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CapA modern, lightning-quick PoW captcha

Cap is a lightweight, modern open-source CAPTCHA alternative using
SHA-256 proof-of-work

Quickstart & docs
Demo
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[?][?]

250x smaller than hCaptcha

Cap's widget library is extremely small, only ~20kb minified
(including WASM)

[?]

Private

Cap's usage of proof-of-work eliminates the need for any tracking,
fingerprinting or data collection



Fully customizable

Cap is self-hostable so you can customize both the backend & frontend
(or you can just use CSS variables)



PoW-based

Cap uses PoW instead of complex puzzles, making it easier for humans
and harder for bots



Standalone mode

Cap offers a standalone mode with Docker, allowing you to use it with
languages other than JS



Invisible mode

Cap can run invisibly in the background using a simple JS API

[?]

Floating mode

Floating mode keeps your CAPTCHA hidden until it's needed



Fully FOSS

Completely open source under the Apache 2.0 license

What is Cap?  

Cap is a lightweight, modern open-source CAPTCHA alternative using
SHA-256 proof-of-work. It's fast, private, and extremely simple to
integrate. Learn more about proof-of-work here.

Cap is built into 2 main parts:

  * @cap.js/widget: A small JavaScript library that renders the
    CAPTCHA and handles solving it using Web Workers and WASM.

  * @cap.js/server: An extremely simple, zero-dependencies library
    that handles creating and validating challenges.

There are also some other helpful packages:

  * @cap.js/solver: Server-side solver for the CAPTCHA in case you
    want to use machine-to-machine.

  * @cap.js/cli: Command-line interface for solving CAPTCHAs made
    with Cap. It's mainly designed for testing and when you need to
    solve these CAPTCHAs in a browser without JavaScript support.

  * Standalone mode: Docker image that helps you use Cap with any
    language or framework. It runs a simple REST API that can be used
    to create and validate challenges and an interactive UI to manage
    your keys.

  * @cap.js/wasm: WASM solvers for Node and Web built with Rust.

We also provide a middleware for a Cloudflare browser checkpoint-like
experience:

  * @cap.js/checkpoint-hono
  * @cap.js/checkpoint-express
  * @cap.js/middleware-elysia
  * more coming soon!

It's designed to be a drop-in replacement for existing CAPTCHA
solutions, with a focus on performance and UX.

Cap is built with JavaScript, runs on any JS runtime (Bun, Node.js,
Deno), and has no dependencies. If you're not using any JS runtime,
you can also use the standalone mode with Docker, which relies
entirely on a simple REST API to create and validate challenges.

Why Cap?  

  * 250x smaller than hCaptcha
    @cap.js/widget is extremely small, only 12kb minified and
    brotli'd.
  * Private
    Cap's usage of proof-of-work eliminates the need for any
    tracking, fingerprinting or data collection.
  * Fully customizable
    Cap's self-hostable so you can customize both the backend &
    frontend -- or you can just use CSS variables
  * Proof-of-work
    Cap uses proof-of-work instead of complex puzzles, making it
    easier for humans and harder for bots
  * Standalone mode
    Cap offers a standalone mode with Docker, allowing you to use it
    with languages other than JS.
  * Invisible mode
    Cap can run invisibly in the background using a simple JS API.
  * Floating mode
    Cap's floating mode keeps your CAPTCHA hidden until it's needed.
  * Fully open-source
    Completely open source under the Apache license 2.0 license.

It's ideal for:

  * Protecting APIs from bots
  * Preventing spam on forms
  * Blocking automated login attempts
  * Securing free-tier abuse

Feature comparison  

                                         Fast   Easy  Small Checkpoint GDPR/CCPA              Hard  Easy to
    CAPTCHA     Open-source Free Private  to    for   error  support   Compliant Customizable for  integrate
                                         solve humans rate                                    bots
Cap                                                                                      
Cloudflare                                                                               
Turnstile
reCAPTCHA                                                                                
hCAPTCHA                                                                                 
Altcha                                                                                   
FriendlyCaptcha                                                                          
MTCaptcha                                                                                
GeeTest                                                                                  
Arkose Labs                                                                              

Alternatives  

Cap is a modern alternative to:

  * reCAPTCHA
  * hCaptcha
  * Cloudflare Turnstile

But unlike them, Cap is computation-bound, not tracking-bound.

Read more about alternatives

License  

Cap is licensed under the Apache License 2.0.

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