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Free CAPTCHA-Service


This server provides CAPTCHA images and audio files you can use in html-forms. CAPTCHA images can prevent forms to be used by robots.

How does it work?

A CAPTCHA image shows a random string which the user has to type to submit a form. This is a simple problem for (seeing) humans, but a very hard problem for computers which have to use character recognition, especially, because the displayed string is alienated in a way, which makes it very hard for a computer to decode. For more information see


To use this technology, your web pages have to be generated dynamically in any programming language. We have implemented the following samples for the most popular programming languages. Please read our HowTo before you start.

How are the images integrated?

Both your webserver and our CAPTCHA server need a common secret key. The picture is integrated quite similar to page counters. As a parameter you have to send a random string within the link. The secret key and the random string are used to generate the password. Both strings are needed to compute the password. The password is computed by the CAPTCHA server to generate the image and by your server to validate the users input. A random string should not be used multiple times.

Where can I register for this service?

For development and test, please use our "demo" user. You get your own secret key for free on our registration page.

What does it cost?

How reliable is this service?

To assure the highest availability we have setup a backup server in another network guaranteeing, that neither hosting nor network problems will disrupt the service. While the standard CAPTCHA image server has the address, the backup server's address is Our samples contain a JavaScript, which automatically loads the image from the backup server, if the standard server should be down.

Since we started this service in 2004, we only had a few minutes downtime of our main server and no downtime of both servers at the same time.


How is the password computed?


The following JavaScript demonstrates the options of our system. (Note: JavaScript is not necessary on your web pages).

Required Parameters
Secret Key:
Random String:
Optional Parameters
Number of Letters:
Width, Height:
CAPTCHA Password:
CAPTCHA Image Link:
CAPTCHA Audio Link:
CAPTCHA Audio: Phonetic Spelling of the CAPTCHA
Spelling in other languages: en de it nl fr
XHTML 1.1 compliant A service of Felix Holderied and Sebastian Wilhelmi (Contact)