http://captchas.net
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Free CAPTCHA-Service

Why http://captchas.net?

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 http://en.wikipedia.org/wiki/Captcha.

Prerequisites?

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.

• PHP
• ASP
• Perl
• Python
• JSP
• Apex
• Ruby (External Link)
• JavaScript, npm (External Link)

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?

• Our service is for free, as well for commercial websites.
• If you like to pay for it, you can use it without "http://captchas.net/" in image: 120 EUR (160$) per year.
• Integration of service by us in your form: 120 EUR (160$).
• Our image generator as easy to install binary for your web server: On request.
• Send your inquiries to mail@captchas.net.

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 image.captchas.net, the backup server's address is image.backup.captchas.net. 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.

References

• Our service is integrated in the CMS Plone and Joomla.
• At address-protector.com this service is used to protect e-mail addresses from being automatically harvested to be used for sending SPAM.
• www.blog.de and www.blog.co.uk prevent comment spam with our service.
• pir.org uses this service to protect its forms.
• Please use Google:"captchas.net" to find further sites and comments on our service.

How is the password computed?

• Concatenate the secret key and the random string (Example: secretRandomZufall).
• If Alphabet or Letters differs from abcdefghijklmnopqrstuvwxyz/6, append both separated by ':' (<secret><random>:<alphabet>:<letters>).
• Take the MD5-sum of the resulting string (Example: 0x4a 0x7d 0x5d 0xf1 0xdd 0xd7 0x88 0xd4 0x7b 0xd5 0x1e 0x66 0x65 0x45 0xe1 0xec).
• Take the first 6 bytes of the resulting 16-byte-long MD5 value (Example: 74 125 93 241 221 215).
• Determine the remainders of this 6 bytes, when dividing by 26 (Example: 22 21 15 7 13 7).
• Every number encodes a character from the chosen alphabet (Example: "wvphnh").

Demonstration

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

Required Parameters
Username:
Secret Key:
Random String:
Optional Parameters
Alphabet:
Number of Letters:
Width, Height:
Color:
CAPTCHA Password:
CAPTCHA Image Link:
CAPTCHA Image:
CAPTCHA Audio Link:
CAPTCHA Audio:	Phonetic Spelling of the CAPTCHA
Spelling in other languages:	en de it nl fr