CAPTCHA: Completely Automated Public Turing test to tell Computers and Humans Apart


CAPTCHA, “Completely Automated Public Turing test to tell Computers and Humans Apart,” is a security feature commonly used on websites and online services to distinguish between automated bots and human users.

Its primary purpose is to prevent automated software programs, often called “bots” or “spiders,” from accessing or abusing a website’s functions or services.

How Does CAPTCHA Work?

  1. Presentation of a Challenge: When a user attempts to perform a specific action on a website, such as submitting a form, creating an account, or making multiple requests in a short time frame, the website may present a CAPTCHA challenge. This challenge typically appears as a distorted image or a sequence of characters.
  2. Solving the Challenge: To proceed with their intended action, the user must solve the CAPTCHA challenge. This usually involves tasks that are easy for humans to perform but difficult for automated scripts.
    Common CAPTCHA types include

    • Image-based CAPTCHAs: Users are asked to identify objects or characters in a series of distorted or obscured images, such as selecting all images containing traffic lights or storefronts.
    • Text-based CAPTCHAs: Users are presented with a series of letters and numbers, often distorted or partially obscured, and they must type the characters correctly into a text box.
    • Checkbox CAPTCHAs: Users may be asked to check a box that says, “I am not a robot.” While simple, this method may also include background checks to determine if the user’s behavior is consistent with human interaction.
    • Puzzle CAPTCHAs: Some CAPTCHAs require users to solve a simple puzzle, such as arranging blocks to complete an image or connecting dots to form a shape.
  3. Verification: Once the user submits their response to the CAPTCHA challenge, the website’s server checks whether the answer is correct. If the response is accurate, the user is allowed to proceed with their intended action.
  4. Failure or Further Testing: If the user fails to solve the CAPTCHA correctly, they may be presented with a new challenge or asked to try again. In some cases, repeated failures may result in the user being temporarily blocked or restricted from accessing certain features.

The idea behind CAPTCHA is to create a task that is easy for humans to perform but challenging for automated bots to complete accurately. By using CAPTCHAs, websites and online services can protect against various forms of abuse, such as spamming, data scraping, and fraudulent account creation.

While CAPTCHAs are effective at their intended purpose, they can sometimes be frustrating for users, particularly if the challenges are overly complex or difficult to decipher. In response to this, some websites have adopted alternative methods of bot detection, such as behavioral analysis and device fingerprinting, to minimize user inconvenience while maintaining security.

CAPTCHA Examples

Google reCAPTCHA

When the user picks the right options, reCAPTCHA figures that the user isn’t a bot.

Google reCAPTCHA v2

Cloudflare CAPTCHA

Cloudflare CAPTCHA

Relationship between the Turing Test and CAPTCHA

CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is inspired by the broader concept of the Turing Test, proposed by British mathematician and computer scientist Alan Turing in 1950. While CAPTCHA and the Turing Test share some similarities, they have distinct purposes and goals.