https://docs.cashu.space/

What is Cashu? - Just a Diffie-Hellman Key Exchange

Cashu is an ecash protocol that is integrated with the Bitcoin protocol. An Ecash system consists of two parts, the mint and the Ecash wallet. Anyone can run a mint for their application, be it a wallet, a web paywall, paid streaming services, or a voucher and rewards system for a super market. Ecash transactions between users or from the user to a service provider respect the user's privacy. A mint does not store a database of user accounts and their activity which protects users of an Ecash system from leaks their private data to hackers and can provide stronger censorship resistant than classical payment systems.