Cap: Open Source CAPTCHA Alternative

Table of Contents

1. Introduction
2. What is Cap?
3. Key Features
4. How Cap Works
5. Conclusion

Introduction

The web is saturated with CAPTCHA systems—those challenges that ask users to identify traffic lights or type distorted text—aimed at separating humans from bots. While effective to some degree, these systems often introduce significant friction into the user experience. Worse, many are heavy, opaque, and invasive—tracking users and leaking privacy for the sake of verification.

Cap is a modern, open-source alternative to traditional CAPTCHA systems. It eliminates these pain points by leveraging a simple yet effective SHA-256 proof-of-work mechanism. Cap doesn’t rely on behavioral tracking, user profiling, or third-party services. Instead, it shifts the burden of verification to computational effort—a task lightweight enough for humans to handle, but prohibitive for bots to scale.

With a minuscule 12KB widget, zero dependencies, and full customizability, Cap is designed for developers who want control, speed, and above all—privacy.

What is Cap?

Cap is a privacy-first, open-source CAPTCHA alternative that uses SHA-256 proof-of-work instead of traditional challenge-response systems. Unlike conventional CAPTCHAs that rely on solving image puzzles or identifying patterns, Cap requires the user’s browser to perform a small, computational task to prove they're not a bot.

This approach is frictionless for users and difficult for automated scripts to exploit at scale. The entire widget weighs just 12KB (minified and compressed) and has zero external dependencies, making it ideal for performance-critical or privacy-conscious applications.

Cap is designed to be self-hostable and fully customizable. You can style it with CSS variables, integrate it with any backend stack, and even run it in different modes—like invisible, floating, or standard—to suit your UX needs.

Key Features

Cap stands out as a CAPTCHA solution by focusing on simplicity, performance, and user privacy. Below are its core features:

🪶 Lightweight

Cap’s frontend widget is just 12KB when minified and compressed. With no external dependencies or heavy assets, it loads quickly and performs smoothly—ideal for modern, high-performance websites.

🛡️ Privacy-First

Cap collects zero user data. There’s no fingerprinting, tracking, or behavioral analysis. This makes it GDPR-friendly and perfect for developers who prioritize user privacy.

⚙️ Proof-of-Work Based

Instead of using quizzes or visual puzzles, Cap uses SHA-256 proof-of-work to verify users. This process is easy for humans but computationally expensive for bots to automate at scale.

🧩 Fully Customizable

Cap is designed to be themed and styled with CSS, giving you full control over the look and feel. You can also modify the backend logic or run it entirely from your own infrastructure.

🧱 Self-Hostable

Cap offers a standalone mode with Docker support. This allows developers to run the entire system independently of third-party services, supporting any backend language or stack.

🫥 Invisible & Floating Modes

Cap can operate in invisible mode—triggered silently in the background—or in floating mode, where it remains hidden until needed. These options offer flexibility in UI integration.

📜 Open Source

Licensed under Apache 2.0, Cap encourages contributions and full transparency. You can audit the code, fork it, or extend it for your specific use case.

How Cap Works

Cap stands out as a lightweight, privacy-focused, and fully customizable alternative to traditional CAPTCHA systems. Unlike reCAPTCHA and hCaptcha, which collect user data and rely on image puzzles or behavioral tracking, Cap does not track or collect any data, offering a more seamless and user-friendly experience. Cap is also significantly smaller in size (12KB) compared to the heavier reCAPTCHA and hCaptcha, ensuring faster page load times. Additionally, while Cap is open-source and fully customizable, allowing for self-hosting, both reCAPTCHA and hCaptcha are proprietary solutions with limited customization and no self-hosting options. This makes Cap an ideal choice for developers looking for a privacy-first and performant CAPTCHA solution.

The following table summarizes the comparison between the major Cap alternatives.

Conclusion

Cap offers a fresh and innovative approach to CAPTCHA verification by prioritizing privacy, performance, and customizability. Unlike traditional CAPTCHA solutions like reCAPTCHA and hCaptcha, Cap empowers developers with a lightweight, open-source alternative that minimizes friction for users while ensuring robust bot protection through a simple proof-of-work mechanism. Whether you're building a privacy-conscious application or seeking an easy-to-integrate solution with full control, Cap provides a flexible and effective option that aligns with modern web development needs.

With its lightweight design, strong privacy focus, and open-source nature, Cap is an ideal choice for developers who value user experience, transparency, and control.