Source code and details: https://git.anarchists.space/nemesis/Vigil

A privacy-friendly anti-spam system for account registration on a public Matrix Synapse homeserver. Before an account is created, the client solves an Argon2id puzzle that is intentionally slow and memory-intensive, taking approximately 30 minutes by default, though the homeserver owner can configure the duration. This makes large-scale bot registration expensive.

Synapse's own open registration gets turned off; Vigil is the only
thing that can create an account, and it only does that once a
client proves it solved a fresh puzzle.

Features

• Privacy-friendly
• A lot harder to bypass than traditional anti-spam systems
• Optional CLI tool to solve cryptographic challenges for people who don't wanna use JavaScript
• Small codebase

neat