Wwohn.com : Captcha alternatives

Posted by Oeroek in maart 3rd, 2008

There are many alternatives that may replace the unreadable captcha or make it more reader friendly. Some of these alternatives can be used without the user noticing, the non interactive alternatives. Others are just variations of the reverse Turing test of which the captcha is also an example. Although I realize that these alternatives are not perfect, the captcha is not perfect either.

Examples of impossible user tests

This person made a test that not only keeps out the spambot but also the visitor.
The ten worst captchas. Thank you Webhypes for the hyperlink.

Non interactive alternatives

The following alternatives may help to detect spambots. Good spammers will probably find their way around these checks just as they will decode the captchas. Nevertheless, you may prevent scriptkiddies, amateurs and old style spambots from entering your site. You can make it more difficult and combine several systems.

Add input fields that are not visible for the user. A bot may try to fill up all fields available, including the hidden ones. By verifying that the hidden field is empty you can detect human visitors. You should probably insert the CSS in an external style sheet. You can also replace {visibility: hidden} with {display:none}. Do not forget to change the name of the field as frequent as possible.

<input name="spam" type="text" style="visibility: hidden">
You can also use a hidden submit button and add a user button with Javascript. A bot may click on the wrong button therewith identifying itself. Thanks to Bram on TBForum.

<input type='submit' name='p' value='go' style='display:none'/> <script language=\"JavaScript1.1\" type=\"text/javascript\"> document.write('<in' + 'put ty'+'pe=\"hid' + 'den\" na'+'me=\"acti'+'on\" va'+'lue=\"addme\">'); document.write('<input t'+'ype=\"sub'+'mit\" na'+ 'me=\"post\" value=\" $[Post] \" />'); </script>
Look for fields other than the e-mail or website field and check for the use of @ or HTTP. There may be bots that fill in an email address or website in as many fields as possible.
Use filters such as the free spam filter of Akismet. I use it on this Wordpress blog an I am very happy. Actually, it is my main and only spam protector.

Variations of the old fashioned captcha

Use logical puzzles such as “2 times 2 = ….”, “How much wheels does a car have?”, etc. We found the following example on linkvendor.
Use trivia such as “fire is not cold but ….”. See an example.
Present pictures of objects and ask visitors questions such as: “how many cats there are in the picture”, “is there a house in the picture”. You could make this multiple choice.
Present several pictures and have visitors click on a specific picture, for example the kitten. After the second/third attempt the message should be in the moderation cue. See an example and read about the creator of this test or a review.
Make a “yes” and a “no” radio button and make “no” the default. Have visitors state that they are not spammers by selecting “yes”.
Make an audio file that reads out loud the figures and have visitors type them in. Although this system helps the blind, it cripples the deaf.

I am going to make this a dynamical document. Please let me know if you know of other tests and have examples, by adding a comment or using the contact form. I will add the best alternatives to this document.

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