CAPTCHA’s are as infuriating, but here’s why they’re everywhere

Bots account for nearly half of global internet traffic, increasing the need for online security. Whether browsing on an Android phone or a Chromebook, you’ve likely encountered the “I’m not a robot” message. A CAPTCHA distinguishes human users from bots. This article explains what a CAPTCHA is, how it works, the available types, and its role.

The meaning behind the CAPTCHA and its purpose

A CAPTCHA test determines whether an online user is a human or a bot by presenting challenges humans solve, but bots do not. CAPTCHA stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart. The Turing test, proposed by Alan Turing in 1950, assesses whether a machine behaves indistinguishably from a human.

In a traditional Turing test, a human evaluator converses with a human and a machine, unaware of which. If the evaluator cannot distinguish the machine from the human, the machine passes the test. CAPTCHA applies a similar principle in a more targeted manner.

Common uses of CAPTCHA in cybersecurity

Companies block bots that spread spam and malware or engage in malicious activities using a CAPTCHA test before signing up for email, social media profiles, or other services. Early adopters used CAPTCHA to stop bots from creating fake email accounts. Vendors use CAPTCHA to block bots from purchasing limited commodities such as concert tickets and reselling them on secondary markets.

Scammers and cybercriminals use comment sections to spread scams and malware. They also spam reviews by posting fake reviews to boost product rankings on e-commerce sites and search engines. Requiring a CAPTCHA before posting comments or reviews mitigates these activities. In brute-force and dictionary attacks, hackers use bots to guess character combinations until they find the correct password. Requiring a CAPTCHA after several unsuccessful login attempts prevents these attacks.

A look at the most common types of CAPTCHA

CAPTCHA systems evolved to counter increasingly advanced bots, creating various types that test human cognitive abilities. Below are the most common types.

Text-based CAPTCHA

Text-based CAPTCHAs are the earliest systems. They display distorted text, often a random sequence of letters and numbers, which users must interpret and enter. These CAPTCHAs work because humans recognize distorted characters, while bots struggle with noisy text.

Image-based CAPTCHA

Image-based CAPTCHAs emerged to address the limitations of text-based systems. Instead of reading distorted text, users see images and must select those containing specific objects, such as traffic lights, crosswalks, or vehicles. This method exploits real-world image complexity, making it harder for bots to identify objects accurately.

Math-based CAPTCHA

Math-based CAPTCHAs present simple arithmetic problems, like addition or subtraction, to verify users. For example, the system asks users to solve 8 + 1 and enter the correct answer. Humans perform these calculations easily, while bots struggle with arithmetic.

Game-Based CAPTCHA

Game-based CAPTCHAs verify users with interactive tasks instead of traditional text or image challenges. They involve games like dragging objects, identifying patterns, or solving puzzles that leverage human cognitive skills that bots cannot replicate. As AI evolves, game-based CAPTCHAs offer a dynamic defense against bots.

Audio-based CAPTCHA

Developers created audio-based CAPTCHAs for visually impaired users. In these systems, users listen to recordings of characters or numbers and enter them into a form. Developers distort the audio or add background noise to block bots with simple speech recognition systems.

Google’s reCAPTCHA

Google’s reCAPTCHA is a widely used system. It monitors user behavior by analyzing mouse movements, typing patterns, and interaction speeds to determine if the user is a human or a bot. If suspicious behavior occurs, the system presents a challenge, often requiring the selection of specific images.

The No CAPTCHA reCAPTCHA system is more user-friendly compared to traditional CAPTCHA. It assesses user interaction in the background by analyzing browsing history, IP address, and on-site behavior to gauge bot activity. Although user-friendly, it relies on background risk assessments and collects personal data for analysis, raising privacy concerns. It does not disclose how it calculates risk scores.

The challenges of using CAPTCHA

CAPTCHA technology has several disadvantages. CAPTCHA challenges add extra steps during registration, login, and form completion. The complexity of CAPTCHAs has increased to counter advanced bots, making them more challenging to solve.

According to Stanford study results, participants agreed on CAPTCHA solutions only 71% of the time. Non-native English speakers struggled more than native speakers. Accessibility is another challenge, especially for visually impaired users, because screen readers cannot read text and image CAPTCHAs. Alternative CAPTCHAs, like audio-based challenges, remain problematic. Users in the study agreed on audio CAPTCHA solutions only 31% of the time.

AI is breaking the CAPTCHA

Artificial intelligence has changed the effectiveness of CAPTCHAs and reCAPTCHAs. Modern machine learning solves CAPTCHAs and reCAPTCHAs with over 70% accuracy. CAPTCHA puzzles provide valuable training data for advanced systems. Even basic AI eventually defeats sophisticated CAPTCHA puzzles. We need better CAPTCHA technology soon, with better accessibility and security in mind.