What Are CAPTCHAs?


Wherever we go on the internet, we encounter CAPTCHAs, those twisted words that block or enable entries on websites. Need to post an ad on Craigslist? There’s a CAPTCHA. Want to comment on an article or blog post? There’s a CAPTCHA.

So, why do we have them?

How do CAPTCHAs work?

CAPTCHAs were invented to block spamming machines from posting wherever they want. In order to keep out spammers, a CAPTCHA has to effectively test if you are human or machine. Computer scientists figured out that one of the easiest ways to do that is to use images of language. In order to deceive spammers, the images of language take randomly generated text and manipulate the image, so that a human can barely read it, but a computer trying to take a picture of it cannot.

Even though we read words on the internet, the internet and computers are not made of words. In fact, computers often have a hard time understanding languages because they do not conform to the hard and fast rules that computer programs demand. That’s why coding languages have to be invented: human languages are too irregular. This is also one of the reasons it is so hard to create an intelligent robot.

So, CAPTCHAs take advantage of the uniquely human ability to see letters that have been stretched or manipulated and still be able to decipher which letters they are.

Why are they called CAPTCHAs?

The term CAPTCHA was first used by computer scientists at Carnegie Mellon University in 2000. CAPTCHA is actually an acronym that stands for “Completely Automated Public Turing test to tell Computers and Humans Apart.” That’s a mouthful.

It’s also a pretty straight-forward title, except for the Turing test part. What exactly is a Turing test? Alan Turing was a computer theorist who invented the Turing test which humans use to see if a machine can converse like a human being. A CAPTCHA is actually an inverted Turing test whereby a machine tests to see if you are human or not, but the core principle remains.

You may wonder why CAPTCHAs don’t use images of things other than letters, like a beach or a dog, but images are harder to have an exact answer for. A picture of a beach could generate a wide variety of responses–sea, sand, sunny, ocean, and so on–but a CAPTCHA that uses letters is paired to a particular answer. Letters, unlike images, are able to be deciphered by the human eye and programmed precisely by whoever creates the CAPTCHA.

Do you think computers will ever be able to decipher CAPTCHAs?

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