Spam protection with Text-Captchas

Most of all webmasters, which have contact forms, registration forms or online polls on their websites, know the problem: in a short time they are receiving spam. The reason is, that spammers are using so-called harvesters (also known as webcrawler, spider, robot, or bot). This is a special software, which is searching automatically through the WWW, analyzes websites and tries to fill out any discovered forms with spam.

With Captchas (Completely Automated Public Turing test to tell Computers and Humans Apart), a method, which is able to distinguish between a computer (bot) and a human, it is possible to prevent these spam in most cases.

Please note: using Text-Captchas on websites is with a view to the Accessibility just a good way. But Spam-Bots could possibly recognize them too. The bot doesn't need any OCR-routines, he only has to discover the calculation like in example #1. So it's a good idea to insert a second verification method into your script. The script could measures (inside a cookie, session, or a hidden form field) the time how long a user (or bot) needs to fill out the complete form. Reason: a human user needs many seconds, and a bot only a fraction of a second.

See also: Complete example (Image captcha) inclusive form and evaluation

Example #1

The use of this Text-Captcha is very simple, easy to understand, and everybody(?) in the whole world can solve the calculation.

Download: example1.zip (form incl. evaluation)

Example #2

That's one of my favorites. You can generate thousands of different questions, which Spam-Bots can't crack.

Example #3

A random string is possible too, but it's no insurmountable hurdle if the input request is always the same.

Ok, that's enough. Use your imagination to create your own questions and answers.