
path.Warp(points, rect,
matrix, WarpMode.Perspective, 0F);
}
g.FillPath(textBrush, path);
//
Random Noise over Text
if
(_noise > 0)
{
int
maxDim = Math.Max(rect.Width, rect.Height);
int
radius = (int)(maxDim * _noise / 3000);
int
maxGran = (int)(rect.Width * rect.Height
/ (100 - (_noise >=
90 ? 90 : _noise)));
for
(int i = 0; i < maxGran; i++)
g.FillEllipse(textBrush,
_random.Next(rect.Width), _random.Next(rect.Height),
_random.Next(radius),
_random.Next(radius));
}
//
Draw Lines over Text
if
(_lines > 0)
{
int
lines = ((int)_lines / 30) + 1;
using
(Pen pen = new Pen(textBrush, 1))
for (int i = 0; i < lines; i++)
{
PointF[] points = new PointF[lines > 2 ? lines - 1 : 2];
for (int j = 0; j
< points.Length; j++)
points[j] = new PointF(_random.Next(rect.Width),
_random.Next(rect.Height));
g.DrawCurve(pen,
points, 1.75F);
}
}
}
finally
{
//
Dispose GDI+ objects
if
(g != null)
g.Dispose();
if
(font != null)
font.Dispose();
if
(textBrush != null)
textBrush.Dispose();
if
(backBrush != null)
backBrush.Dispose();
}
return
bitmap;
}
}
private int _width = 100;
[Category("Bitmap")]
[Description("Image
Width")]
public int Width
{
get { return _width; }
set
{
if
(value < 10 || value
> 1000)
throw
new Exception("Image Width must be between 10 and 1000");
_width = value;
}
}
private int _height = 60;
[Category("Bitmap")]
[Description("Image
Height")]
public int Height
{
get { return _height; }
set
{
if
(value < 10 || value
> 1000)
throw
new Exception("Image Height must be between 10 and 1000");
_height = value;
}
}
#endregion
#region Effects
private int _noise = 100;
[Category("Effects")]
[Description("Noise
Factor")]
public int NoiseFactor
{
get { return _noise; }
set
{
if
(value < 0 || value
> 100)
throw
new Exception("Noise Factor must be between 0 and 100");
_noise = value;
}
}
private int _warp = 100;
[Category("Effects")]
[Description("Warp
Factor")]
public int WarpFactor
{
get { return _warp; }
set
{
if
(value < 0 || value
> 100)
throw
new Exception("Warp Factor must be between 0 and 100");
_warp = value;
}
}
private int _lines = 100;
[Category("Effects")]
[Description("Lines
Factor")]
public int LinesFactor
{
get { return _lines; }
set
{
if
(value < 0 || value
> 100)
throw
new Exception("Lines Factor must be between 0 and 100");
_lines = value;
}
}
#endregion
#region Brushes
public
HatchStyle _textStyle = HatchStyle.NarrowHorizontal;
[Category("Hatch
Brush Text")]
[Description("Hatch
Brush Text Style")]
public
HatchStyle TextStyle
{
get { return _textStyle; }
set {
_textStyle = value; }
}
private
Color _textForeColor = Color.Black;
[Category("Hatch
Brush Text")]
[Description("Hatch
Brush Text Fore Color")]
public
Color TextForeColor
{
get { return _textForeColor; }
set {
_textForeColor = value; }
}
private
Color _textBackColor = Color.DarkGray;
[Category("Hatch
Brush Text")]
[Description("Hatch
Brush Text Background Color")]
public
Color TextBackColor
{
get { return _textBackColor; }
set {
_textBackColor = value; }
}
public
HatchStyle _backStyle = HatchStyle.DottedGrid;
[Category("Hatch
Brush Background")]
[Description("Hatch
Brush Background Style")]
public
HatchStyle BackgroundStyle
{
get { return _backStyle; }
set {
_backStyle = value; }
}
private
Color _backForeColor = Color.LightGray;
[Category("Hatch
Brush Background")]
[Description("Hatch
Brush Background Fore Color")]
public
Color BackgroundForeColor
{
get { return _backForeColor; }
set {
_backForeColor = value; }
}
private
Color _backBackColor = Color.White;
[Category("Hatch
Brush Background")]
[Description("Hatch
Brush Background Background Color")]
public
Color BackgroundBackColor
{
get { return _backBackColor; }
set {
_backBackColor = value; }
}
#endregion
#region Text
private string _text = "";
[Category("Text"),
Description("Text
string. Empty to auto-generate random string.")]
