Telerik RadCaptcha is UI control that provides two major strategies for protection against automated form submissions:
- Image With Modified Symbols (Captcha Image) - They are displayed in a form, and the user is required to input the symbols in a textbox. The Image is generated with an HttpHandler.
- Automatic Robots Discovery - this strategy uses predefined rules which decide whether the input comes from a robot or not. At this point, there are two implemented rules that could be applied either separately or simultaneously.
- Minimum form submission time - the presumption is that a human cannot input the fields in a form correctly for a time less than 3 seconds (this is set by default, and could be modified). If the submission is executed faster than the predefined value, it is assumed that the executor is a robot.
- Hidden textbox in the form (the so-called "honeypot") - this rule requires the insertion of a textbox which is not visible when the form is styled. Still, it will be detected by a robot, and therefore if any data is entered, the executor is considered to be a robot.
- Three Modes for Protection - you can easily define which spam protection strategies to be used for spam protection. For maximum security, by default, all of them are used.
- Set Custom Error Message - the error message that is displayed when the condition being validated fails. Depending on which protection strategy is used, the respective error message is displayed.
- Background and Line Noise Level of the Captcha Image - you can easily control the background and the line noise of the Image by setting the respective value (None, Low, Medium, High or Extreme). The default level of the background noise is Low and of the line noise is None.
- Font Family and Font Warp of the Catcha Image - you can easily choose which font family to be used for the Image text. Arial is used as a default value for the font family. Furthermore, the amount of random font warping to apply to the rendered text can be changed by setting the CaptchaFontWarp property. The default amount of font warping is Low.
- Text Length and Possible Characters of the Catcha Image - the default length of the text is 5 characters, and the characters could be either letters or either numeric characters. Alternatively, you can choose what kind of characters to be used (only letters or only numeric characters), and change the length of the text.
- Maximum time interval of the Captcha Image - the maximum number of minutes the Captcha Image will be cached and valid.
- Minimum Timeout - minimum number of seconds the form must be displayed before it is valid. If you're too fast, you must be a robot. This is set when “Minimum form submission time” mode is used for Spam Protection.
To get familiar with the control please follow these online resources: