What is CAPTCHA and How Does It Work?

What is CAPTCHA and How Does It Work?

You've probably come across a CAPTCHA at least once on the Internet. But what exactly is a CAPTCHA and how does it work? Lets dig deep in simple words

What technology is used on blog posts and certain web search methods when a visitor is shown a box-shaped icon with alphabets?

Then, the user has to retype the details shown to prove his/her identity and that s/he is the real receiver. This approach is termed CAPTCHA, which stands for "Completely Automated Public Turing test to tell Computers and Humans Apart."

Today, in this blog post we will cover the exact intent of using CAPTCHA and its working methodologies.

Why is CAPTCHA important and what is its modus operandi?

Most of the time, a CAPTCHA is a picture of a bunch of messed-up characters on a distorted background. Many CAPTCHA challenges ask people to identify pictures, do simple math problems, respond to a short audio clip, or just tap a box saying "I'm not a robot."

okta captcha

Source: Okta

The key purpose of CAPTCHA

The letter "P" in CAPTCHA means that this algorithm is open to the public. 

The assessment was made in different ways around 1996. However, in 2020, Carnegie Mellon University's researchers, alongside those from IBM, gave it its unique name. This algorithm is used only to generate random sequences of numbers and letters in an image, so busting it won't necessarily render the CAPTCHA susceptible. The framework works since people and computer systems think about character strings in different ways.

Among the most crucial aspects of CAPTCHA is offering protection against commercial spamming, which uses comments on websites to advertise their frauds, which leads to an increase in cyber-related crimes, threats, and attacks.

Admins can keep out internet trolls who try to digitize their actions by making all visitors go through the CAPTCHA verification. CAPTCHA technology makes sure that a real human is attempting to access the online content. This is done to stop bots and spammers, which attempt to automatically extract email accounts or register on websites, forums, or blogs without a person's help. 

Automated systems cannot get past CAPTCHA because they are not able to read the skewed texts in the picture.

How do different types of CAPTCHAs work?

1. What is Traditional CAPTCHA?

Conventional CAPTCHAs, which are still used on some websites, ask the user to pick out letters. The texts are messed up to ensure that bots cannot figure out what they are. To qualify, users must figure out what the messed-up text says, type the right characters into a web form, and send the form. 

Users are asked to try again if the texts don't match. Online and account registration forms, online surveys, and checkout sections for online shopping often have these kinds of tests. The concept is that software programs such as bots won't be capable of figuring out what the messed-up letters mean.

However, a person, who is acquainted with regularly seeing and deciphering letters in various font styles, handwriting, and so forth, will generally be able to determine what they are.

Numerous bots are capable of typing in random letters, which makes it less probable to enter the exact sequence and crack the test. So, bots fail and can't use the app or website, while humans can keep using it as usual.

2. What is Google’s reCAPTCHA?

recaptcha types

Source: Google

Advanced bots can use ML (machine learning) to figure out what these misshapen letters are, so such types of CAPTCHA tests are gradually being phased out in favor of more difficult ones. Google's reCAPTCHA has created a bunch of additional tests to tell the difference between people and bots.

Google's reCAPTCHA arrives as a free resource that can be used instead of classic CAPTCHAs. Research teams from Carnegie Mellon University came up with the reCAPTCHA innovation, and in 2009, Google bought it.

reCAPTCHA is smarter than ordinary CAPTCHA tests. Several reCAPTCHAs, the same as CAPTCHA, ask the user to type in pictures of text, that computers can't read. reCAPTCHA is different from other CAPTCHAs because the text comes from photographs of real-world things, including street names, published works, old newspapers, etc.

Google has added more features to reCAPTCHA over the years. Now, they do not require the use of the same old method of finding pixilated or contorted text. There are other kinds of reCAPTCHA evaluations, such as:

  • Image recognition
  • Checkbox
  • General evaluation of user behavior

Over time, Google has also updated reCAPTCHA versions: such as v2 and v3. If you want to learn more about reCAPTCHA V2 vs V3, then you can check out the link above – understand what reCAPTCHA v3 can and cannot do for your website security. 

