What is CAPTCHA?
CAPTCHAs are tools you can use to differentiate between real users and automated users such as bots. And it is a type of challenge-response test used in computing to determine whether or not the user is human.

  • CAPTCHAs Provide challenges that are difficult for computers to perform but relatively easy for humans.
  • Examples of CAPTCHAs are Clicking in a specific area, identifying audio clips, identifying stretched letters or numbers.
  • The term CAPTCHA was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford.
  • The most common type of CAPTCHA(displayed as Version 1.0) was first invented in 1997 by two groups working in parallel.
  • This form of CAPTCHA requires someone to correctly evaluate and enter a sequence of letters or numbers perceptible in a distorted image displayed on their screen. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human.
  • CAPTCHA is sometimes described as a reverse Turing test.

CAPTCHA stands for the Completely Automated Public Turing test to tell computers and Humans Apart.

How does a CAPTCHA work?
Classic CAPTCHAs, which are still in use on some web properties today, involve asking users to identify letters. The letters are distorted so that bots are not likely to be able to identify them. To pass the test users have to interpret the distorted text, type the correct letters into a form field, and submit the form. If the letters don't match, users are prompted to try again.
These types of tests are common in login forms, account signup forms, online polls, and e-commerce checkout pages.

Computer programs such as bots are unable to interpret distorted letters, while a human being, who is used to seeing and interpreting letters in all kinds of contexts-different fonts, different handwritings, etc. Will usually be able to identify them. Thus, bots fail the test and are blocked from interacting with the website or application, while humans are able to continue using it like normal.

Applications of CAPTCHAs
  • Prevent comment spam in blogs:-By using a CAPTCHA, only humans can enter comments on a blog. There is no need to make users sign up before they enter a comment, And no legitimate comments are ever lost.
  • Protecting Website Registration:-Several companies offer free email services. Most of these services suffered from a specific type of attack "bots" that would sign up for thousands of email accounts every minute. The solution to this problem was to use CAPTCHAs to ensure that only humans obtain free accounts.
  • Maintaining online poll accuracy:-CAPTCHAs can prevent poll skewing by ensuring that each vote is entered by a human.
  • Protecting Email Addresses From Scrapers:-Spammers crawl the Web in search of email addresses posted in cleartext. CAPTCHAs provide an effective mechanism to hide your email address from web scrapers.
  • Limiting registration for services:-Services can use CAPTCHAs to prevent bots from spamming registration systems to create fake accounts.
  • Preventing ticket inflation:-Ticketing systems can use CAPTCHA to limit scalpers from purchasing large numbers of tickets for resale. It can also be used to prevent false registrations to free events.
Drawbacks of using the CAPTCHA
  • Some CAPTCHA types do not support all browsers.
  • It May be difficult to understand or use for some audiences.
  • When a website first starts using CAPTCHAs, they notice a considerable drop in traffic.
  • Disruptive and frustrating for users.
  • Some CAPTCHA types are not accessible to users who view a website using screen readers or assistive devices.
Types of CAPTCHA
Modern CAPTCHAs fall into three main categories:-
  • Text-based CAPTCHA
  • Image-based CAPTCHA
  • Audio-based CAPTCHA
  • Math or Word Problems based CAPTCHA
Text-based CAPTCHA
These CAPTCHAs use known words or phrases, or random combinations of digits and letters. Text-based CAPTCHAs are the original way in which humans were verified. The CAPTCHA presents these characters in a way that is alienated and requires interpretation.
Alienation can involve scaling, rotation, distorting characters. This alienation provides protection against bots with insufficient text recognition algorithms but can also be difficult for humans to interpret.

Image-based CAPTCHA
Image-based CAPTCHAs use recognizable graphical elements, such as photos of animals, shapes, or scenes. And these types of CAPTCHAs are developed to replace text-based ones. Typically, image-based CAPTCHAs require users to select images matching a theme or to identify images that don't fit. Image-based CAPTCHAs are typically easier for humans to interpret than text-based ones. For bots, image-based CAPTCHAs are more difficult than text to interpret because these tools require both image recognition semantic classification.

Audio-based CAPTCHA
These types of CAPTCHAs are developed as an alternative that grants accessibility to visually impaired users. Audio-based CAPTCHAs are often used in combination with text or image-based CAPTCHAs. Audio CAPTCHAs present an audio recording of a series of letters or numbers which a user then enters.

Math or Word problems based CAPTCHA
some CAPTCHA mechanisms ask users to solve a simple mathematical problem such as "1+3" or "10-3". The assumption is that a bot will find it difficult to identify the question and devise a response. And another variant is a word problem, asking the user to type the missing word in a sentence or complete a sequence of several related terms. These types of problems are accessible to vision-impaired users, but at the same time, they may be easier for bad bots to solve.

Advanced bots are able to use machine learning to identify these distorted letters, so these kinds of CAPTCHA tests are being replaced with more complex tests. Google reCAPTCHA has developed a number of other tests to sort out human users from bots.

What is reCAPTCHA?
reCAPTCHA is more advanced than the typical CAPTCHA tests. reCAPTCHA is a free service Google offers as a replacement for traditional CAPTCHAs. This technology was developed by researchers at Carnegie Mellon University, then acquired by Google in 2009. Like CAPTCHA, some reCAPTCHA requires users to enter images of text that computers have trouble deciphering. unlike regular CAPTCHAs, reCAPTCHA sources the text from real-world images like pictures of street addresses, text from old newspapers, and so on.

Over time Google has expanded the functionality of reCAPTCHA tests so that they no longer have to rely on the old style of identifying blurry or distorted text. Other types of reCAPTCHA tests include image recognition, checkbox.

How does reCAPTCHA work without any user interaction?
reCAPTCHA can take a holistic look at a user's behavior and history of interacting with content on the internet. Most of the time, the program can decide based on those factors whether or not the user is a bot, without providing the user with a challenge to complete. If not, then the user will get a typical reCAPTCHA challenge.

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