Mon Jan 3rd 2005, 12:59am
CAPTCHA: Completely Automated Public Turing test to tell Computers and Humans Apart

Captcha is quite clever; it’s a great way of blocking out automated spam bots from posting to web pages. A human, if asked, can read the above as “smwm”, but surely no computer could. If you gate your web forms with this kind of test, then you can be reasonably confident that any posting must be done by a human.

Wikipedia has a good writeup on Captcha. I especially like the discussion on circumvention techniques, for example how spammers use unwitting visitors to porn sites to “crack” Captcha images at various popular email providers for them. Wanna see the pictures? Enter the word below. Ha ha, great stuff.

I haven’t had a problem with spam bots here at my site, given that it’s a very small out of the way site, and it probably helps that I use my own custom blog engine that none of the ready-made spambots support out of the box (unlike users of popular blog engines like Movable Type). But hearing more and more lately about spambots defacing people’s web sites made me realize that if someone wanted to, it would be completely trivial to write a perl script to spam my site with billions of comments and bring it to its knees. So I found a really simple Captcha implementation at SourceForge and threw it up as a second challenge page whenever someone posts a comment. Again, it’s very simple, but it seems like it’ll be very effective.

Een captcha (een afkorting van “completely automated public Turingtest to tell computers and humans apart”) is een reactietest die in de gegevensverwerking wordt gebruikt om te bepalen of er al dan niet sprake is van een menselijke gebruiker.


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