What Is a 'CAPTCHA' Test? How Do CAPTCHAs Work?

CAPTCHA = Completely Automated Public Turing test to tell Computers and Humans Apart
CAPTCHA = Completely Automated Public Turing test to tell Computers and Humans Apart. screenshot

Question: What Is a 'CAPTCHA' Test? 


A CAPTCHA is a short online typing test that is easy for humans to pass but difficult for robotic software programs. The purpose of a CAPTCHA is to discourage hackers and spammers from using auto-filling software programs at that web page.

Why Are CAPTCHAs Necessary Online?

CAPTCHAs are used to deter hackers from abusing online services. Specifically: hackers and spammers will undertake unethical online activities, like:

  • Swaying an online poll by robotically submitting hundreds of false responses.
  • Brute-force opening someone's online account by repeatedly attempting different passwords.
  • Signing up for hundreds of free email accounts.
  • Spamming blogs and news stories with dozens of bogus comments and search engine links.
  • Scraping (copying) people's email addresses from web pages, to use them later in spam attacks.
  • Falsifying torrent seed counts and positive feedback in order to lure people into downloading a trojan payload.

CAPTCHA tests can stop 90% of the above hacker attacks by blocking the robot software from submitting online requests.

What Does CAPTCHA Stand For?

CAPTCHA is an acronym from Carnegie Mellon University. It stands for "Completely Automated Public Turing test to tell Computers and Humans Apart". This is a rather obnoxious acronym, but the term has become the standard moniker for these simple human-or-robot tests.

How Do CAPTCHAs Work?

CAPTCHAs work by asking you to type a phrase that a robot would be hard-pressed to read. Commonly, these CAPTCHA phrases are .gif pictures of scrambled words, but can also be .mp3 voice recordings. These pictures and recordings are very hard for conventional software programs to understand, and hence, robots are usually unable to type the phrase in response to the picture or recording.

What Are Some Example CAPTCHAs?

You can see some examples of CAPTCHAs, and even get your own CAPTCHA tests for your website, here.


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