public string Text
{
get { return _text; }
set
{
if
(value == null
|| (value.Length
!= 0 && value.Length < 2)
|| value.Length
> 10)
throw
new Exception("Text Length must be between 2 and 10");
_text = value;
if
(_text.Length == 0)
for
(int i = 0; i < _length; i++)
{
char
c;
do
c =
_charSet[_random.Next(0, _charSet.Length)];
while
(_unique && _length <= _charSet.Length && _text.IndexOf(c)
>= 0);
_text += c;
}
_length = _text.Length;
}
}
private string _charSet = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ";
[Category("Text")]
[Description("Character
set for auto-text generation")]
public string CharacterSet
{
get { return _charSet; }
set
{
if
(value == null
|| value.Length < 2)
throw
new Exception("Minimum character set size is 2");
_charSet = value;
Text = "";
}
}
private int _length = 5;
[Category("Text")]
[Description("Text
Length")]
public int Length
{
get { return _length; }
set
{
if
(value < 2 || value
> 10)
throw
new Exception("Text Length must be between 2 and 10");
_length = value;
Text = "";
}
}
private bool _unique = true;
[Category("Text")]
[Description("Unique
characters or not")]
public bool Unique
{
get { return _unique; }
set
{
_unique = value;
Text = "";
}
}
#endregion
#region Font
private string _fontName = "Tahoma";
[Category("Font")]
[Description("Font
Name")]
[TypeConverter(typeof(FontConverter.FontNameConverter))]
[Editor("System.Drawing.Design.FontNameEditor",
typeof(UITypeEditor))]
public string FontName
{
get { return _fontName; }
set {
_fontName = value; }
}
private int _fontSize = 25;
[Category("Font")]
[Description("Font
Size")]
public int FontSize
{
get { return _fontSize; }
set {
_fontSize = value; }
}
private
FontStyle _fontStyle = FontStyle.Bold;
[Category("Font")]
[Description("Font
Style")]
public
FontStyle FontStyle
{
get { return _fontStyle; }
set {
_fontStyle = value; }
}
#endregion
}
First
we need to create the defult.aspx page as shown below
<table id="tblCaptcha" runat="server">
<tr>
<td >
<img id="imgCP" runat="server"
border="0"
height="70"
src="default.aspx?captcha=true"/>
</td>
<td>
<asp:TextBox ID="CaptchaCode"
runat="server"></asp:TextBox> <asp:ImageButton ID="imgbtnRefresh"
runat="server"
Height="20px"
ImageUrl="~/Images/refresh_but.png"
OnClick="imgbtnRefresh_Click"
/>
<asp:Label ID="Label2" runat="server"
Text="Label"
></asp:Label><br />
<span>(Enter the
letters displayed in the image above. If you cannot read the letters, press the
Refresh button.)</span>
</td>
</tr>
</table>
If you use the above code the design will be like
this
Next
we need to write Code i.e, aspx.cs file as follows:
protected void Page_Load(object
sender, EventArgs e)
{
if
(Request.QueryString["captcha"] ==
"true")
GetCaptchaImage();//th
else
{
if
(IsPostBack)
_result = (IsValidCaptcha()
? "Valid Code" : "Invalid Code");
CreateCaptchaImage();
}
}
// To Show
the Generated CaptchaImage
private
void GetCaptchaImage()
{
CaptchaImage
image = Session[CAPTCHA_ID] as CaptchaImage;
Debug.Assert(image
!= null);
Response.Clear();
Response.ContentType = "image/jpeg";
image.Image.Save(Response.OutputStream,
ImageFormat.Jpeg);
}
// Generate a
new CaptchaImage instance, with custom values
private
void CreateCaptchaImage()
{
CaptchaImage
image = new CaptchaImage();
Session[CAPTCHA_ID] = image;
//
customize CaptchaImage instance here
//if
u give only numerical then you will get the captcha with numbers only. If you include the alphabets along with
numerical then will get the captcha image with numbers & alphabet.
image.CharacterSet = "2345678910";
}
// Returns
true if valid entered code (case insensitive comparison!)
private
bool IsValidCaptcha()
{
CaptchaImage
image = Session[CAPTCHA_ID] as CaptchaImage;
return
(String.Compare(image.Text,
CaptchaCode.Text, true) == 0);
}
protected
void imgbtnRefresh_Click(object sender, ImageClickEventArgs
e)
{
CaptchaCode.Text = string.Empty;
}
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