Simple Validation Image Demo

Anyone who has ever had any sort of guestbook or comments page on their site which takes user input without requiring registration or without new posts being checked before being set live will know that within a few days you will be hosting several thousand adverts for Viagra, Casinos and Russian pornography!
The same goes for contact forms where a user enters a message and it is sent to the webmaster.

What can be done?

You can force people to signup for a login in order to post messages, contact you or leave comments, but you just end up putting people off bothering.
Another approach would be to have a system that sends you an automated message with the contents of the user's (or spam bots) post to either allow or deny it, but personally I hate being spammed either directly or indirectly by my own website forwarding the junk on to me for approval!

The other alternative is to use validation images (CAPTCHAs).

The basic idea is that you generate an image that is displayed in whatever user input (comments, contact, registration) section you are trying to protect, and the user must enter the numbers or letters shown in the image in order to proceed.

These images are known as CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart*) and vary from very simple (such as the example presented here) to more complex (that the likes of Yahoo and Paypal use), the latter using character distortion, and other artifacts like background noise.

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Implementation

The way that this verification demo has been implemented is to use an ASPX page as an image source for the validation code picture.
You pass the various characteristics of the number (size, color, background, etc) to the image generating ASP.NET page by means of server side session variables, and an image containing this number is then generated and pushed to the output stream as an Gif image.

The displaying page generates the number randomly before passing it to the image generator, and then verifies that the users input and the original number generated match upon the forms submission.

Download Source and Example

* CAPTCHA is a trademark of the Carnegie Mellon Institute.