CAPTCHA! Here's everything you need to know about it

CAPTCHA or Captcha (pronounced as cap-ch-uh) which stands for “Completely Automated Public Turing test to tell Computers and Humans Apart” is a type of challenge-response test to ensure that the response is only generated by humans and not by a computer.

You go to a website. You decide to sign up for a new account there. You’re taken to a screen where you meticulously enter your details, making sure you don't leave out any required fields (or else you’ll have to retype your password… twice). And right before you are allowed to hit “Submit” you see the final challenge of registering for something online– a box with some strange symbols all jumbled up (possibly incomprehensible upon first glance) with the instructions “type what’s in the box.”

It's not a new scenario– in fact, it’s probably something most people have had to deal with online since Captcha really got kicked off in the late 90s. In general, a Captcha is a challenge-response test that is designed to make sure the user taking the test is actually a human. This is based on the assumption that humans are better at character recognition than machines. Indeed, the algorithms for optical character recognition (OCR) weren't very good at figuring out Captchas when they were first introduced. Therefore, Captchas originally provided a good defense against spam robots or automated programs that wished to abuse features of online services that were designed or intended to be used only by real people.

But how useful is Captcha nowadays?

With the widespread proliferation of spam and bots (especially among social networks and forums), I would say the purpose of Captcha is still very important—almost essential. For at least the near term, we will continue to need a way to verify that the people using a website are actually people. But Captcha has its problems—it’s a usability liability (people don’t want to be bothered with “guess-this-word” puzzles all out the random). Also, Captcha isn’t very friendly to the visually impaired (although some implementations offer the option of playing an audio recording of the words or letters to address this issue).

This begs the question: is there something else that can achieve this purpose better than Captcha? If so, then perhaps Captcha itself has outlived its usefulness. If not, then maybe we still need it around.

There have been lots of attempts to create alternatives, from identifying kittens to a type of image-word association.

Most attempts look OK and work OK… but not really all that much better than regular text/image Captcha. Many even suffer from the exact same problems.

And then there is the constantly improving OCR technology. There is only so much you can do to mangle an image before even humans can’t easily read it. This is a potential upper bound, and pretty soon, the algorithms might be just as good (if not better) than human recognition. Even GMails Captcha was broken recently.

So perhaps Captcha really is on its way out— it will be interesting to see what will take its place in the future. Maybe the underlying purpose behind Captcha will change somehow. Or maybe a truly innovative technology will emerge. But for now… I, for one, am glad it is around to help keep our Internets clean of the riffraff. At least for the short-term, its usefulness is certainly nothing to be contested.

Designing a CAPTCHA System:

CAPTCHAs are designed on the fact that, the computers lack the ability that human beings have when it comes to processing visual data. It is more easily possible for humans to look at an image and pick out the patterns than a computer. This is because computers lack the real intelligence that humans have by default. CAPTCHAs are implemented by presenting users with an image which contains distorted or randomly stretched characters which only humans should be able to identify. Sometimes, characters are stroked out or presented with a noisy background to make it, even more, harder for computers to figure out the patterns.

Most, but not all, CAPTCHAs rely on a visual test. Some Websites implement a totally different CAPTCHA system to tell humans and computers apart. For example, a user is presented with 4 images in which 3 contains pictures of animals and one contains a flower. The user is asked to select only those images which contain animals in them. This Turing test can easily be solved by any human, but almost impossible for a computer.

Breaking the CAPTCHA:

The challenge in breaking the CAPTCHA lies in the real hard task of teaching a computer how to process information in a way similar to how humans think. Algorithms with artificial intelligence (AI) will have to be designed in order to make the computer think like humans when it comes to recognizing the patterns in CAPTCHA images.

However, there is no universal algorithm that could pass through and break any CAPTCHA system. Thus, each CAPTCHA algorithm must have to be tackled individually. It might not work 100 percent of the time, but it can work often enough to be worthwhile to the spammers.