What's a Captcha?

One of the oldest problems in computer science is to build a computer or software that mimics "thinking" like a human, and does it so well that you couldn't tell the difference. You could ask it a series of questions, and you wouldn't be able to tell whether the responses came from a real human or a computer. That's referred to as a form of "Turing test", after the computer scientist Alan Turing.

A Captcha is a kind of "reverse Turing test". In essence it's a way of proving that you're human.

You know those images with slightly scrambled letters, where you're supposed to type in what you see? That is a Captcha. Deciphering those letters is currently beyond the ability of contemporary mass market computers and software. Since you and I can typically make out what those letters are, and then type them in correctly, we must not be computers. We've proven that we're human.

Why do we care?

As with so many things these days, it mostly comes back to spam.

Here's one example: without a Captcha, it would be easy to write a computer program to open hundreds or thousands of Hotmail accounts, and start spamming from them. Once the accounts are blocked, the program can just as easily start creating thousands more. It really would be as easy as it sounds for a reasonably competent programmer.

However, at some point along the account creation process, Hotmail presents a Captcha, saying in effect "prove to me that you are not a computer, and I'll let you create this account". The computer program is stopped dead in its tracks.

I suspect that you actually experienced something similar. GMail apparently tries to prevent similar abuse by preventing you from accessing your POP3 mail "too often" (whatever that means). Apparently you tripped the limit, and GMail locked out your POP3 access. In order to restore that access you had to - you guessed it! - prove that you're human. That "UnlockCaptcha" page has two things on it: a place for you to enter your account name and password, and the Captcha: a bunch of distorted text for you to type in.

Captcha's have one big drawback: they assume you can see. Blind computer users, of which there are many, can by definition not complete a visually oriented Captcha. There are alternative types of Captchas - some using images (match the word to the picture), or even simple math (what's the answer when you add these two numbers) that can, sometimes, work better. Alternatively some sites include a link to an completely different verification mechanism for those with vision problems. The Google page you mentioned includes an icon that, when clicked on, plays an audio version of the Captcha. (Sadly, it requires plugins and ActiveX controls, so it's still not particularly seamless, but it's better than nothing at all.)

The future of Captcha is also interesting. There's no doubt that image processing software and computers themselves will become more powerful and eventually will be able to automatically decipher today's Captcha images. What than?