Posted by Michael Fesser on 10/18/07 20:15

..oO(laredotornado@zipmail.com)

>I'm using PHP 4.4.4. I want to block automatic submissions from
>spammers on our web site, so I'd like to use some PHP libraries that
>generate those image graphics and require the user to type the
>characters in the graphics.

That's called a CAPTCHA, in this case a graphical one.

>Do you have any recommendations on things
>you have used that are easy to implement and effective?

No, and I won't use them. There are various accessibility problems with
these graphical CAPTCHAs:

Inaccessibility of CAPTCHA
http://www.w3.org/TR/turingtest/

This link also discusses some alternatives to the inaccessible graphical
CAPTCHAs.

Additionally many of the available implementations are already broken
and can be read by machines:

Breaking a Visual CAPTCHA
http://www.cs.sfu.ca/~mori/research/gimpy/

PWNtcha - captcha decoder
http://sam.zoy.org/pwntcha/

aiCaptcha - Using AI to beat CAPTCHA and post comment spam
http://www.brains-n-brawn.com/default.aspx?vDir=aicaptcha

XRumer (CAPTCHA breaking spam poster)
http://pandalabs.pandasecurity.com/archive/XRumer.aspx

Micha

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