Captcha in VB.NET

[CWA]

This tutorial will show you how to setup a Captcha ("Completely Automated Public Turing test to tell Computers and Humans Apart") for your website’s signup or email forms, or for whatever purpose using VB.NET. Simply follow the instructions below and you’ll have a professional looking captcha setup in only a few minutes. The complete code can be downloaded here, or you can simply copy and paste the code below. The Captcha is entirely customizable from all colors, background noise, and number of characters to display. Click here for an example of the Captcha using random colors with a striped background noise.

First off, as the characters are written on an image, we need to import the following Namespaces:

<%@ import Namespace="System.Drawing.Imaging" %>
<%@ import Namespace="System.Drawing" %>
<%@ import Namespace="System.Drawing.Drawing2D" %>
<%@ import Namespace="System.Drawing.Text" %>

Next, the following lines needs to go into the “If Not IsPostback” section of your Page_Load Subroutine to generate the Captcha’s image the first time the page loads. You need to allow write access for your web server (Usually IWPD user in IIS) to the directory mentioned (“/captchaimg”). This allows an image file with the captcha characters on to be written to this folder to display to the user.

        Dim strPathToImage As String = "captchaimg/captcha.gif" ' Make sure the directory is writable by your web server!
        Dim strText As String = fncDrawCaptcha(Server.MapPath(strPathToImage)) 
        imgCaptcha.ImageUrl = strPathToImage
        Session.Add("strText", strText)

Now, copy the functions and subroutines below into your VB code. In the function (fncDrawCaptcha) you’ll see lines marked “Editable Values”. These values can be edited to personalize how your Captcha image will look like:

BackgroundColor, BackgroundNoiseColor and TextColor - The values (255, 255, 255) represent the RGB (Red, Green, Blue) value for each part of the Captcha image.
RandomBackgroundNoiseColor and RandomTextColor - If the values are set to True, the BackgroundNoiseColor and TextColor values mentioned above will be ignored and random colors will be used.
BackgroundNoiseTexture - As with all Captcha’s, you need a background distortion to make it difficult for programs to decipher the text. There are a lot of options to choose from (see the code).
length – The number of characters to display on the Captcha image.

Your code for when the form is submitted should go in the “subSubmitForm” as indicated.

Function fncDrawCaptcha(ByVal path As String) As String 

    '*** Editable Values
    Dim BackgroundColor As [String]() = New [String]() {255, 255, 255} ' The 3 numbers represent in order RED, GREEN, BLUE for the captcha's background color
    Dim RandomBackgroundNoiseColor As Boolean = True ' True / False. If you choose True, BackgroundNoiseColor will not apply 
    Dim RandomTextColor As Boolean = True ' True / False. If you choose True, TextColor will not apply 
    Dim BackgroundNoiseColor As [String]() = New [String]() {150, 150, 150} ' The 3 numbers represent in order RED, GREEN, BLUE
    Dim TextColor As [String]() = New [String]() {200, 200, 200} ' The 3 numbers represent in order RED, GREEN, BLUE
    Dim BackgroundNoiseTexture As HatchStyle = HatchStyle.Min ' replace ".Min" with any of the following: Horizontal, Vertical, ForwardDiagonal, BackwardDiagonal, Cross, DiagonalCross, Percent05, Percent10, Percent20, Percent25, Percent30, Percent40, Percent50, Percent60, Percent70, Percent75, Percent80, Percent90, LightDownwardDiagonal, LightUpwardDiagonal, DarkDownwardDiagonal, DarkUpwardDiagonal, WideDownwardDiagonal, WideUpwardDiagonal, LightVertical, LightHorizontal, NarrowVertical, NarrowHorizontal, DarkVertical, DarkHorizontal, DashedDownwardDiagonal, DashedUpwardDiagonal, DashedHorizontal, DashedVertical, SmallConfetti, LargeConfetti, ZigZag, Wave, DiagonalBrick, HorizontalBrick, Weave, Plaid, Divot, DottedGrid, DottedDiamond, Shingle, Trellis, Sphere, SmallGrid, SmallCheckerBoard, LargeCheckerBoard, OutlinedDiamond, SolidDiamond, LargeGrid, Min, Max
    Dim length as integer = 6 ' Number of characters to generate
    '*** END Editable Values
    
    Dim height as integer = 100
    Dim width as integer = 200
    width = width + ((length - 6) * 30)
    Dim ranRotate As New Random
    Dim strText As String = left(replace(System.Guid.NewGuid().ToString(), "-", ""), length)
    Dim bmpCanvas As New Bitmap(width, height, PixelFormat.Format24bppRgb)
    Dim graCanvas As Graphics = Graphics.FromImage(bmpCanvas)
    Dim recF As New RectangleF(0, 0, width, height)
    Dim bruBackground As Brush
    Dim letterBrush as SolidBrush

    graCanvas.TextRenderingHint = TextRenderingHint.AntiAlias 

    If RandomBackgroundNoiseColor = true Then
        bruBackground = New HatchBrush(BackgroundNoiseTexture, Color.FromArgb((ranRotate.Next(0,255)), (ranRotate.Next(0,255)), (ranRotate.Next(0,255)) ), Color.FromArgb(BackgroundColor(0),BackgroundColor(1),BackgroundColor(2)))
    Else
        bruBackground = New HatchBrush(BackgroundNoiseTexture, Color.FromArgb(BackgroundNoiseColor(0), BackgroundNoiseColor(1), BackgroundNoiseColor(2) ), Color.FromArgb(BackgroundColor(0),BackgroundColor(1),BackgroundColor(2)))
    End If
    
    graCanvas.FillRectangle(bruBackground, recF)

