Following ASP.NET (C#) code below will create, display and verify Math Captcha (Mathematical Captcha) for you be able to rapidly integrate into aspx page functions such as login, registration, comment, forgot password, etc to prevent SPAM.
We’re creating 2 files: one for creating the Math Captcha image and one for display and verifying the Math Captcha.
Create Math Captcha With ASP.NET C#
1. Create The Math Captcha Image
Create an aspx file named for example simple-math-captcha.aspx and code behind file named simple-math-captcha.aspx.cs – Visual C# with content as below:
+ simple-math-captcha.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="simple-math-captcha.aspx.cs" Inherits="simple_math_captcha" %> |
+ simple-math-captcha.aspx.cs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 | using System; using System.Collections.Generic; using System.Linq; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; using System.Drawing; using System.Drawing.Imaging; using System.Drawing.Text; using System.IO; public partial class simple_math_captcha : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { Response.AddHeader("p3p", "CP=\"CAO PSA OUR\""); Color myColor = System.Drawing.Color.Red; Bitmap objBMP = new System.Drawing.Bitmap(80, 30); Graphics objGraphics = System.Drawing.Graphics.FromImage(objBMP); objGraphics.Clear(Color.Transparent); Font objFont = new Font("Arial", 12, FontStyle.Regular); string randomStr = ""; Random r = new Random(); int a = r.Next(1, 99); int b = r.Next(1, 99); int c = a + b; randomStr = a.ToString() + " + " + b.ToString() + " = "; Session["LoyatyMathCaptcha"] = c.ToString(); SolidBrush myBrush = new SolidBrush(myColor); objGraphics.DrawString(randomStr, objFont, myBrush, 3, 3); Response.ContentType = "image/png"; System.IO.MemoryStream mem = new MemoryStream(); objBMP.Save(mem, ImageFormat.Png); mem.WriteTo(Response.OutputStream); objFont.Dispose(); objGraphics.Dispose(); objBMP.Dispose(); } } |
Notes:
- Set allow session on IE load by Iframe/ Frame, line 16
- Math Captcha image text color: Red, line 18
- Math Captcha image size is 80×30, line 20
- Text font for the Math Captcha image: Arial and size: 12, line 24
- Create the math, ex: A + B = C, from line 26 to 35
- Create a Session named for example “LoyatyMathCaptcha” to store the right result, will use to verify the math captcha, line 37
- Set the content type: PNG and return the image, line 43
2. Display and verify the Math Captcha Image
Create an aspx file named for example simple-math-captcha-demo.aspx and code behind file named simple-math-captcha-demo.aspx.cs – Visual C# with content as below:
+ simple-math-captcha-demo.aspx
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | <%@ Page Language="C#" AutoEventWireup="true" CodeFile="simple-math-captcha-demo.aspx.cs" Inherits="simple_math_captcha_demo" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> </head> <body> <form id="form1" runat="server"> <table> <tr> <td colspan="2">Enter Captcha (with number)</td> </tr> <tr> <td colspan="2"><asp:Label ForeColor="Red" runat="server" ID="lblMessage"></asp:Label></td> </tr> <tr> <td><img src="simple-math-captcha.aspx" /></td> <td><asp:TextBox runat="server" ID="txtCaptcha"></asp:TextBox></td> </tr> <tr><td colspan="2"><asp:Button runat="server" ID="btnSubmit" Text="Submit" onclick="btnSubmit_Click" /></td></tr> </table> </form> </body> </html> |
Note: in order to increase the user experience, we should implement JQuery allows only numbers in the textbox.
+ simple-math-captcha-demo.aspx.cs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 | using System; using System.Collections.Generic; using System.Linq; using System.Web; using System.Web.UI; using System.Web.UI.WebControls; public partial class simple_math_captcha_demo : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { } protected void btnSubmit_Click(object sender, EventArgs e) { string captcha = txtCaptcha.Text; if (captcha == Session["LoyatyMathCaptcha"].ToString()) { lblMessage.Text = "Correct!"; } else { lblMessage.Text = "InCorrect!"; } } } |
If we need to implement a more complicate/securier Captcha, consider to implement Google Captcha also with ASP.NET (C#).
