What is a CAPTCHA anti-spam system?

Ah, spam, the bane of our collective online existence. It's a pain and it's frustrating how much time and effort we have to collectively expend trying to minimize its impact on our online experiences. Blech.

Here's a fun fact that most people don't know: CAPTCHA is actually an acronym. It stands for "Completely Automated Public Turing test to tell Computers and Humans Apart." (Turing here refers to the brilliant computer scientist Alan Turing, who spent much time thinking about how to differentiate really smart computers from humans, to so-called "Turing test").

I know you've seen these. Here's a typical CAPTCHA from Google's site:

The basic idea is that it's some twisted or distorted or obfuscated text that you, as a human, will have no problem recognizing, but that a computer program would have a very tough time interpreting: if it can't figure out the words shown, it can't pretend to be a human and therefore can't spoof the system.

Here's another example, from Craigslist:

This one is more interesting in some sense because it's the same basic idea, but this recaptcha is actually also a distributed solution to interpreting difficult-to-read scanned text in old books being digitized. Nice win:win!

Lots of sites use these, actually. Here's one from Facebook:

Unfortunately, the simpler CAPTCHA systems have been cracked through various means, ranging from smart image scanning and analysis software to brute-force guessing (mapping captcha image file name with a solution) to simply showing the CAPTCHA in another context on an apparently innocent Web site and storing the values [innocent] people type in.

MySpace is late to the party, but its system is pretty typical:

Earthlink is a bit more colorful:

And, finally, here's what Yahoo uses:

They're all using the same basic concept and it works, though it can definitely be frustrating when they twist or distort the text too much and just cause a headache.

I would say that if you are having spam problems on your site, it's quite probably from automated tools pretending to be humans, and a CAPTCHA system might well be just what you need to make the problem disappear.

If only it were so easy to make the spammers disappear.