ALTCHA – privacy-first CAPTCHA alternative

ALTCHA is a self-hosted CAPTCHA alternative designed to protect websites, forms, APIs and other online services from automated abuse without relying on traditional image-selection puzzles.

The software uses browser-based proof-of-work to verify requests while keeping normal interaction with the user to a minimum. It is supplied as a Web Component, making it suitable for adding to existing forms while leaving the challenge generation and verification infrastructure under the operator’s control.

This is free and open source software.

Key Features

• Offers PBKDF2, SHA, Argon2id and Scrypt challenge algorithms, including memory-bound choices intended to reduce the advantage of specialised hardware.
• Operates without tracking, fingerprinting or collecting visitor data.
• Can be entirely self-hosted without dependence on an external CAPTCHA service.
• Provides standard, bar, floating, overlay and invisible display options together with checkbox and switch interaction styles.
• Includes accessible fallback challenges with audio support where an explicit challenge is required.
• Designed around WCAG accessibility requirements and compliance with the European Accessibility Act.
• Uses Web Workers to perform computational challenges without blocking the main browser interface.
• Includes extensive internationalisation with automatic language detection and support for importing individual translations.
• Offers server-side integration libraries covering TypeScript, Go, Python, PHP, Java, Ruby, Rust, Dart, Elixir and other languages.
• Provides configuration controls for challenge behaviour, timeouts, worker counts, themes and custom verification handlers.

Website: github.com/altcha-org/altcha
Support:
Developer: ALTCHA
License: MIT License

ALTCHA is written in TypeScript. Learn TypeScript with our recommended free books and free tutorials.

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