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altcha-org

altcha-org/altcha

Self-hosted, cookie-free CAPTCHA alternative that uses proof-of-work to block spam and bots without visual puzzles.

Open-source alternative to:reCAPTCHAhCaptchaCloudflare Turnstile

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What is it?

ALTCHA is a self-hosted anti-spam and anti-abuse widget for websites, APIs and online services. Instead of asking users to solve image puzzles, it runs a background proof-of-work computation in the browser and verifies the result on your server, so legitimate users pass through with little friction.

The widget is distributed as a Web Component (<altcha-widget>) that works with any framework, and it ships with server libraries for TypeScript, Python, Go, PHP, Java, Rust and several other languages. It supports PBKDF2 and SHA algorithms by default, with Argon2id and Scrypt available as memory-bound options that resist ASICs and GPUs. Accessibility features include a code-challenge fallback with audio support.

The project is designed around privacy: it sets no cookies by default and does no fingerprinting or tracking. It is configurable through JavaScript or HTML attributes, supports over 50 UI languages, and offers several display modes such as checkbox, floating, bar and invisible.

Key features

  • 01Uses background proof-of-work instead of visual puzzles.
  • 02Supports Argon2id and Scrypt memory-bound algorithms against hardware acceleration.
  • 03Sets no cookies by default and performs no fingerprinting.
  • 04Includes an accessible code-challenge fallback with audio.
  • 05Ships server libraries for many languages and supports 50+ UI languages.
  • 06Offers multiple display modes and an optional data obfuscation plugin.

Use cases

  • Protecting contact, signup and comment forms from spam bots.
  • Adding bot protection to public APIs without third-party requests.
  • Meeting privacy or accessibility requirements that rule out traditional CAPTCHAs.
  • Shielding email addresses from scrapers with the obfuscation plugin.

Topics

#captcha#proof-of-work#anti-spam#anti-bot#privacy#web-components#accessibility#self-hosted#anti-abuse#antispam

FAQ

What is altcha?
Self-hosted, cookie-free CAPTCHA alternative that uses proof-of-work to block spam and bots without visual puzzles. ALTCHA is a self-hosted anti-spam and anti-abuse widget for websites, APIs and online services. Instead of asking users to solve image puzzles, it runs a background proof-of-work computation in the browser and verifies the result on your server, so legitimate users pass through with little friction.
Is altcha free to use? What license does it have?
altcha is open source under the MIT license. Check the license terms before using it in commercial products.
Is altcha still maintained?
Actively maintained. The last commit to altcha was on Oct 19, 2026.
What can I use altcha for?
Protecting contact, signup and comment forms from spam bots. Adding bot protection to public APIs without third-party requests. Meeting privacy or accessibility requirements that rule out traditional CAPTCHAs. Shielding email addresses from scrapers with the obfuscation plugin.
Where can I get altcha?
The source code and installation instructions are on GitHub: https://github.com/altcha-org/altcha

Summary generated by AI from the project's README. GitHub stats refresh daily.

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