CAPTCHA is an acronym for Completely Automated Public Turing test to tell Computers and Humans Apart, which is a security measure to distinguish between humans and bot programmes — a software application that performs automated repetitive tasks on the internet. The purpose of this test, as the name suggests, is to identify and prevent spam and password decryption (it is the process of converting encrypted data to its original readable format.) Although CAPTCHAs are designed as a security measure to block automated bots, they themselves are automated and pop up on certain websites, where they automatically fail or pass users. CAPTCHAs commonly appear on login pages, websites, registration portals, and comment sections. The test also appears when creating accounts, especially on Google.
The origin
The earliest form of CAPTCHA technology, which included the use of random wrapped texts and a combination of digits and letters, was developed in the early 2000s to combat widespread nefarious activities of hackers on the internet. During this period, web service company Yahoo faced the brunt of malicious attacks from bots. Spam accounts were created, especially in the chat rooms, causing trouble for the company. This was when Luis von Ahn, a 22-year-old software developer with the support of his professor Manuel Blum, developed CAPTCHA to tackle the widespread attack.
How do they work?
To prevent spam and password decryption, CAPTCHA asks you to answer a short test that verifies you are a human and not a computer attempting to access a password-protected account.
The test has two parts: a string of randomly generated letters or numbers that appear as a distorted image and a text box. The individual has to type the characters seen in the image into the text box to pass the test. Although this is normally how CAPTCHA works, there are other methods to prove you are a human.
Did you know?
The ‘T’ in CAPTCHA, which stands for Turing, is named after Alan Turing, an English mathematician and computer scientist. In the 1950s, he developed a Turing test that involved an interrogator asking questions to a human and a computer through a text-only chat. If the computer is able to present itself as a human and the interrogator doesn’t find any difference between both their responses, then the test is considered successful. Turing dubbed it ‘the imitation game’.
Types of CAPTCHA
Text-based CAPTCHA: It goes without saying that these are the oldest forms of CAPTCHA, which we randomly encounter even today. These are presented in a distorted way, making it incomprehensible for a bot to interpret the text. Not to mention, sometimes we need to pop our eyes out to see these texts.
Image-based CAPTCHA: We have all come across pictures of traffic signals, cars, bicycles, etc., at some point. This test involves asking users to complete a picture-based challenge where a prompt is given to tick images on the grid.
Audio CAPTCHA: Audio CAPTCHAs are designed as an alternative to text- and image-based CAPTCHAs to cater to users who have trouble seeing. They are presented as an audio recording containing a series of numbers and texts.
Birth of reCAPTCHA
With the success of CAPTCHA, Luis von Ahn seized the opportunity to make more user-friendly CAPTCHAs with a capability of detecting advanced bots by using advanced algorithms. This came to be known as reCAPTCHA. reCAPTCHA is an upgraded version that asks users to click on the checkbox to prove they are not a robot. Due to the novelty of the concept, Google purchased reCAPTCHA in 2009.
Over the years, CAPTCHA has surely evolved through complex algorithms. With the advancements in AI, websites might turn to use biometric CAPTCHAs, though these flag privacy concerns.
Published – August 18, 2025 03:24 pm IST