RadCaptcha allows you to easily control the character set from which the CaptchaImage text is randomly generated.
There are three predefined sets to choose from:
If you wish to specify your own character set, choose CustomCharSet for the CaptchaImage.TextChars property and set the desired character sequence to the CaptchaImage.CharSet property. Please note that for security reasons you need to provide at least 15 characters.
Additionally, you can tell the RadCaptcha to respect/ignore the letter case by setting the IgnoreCase property. By default the letter case is ignored (i.e., IgnoreCase="true").