    If RandomTextColor = true Then
        letterBrush = new SolidBrush(Color.FromArgb((ranRotate.Next(0,255)), (ranRotate.Next(0,255)), (ranRotate.Next(0,255)) ))
    Else
        letterBrush = new SolidBrush(Color.FromArgb(TextColor(0), TextColor(1), TextColor(2))) 
    End If
    
    Dim matRotate As New System.Drawing.Drawing2D.Matrix 
    Dim i As Integer
    For i = 0 To Len(strText) - 1 
        matRotate.Reset()    
        matRotate.RotateAt(ranRotate.Next(-30, 30), New PointF(width/(Len(strText) + 1) * i, height *  0.5)) 
        graCanvas.Transform = matRotate
        If i = 0 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Comic Sans MS", 25, FontStyle.Italic), letterBrush, width/(Len(strText) + 1) * i, height * 0.4) 'draw ‘the text on our image
        ElseIf i = 1 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Arial", 30, FontStyle.Bold), letterBrush, width/(Len(strText) + 1) * i, height * 0.1) 'draw ‘the text on our image
        ElseIf i = 2 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Times New Roman", 25, FontStyle.Italic), letterBrush, width/(Len(strText) + 1) * i, height * 0.5) 'draw ‘the text on our image
        ElseIf i = 3 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Georgia", 35, FontStyle.Bold), letterBrush, width/(Len(strText) + 1) * i, height * 0.1) 'draw ‘the text on our image
        ElseIf i = 4 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Verdana", 25, FontStyle.Italic), letterBrush, width/(Len(strText) + 1) * i, height * 0.5) 'draw ‘the text on our image
        ElseIf i = 5 Then
            graCanvas.DrawString(strText.Chars(i), New Font("Geneva", 30, FontStyle.Bold), letterBrush, width/(Len(strText) + 1) * i, height * 0.1) 'draw ‘the text on our image
        Else
            graCanvas.DrawString(strText.Chars(i), New Font("Arial", 30, FontStyle.Italic), letterBrush, width/Len(strText) * i, height * 0.5) 'draw ‘the text on our image
        End If
        graCanvas.ResetTransform()
    Next

    bmpCanvas.Save(path, ImageFormat.Gif) 
    graCanvas.Dispose()
    bmpCanvas.Dispose()
        
    Return strText

End Function 

Sub subCheckCaptcha(sender As Object, e As System.EventArgs)

    If Not txtCaptcha.Text = Session.Item("strText") Then
        lblMessage.text = "The characters does not match"
    Else
        lblMessage.text = ""
        subSubmitForm()
    End If
        
End Sub

Sub subSubmitForm()
    'Your code in here
    response.write("Captcha text entered successfully")
End Sub

Use the following controls in your form section to display the Captcha to your website users

  <p>
    <asp:Image ID="imgCaptcha" runat="server" />
  </p>
  <p>
    <asp:TextBox ID="txtCaptcha" runat="server" />
  </p>
  <p>
    <asp:Label ID="lblMessage" runat="server" CssClass="red" />
  </p>
  <p>
    <asp:Button ID="btnSubmit" runat="server" OnClick="subCheckCaptcha" Text="Submit" />
  </p>

Enjoy!

* This code is provided for free. If you like or use it, a link to this post would be appreciated (but you are under no obligation to do so).

[ebrain24]

Does Captcha on every website is made by VB.Net?

[EtheelLambert]

The code Project and added accessible sources accept a amount of cipher snippets for bearing a captcha image on a web page. However, the majority are old and use moderately circuitous JavaScript and/or C#.

[suryaprasath]

Very nice quote.. im understood the concept of captcha.thanks.

[Maddy]

Thanks for such an awesome information.. Really appreciate it!

[DM613]

Project could increase the availability of sources and accept the number of bits and pieces of encryption with the captcha image on a web page. However, most of the old and the moderate use of JavaScript, tortuous and / or C #.

[gggmarketing]

I have found you shared post so informative. Thanks for sharing your concept about Captcha.

[shagovind]

thank you for the example. it helps alot

[kharen]

Hi, I am a junior programmer.
Thanks for the CODE. Does anyone can tell how i should do the option to regenerate the image?? I mean, In my application i have an aditional option so User can click it to re generate the image to get an easier image to read. Thanks for your help =)