mCaptcha
mCaptcha is a privacy-respecting, free CAPTCHA system backend that uses SHA256-based proof-of-work to rate limit and verify users. Instead of forcing users to solve visual puzzles, mCaptcha runs computational work in the browser automatically after a single click, requiring no image recognition or manual interaction. When a user submits a request, their client generates a proof-of-work and sends it to the mCaptcha server for validation. Upon successful validation, the server issues a token that the protected website then verifies with mCaptcha before granting access. PoW difficulty adjusts dynamically, causing minimal delay for legitimate users during normal traffic and requiring more work from attackers during high load. Key features include seamless one-click user experience, no cookies or user tracking, IP-independent operation suitable for users behind NAT, and resistance to replay attacks through short-lived, single-use proof configurations. mCaptcha is free software licensed under AGPL and is intended f