Some MyCaptcha Improvements

Hi All,

I recently received a review about MyCaptcha control in CodePlex that proposed a couple of improvements in the source code.  This review indicates that :

  1. Thread.Sleep is not a good way to ensure that unique characters are selected for teh Captcha text.
  2. Sending the Captcha text to rendering component (.ashx file) is insecure.

Based on the above objections, I modified the source code and re-uploaded it to Codeplex. The modifications are as follows:

First, I removed Thread.Sleep thing from MyCaptcha.ascx.cs and replaced it with a do {..} while (…) loop to ensure that no letter is duplicate:

char newChar = (char)0;
newChar = Char.ToUpper(Valichars[new Random(DateTime.Now.Millisecond).Next(Valichars.Count() – 1)]);
while (Captcha.Contains(newChar));
Captcha += newChar;

To improve security when passing the Captcha text to rendering component (GetImgText.ashx) to ways come to my mind:

  • To include text generation logic into .ashx file
  • To encrypt captcha text before passing it to .ashx file.

As I mentioned before, generating a random text should not be depended on the way we display it. Therefore I don’t like mingling up the two codes. So I added some code to use a Symmetric algorithm to encrypt/decrypt Captcha text.

Thus, I added a new static class called SecurityHelper which includes two encryption and decryption methods . Symmetric algorithms  require a 16-letter key. Therefore I added a private property to give back the key. This key is optional and you can change it to whatever you would like:

private static byte[] SymmetricKey
return  Encoding.UTF8.GetBytes(“1B2c3D4e5F6g7H81”);

This post is too short to include the body of EncryptString and DecryptString methods. You can download the source code and see to it.

Back to MyCaptcha.ascx.cs , we need to use SecurityHelper.EncryptString to encrypt Captcha text before passing it to GetImgText.ashx generic handler:

ImgCaptcha.ImageUrl = “GetImgText.ashx?CaptchaText=” + SecurityHelper.EncryptString(Captcha);

We also need to change GetImgText.ashx.cs to make it decrypt QueryString value:

var CaptchaText = SecurityHelper.DecryptString(

This will make sure that nobody can sniff the captcha text. If you get the Captcha image properties in your browser, it will look like this:


I also tried to make the text more hard-to-read and a bit prettier so I changed the GetImgText.ashx file to draw an image background for Captcha. Background images come from /images/captcha folder so you need to create such a folder in your web site or modify the source code to point to an appropriate location. I have also added three sample background images, like a Valentine Day one:


try other background images and decide which one do you like the most.

Please visit CodePlex page of this project and download the source code.


9 thoughts on “Some MyCaptcha Improvements

  1. Also, I was able to get your first version to function…just couldn’t get it working with the CreateUserWizard, but after downloading the updated files, I keep getting this error:

    The name ‘SecurityHelper’ does not exist in the current context

    Source File: MyCaptcha.ascx.cs Line: 55

  2. Ah, I didn’t have SecurityHelper.cs in the App_Code folder! Thanks!

    It’s not a problem with your code, I’m just not sure how to use it with CreateUserWizard, because I can’t just add the code in the wizard (& can’t add onclick to button). I was just hoping you or someone else has done it and could provide direction 🙂

  3. I´m sorry, I´m a trying to probe the example demo than you provide,but i couldn´t get to work.
    I´m using Visual Studio Web Developer 2009 and 1º error which appears it´s than imgCaptcha doesn´t exisit. This element exists in the “Mycaptcha.aspx” file. I´m a new with visual studio web devolper,c#, and so on. I would apprecite any help.


  4. Dont´worry. I have get to finally work, the only thing was than i should change the web.config because as you explain in disccusion “” GetImgText.ashx need to have Authorization .

    Great Work your MyCaptcha Project.

  5. Aref,

    Thank you for this great component.
    I already used (the dll) in my local project, but I have one problem with it..
    Sometimes when I open the page (which contains captcha component) after a couple of hours, it takes some time (about 10-20 seconds) to load the captcha image, but after that it loads immediately.

    What’s wrong here?

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