Altcha is a similar thing: https://github.com/altcha-org/altcha https://github.com/altcha-org/altcha I recently implemented a very similar thing to its obfusca

Altcha is a similar thing: https://github.com/altcha-org/altcha https://github.com/altcha-org/altcha I recently implemented a very similar thing to its obfuscation via proof-of-work (https://altcha.org/docs/obfuscation/ https://altcha.org/docs/obfuscation/) in my C++ REST backend and flutter front-end, and use it for rate-limiting on APIs that allow creation of a new account or sending sign-up e-mails. I have an authentication token that's then wrapped with AES-GCM using a random IV and the client is given the key, IV stem and a maximum count for the IV.