mCaptcha – privacy-respecting proof-of-work CAPTCHA

mCaptcha is a self-hosted CAPTCHA system designed to make abusive automated requests computationally expensive without forcing legitimate users to solve image or text puzzles.

When someone accesses a protected operation, the client generates a proof-of-work and sends it to mCaptcha for validation. Successful validation produces a token which accompanies the user’s request to the protected service. The application can then verify that token before processing the operation. Most of this workflow is automated from the visitor’s point of view.

This is free and open source software.

Key Features

• Uses SHA-256 based proof-of-work as the basis of its challenge mechanism.
• Supports variable computational difficulty so the amount of work required can respond to operating conditions.
• Issues verification tokens which applications can validate before accepting protected requests.
• Uses short-lived proof-of-work configurations which can only be successfully submitted once.
• Protects against replay attacks by rejecting expired or previously used challenge configurations.
• Does not depend on visitor IP addresses, making it suitable for users sharing addresses through NAT.
• Keeps CAPTCHA routes cookie-free and avoids visitor tracking.
• Requires only a simple user action to start verification, with the computational challenge subsequently handled automatically.
• Can be deployed on infrastructure controlled by the website operator rather than depending on a proprietary CAPTCHA provider.
• Separates proof generation, proof validation and application-side token verification, allowing the protected service to retain control over whether a request is accepted.

Website: github.com/mCaptcha/mCaptcha
Support:
Developer: mCaptcha
License: GNU Affero General Public License v3.0

mCaptcha is written in Rust. Learn Rust with our recommended free books and free tutorials.

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