The guide additionally recaps the differences between reCAPTCHA v2 vs. v3, uncovers the pitfalls of reCAPTCHA v3 configuration, and sums up what a truly effective bot protection and mitigation solution must deliver.

google captcha

3. CAPTCHA test that looks for images

Users are usually shown nine or sixteen square-shaped pictures during a CAPTCHA test for image recognition. The pictures could all be parts of one big picture, or they could all be distinct. 

The visitor has to find pictures that have certain things in them like living creatures, plants, or signboards. If their answer is identical to most of the other answers to that same test, it is regarded as "correct," and the viewer passes.

It's tough for computer systems to figure out how to select certain things from grainy pictures. AI (artificial intelligence) programs that are very smart have trouble with it, and hence, a bot faces the same problem. A person, on the other hand, is able to do the above pretty easily, since people are used to seeing everyday items in a wide variety of circumstances.

image as captcha

4. CAPTCHA tests that only have one box to check

Some CAPTCHA tests just ask users to tick a box beside the phrase "I'm not a robot." But the test isn't the process of tapping the tick box; it is about everything that happens before you press the checkbox.

This CAPTCHA test looks at how the person's cursor moves as it gets close to the tick box. Even when a person moves in a straight line, there is some variation at the microscale, in the form of discrete, subconscious moves that are hard for bots to copy. 

If any of this randomness can be seen in the way the cursor moves, the test determines that the visitor is likely genuine. This test may also look at the person's browser's cookies as well as the computer's history to figure out whether the user can be a bot.

If the assessment still can't tell whether the visitor is a real human, it might give the user extra challenges such as image recognition tests. Mostly, though, a user's cursor moves, cookies, and gadget history are sufficient to figure out who they are.

5. CAPTCHAs where the user doesn't do anything

The newest variants of CAPTCHA can check a user's behavior and background of how they engage with content online as a whole. 

Usually, the software can figure out if users are bots or not, depending on these factors without giving them a task. If not, they will see a standard CAPTCHA test. Some websites have CAPTCHAs set up by default as a way to stop bots from getting in. Otherwise, a test could be run if a person's actions look like those of a spammer. 

For example, a test might be run when a user accesses pages or clicks links at a much faster rate than usual.

6. CAPTCHA tests and projects that use AI

AI software applications get better at reading hard-to-read words and finding items in grainy photos as thousands of people use them to do these things. This data is then entered into the AI system.

Generally, it's hard for computers to figure out letters and objects in different situations since context can alter so much in reality. 

For example, a stoplight is red in color with "STOP" written in the white alphabet on an octagon-shaped shape. 

A structure and text mixture like that would be easy for a computer algorithm to figure out.  Based on the photo's angle, lighting, weather, and other factors, a stoplight in a picture could appear differently than how it sounds in the description above.


With ML, AI can get great at getting around these problems. In the case of a stop sign, a programmer will give the AI software a lot of information about the various features of a stop sign. 

For it to work, they require multiple examples of pictures, which have or don't have stop signs. They need people to pick them out until the software has enough information to become efficient in this certain task.

This requirement is met by CAPTCHA, which gets individuals to recognize items and messages. Over time, this gives sufficient data to create strong AI algorithms.

7. CAPTCHA and Turing tests

Turing tests measure how well a computer can act like a person. In 1950, Alan Turing, who was among the first people to work with computers, came up with the idea of this test.  

A code "passes" the test if it acts just like how a real person would during the assessment and can't be told apart from an individual. It doesn't matter if the responses are correct or incorrect for the Turing test; what matters is, how "human" an answer sounds.

Although termed as "Public Turing tests," a CAPTCHA is literally the reverse of the former. Rather than having to figure out if a device is sentient, it checks whether a "human" visitor is a bot in reality.  

To do this, CAPTCHA tests need to give a short task that most people are good at but that computers have trouble with. Most of the time, recognizing images and text fits these criteria.


Use CAPTCHA on websites that accept information from people who haven't signed in. CAPTCHA is generally not required to accept user input from people who already are registered in their accounts. 

However, CAPTCHA is able to slow down unverified users, such as bots that attempt to share spam messages in forums and blogs without needing to be validated as real users.

Jinal Lad Mehta is a digital marketing enthusiast. She is known for writing creative and engaging content. She loves to help entrepreneurs get their message out into the world. You can find her looking for ways to connect people, ideas, and products.

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